ConcurrentHashMap.java

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamField;
import java.io.Serializable;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.AbstractMap;
import java.util.Arrays;
import java.util.Collection;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.Spliterator;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.DoubleBinaryOperator;
import java.util.function.Function;
import java.util.function.IntBinaryOperator;
import java.util.function.LongBinaryOperator;
import java.util.function.Predicate;
import java.util.function.ToDoubleBiFunction;
import java.util.function.ToDoubleFunction;
import java.util.function.ToIntBiFunction;
import java.util.function.ToIntFunction;
import java.util.function.ToLongBiFunction;
import java.util.function.ToLongFunction;
import java.util.stream.Stream;
import jdk.internal.misc.Unsafe;

/**
 * A hash table supporting full concurrency of retrievals and
 * high expected concurrency for updates. This class obeys the
 * same functional specification as {@link java.util.Hashtable}, and
 * includes versions of methods corresponding to each method of
 * {@code Hashtable}. However, even though all operations are
 * thread-safe, retrieval operations do <em>not</em> entail locking,
 * and there is <em>not</em> any support for locking the entire table
 * in a way that prevents all access.  This class is fully
 * interoperable with {@code Hashtable} in programs that rely on its
 * thread safety but not on its synchronization details.
 *
 * <p>Retrieval operations (including {@code get}) generally do not
 * block, so may overlap with update operations (including {@code put}
 * and {@code remove}). Retrievals reflect the results of the most
 * recently <em>completed</em> update operations holding upon their
 * onset. (More formally, an update operation for a given key bears a
 * <em>happens-before</em> relation with any (non-null) retrieval for
 * that key reporting the updated value.)  For aggregate operations
 * such as {@code putAll} and {@code clear}, concurrent retrievals may
 * reflect insertion or removal of only some entries.  Similarly,
 * Iterators, Spliterators and Enumerations return elements reflecting the
 * state of the hash table at some point at or since the creation of the
 * iterator/enumeration.  They do <em>not</em> throw {@link
 * java.util.ConcurrentModificationException ConcurrentModificationException}.
 * However, iterators are designed to be used by only one thread at a time.
 * Bear in mind that the results of aggregate status methods including
 * {@code size}, {@code isEmpty}, and {@code containsValue} are typically
 * useful only when a map is not undergoing concurrent updates in other threads.
 * Otherwise the results of these methods reflect transient states
 * that may be adequate for monitoring or estimation purposes, but not
 * for program control.
 *
 * <p>The table is dynamically expanded when there are too many
 * collisions (i.e., keys that have distinct hash codes but fall into
 * the same slot modulo the table size), with the expected average
 * effect of maintaining roughly two bins per mapping (corresponding
 * to a 0.75 load factor threshold for resizing). There may be much
 * variance around this average as mappings are added and removed, but
 * overall, this maintains a commonly accepted time/space tradeoff for
 * hash tables.  However, resizing this or any other kind of hash
 * table may be a relatively slow operation. When possible, it is a
 * good idea to provide a size estimate as an optional {@code
 * initialCapacity} constructor argument. An additional optional
 * {@code loadFactor} constructor argument provides a further means of
 * customizing initial table capacity by specifying the table density
 * to be used in calculating the amount of space to allocate for the
 * given number of elements.  Also, for compatibility with previous
 * versions of this class, constructors may optionally specify an
 * expected {@code concurrencyLevel} as an additional hint for
 * internal sizing.  Note that using many keys with exactly the same
 * {@code hashCode()} is a sure way to slow down performance of any
 * hash table. To ameliorate impact, when keys are {@link Comparable},
 * this class may use comparison order among keys to help break ties.
 *
 * <p>A {@link Set} projection of a ConcurrentHashMap may be created
 * (using {@link #newKeySet()} or {@link #newKeySet(int)}), or viewed
 * (using {@link #keySet(Object)} when only keys are of interest, and the
 * mapped values are (perhaps transiently) not used or all take the
 * same mapping value.
 *
 * <p>A ConcurrentHashMap can be used as a scalable frequency map (a
 * form of histogram or multiset) by using {@link
 * java.util.concurrent.atomic.LongAdder} values and initializing via
 * {@link #computeIfAbsent computeIfAbsent}. For example, to add a count
 * to a {@code ConcurrentHashMap<String,LongAdder> freqs}, you can use
 * {@code freqs.computeIfAbsent(key, k -> new LongAdder()).increment();}
 *
 * <p>This class and its views and iterators implement all of the
 * <em>optional</em> methods of the {@link Map} and {@link Iterator}
 * interfaces.
 *
 * <p>Like {@link Hashtable} but unlike {@link HashMap}, this class
 * does <em>not</em> allow {@code null} to be used as a key or value.
 *
 * <p>ConcurrentHashMaps support a set of sequential and parallel bulk
 * operations that, unlike most {@link Stream} methods, are designed
 * to be safely, and often sensibly, applied even with maps that are
 * being concurrently updated by other threads; for example, when
 * computing a snapshot summary of the values in a shared registry.
 * There are three kinds of operation, each with four forms, accepting
 * functions with keys, values, entries, and (key, value) pairs as
 * arguments and/or return values. Because the elements of a
 * ConcurrentHashMap are not ordered in any particular way, and may be
 * processed in different orders in different parallel executions, the
 * correctness of supplied functions should not depend on any
 * ordering, or on any other objects or values that may transiently
 * change while computation is in progress; and except for forEach
 * actions, should ideally be side-effect-free. Bulk operations on
 * {@link Map.Entry} objects do not support method {@code setValue}.
 *
 * <ul>
 * <li>forEach: Performs a given action on each element.
 * A variant form applies a given transformation on each element
 * before performing the action.
 *
 * <li>search: Returns the first available non-null result of
 * applying a given function on each element; skipping further
 * search when a result is found.
 *
 * <li>reduce: Accumulates each element.  The supplied reduction
 * function cannot rely on ordering (more formally, it should be
 * both associative and commutative).  There are five variants:
 *
 * <ul>
 *
 * <li>Plain reductions. (There is not a form of this method for
 * (key, value) function arguments since there is no corresponding
 * return type.)
 *
 * <li>Mapped reductions that accumulate the results of a given
 * function applied to each element.
 *
 * <li>Reductions to scalar doubles, longs, and ints, using a
 * given basis value.
 *
 * </ul>
 * </ul>
 *
 * <p>These bulk operations accept a {@code parallelismThreshold}
 * argument. Methods proceed sequentially if the current map size is
 * estimated to be less than the given threshold. Using a value of
 * {@code Long.MAX_VALUE} suppresses all parallelism.  Using a value
 * of {@code 1} results in maximal parallelism by partitioning into
 * enough subtasks to fully utilize the {@link
 * ForkJoinPool#commonPool()} that is used for all parallel
 * computations. Normally, you would initially choose one of these
 * extreme values, and then measure performance of using in-between
 * values that trade off overhead versus throughput.
 *
 * <p>The concurrency properties of bulk operations follow
 * from those of ConcurrentHashMap: Any non-null result returned
 * from {@code get(key)} and related access methods bears a
 * happens-before relation with the associated insertion or
 * update.  The result of any bulk operation reflects the
 * composition of these per-element relations (but is not
 * necessarily atomic with respect to the map as a whole unless it
 * is somehow known to be quiescent).  Conversely, because keys
 * and values in the map are never null, null serves as a reliable
 * atomic indicator of the current lack of any result.  To
 * maintain this property, null serves as an implicit basis for
 * all non-scalar reduction operations. For the double, long, and
 * int versions, the basis should be one that, when combined with
 * any other value, returns that other value (more formally, it
 * should be the identity element for the reduction). Most common
 * reductions have these properties; for example, computing a sum
 * with basis 0 or a minimum with basis MAX_VALUE.
 *
 * <p>Search and transformation functions provided as arguments
 * should similarly return null to indicate the lack of any result
 * (in which case it is not used). In the case of mapped
 * reductions, this also enables transformations to serve as
 * filters, returning null (or, in the case of primitive
 * specializations, the identity basis) if the element should not
 * be combined. You can create compound transformations and
 * filterings by composing them yourself under this "null means
 * there is nothing there now" rule before using them in search or
 * reduce operations.
 *
 * <p>Methods accepting and/or returning Entry arguments maintain
 * key-value associations. They may be useful for example when
 * finding the key for the greatest value. Note that "plain" Entry
 * arguments can be supplied using {@code new
 * AbstractMap.SimpleEntry(k,v)}.
 *
 * <p>Bulk operations may complete abruptly, throwing an
 * exception encountered in the application of a supplied
 * function. Bear in mind when handling such exceptions that other
 * concurrently executing functions could also have thrown
 * exceptions, or would have done so if the first exception had
 * not occurred.
 *
 * <p>Speedups for parallel compared to sequential forms are common
 * but not guaranteed.  Parallel operations involving brief functions
 * on small maps may execute more slowly than sequential forms if the
 * underlying work to parallelize the computation is more expensive
 * than the computation itself.  Similarly, parallelization may not
 * lead to much actual parallelism if all processors are busy
 * performing unrelated tasks.
 *
 * <p>All arguments to all task methods must be non-null.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */
/*
 * ConcurrentHashMap结构：哈希数组+链表/红黑树，key和value均不能为null
 * ConcurrentHashMap的操作方式跟HashMap是基本统一的，
 * 不同之处在于，ConcurrentHashMap是线程安全的，其中用到了无锁编程
 */
public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable {
    
    /*
     * Overview:
     *
     * The primary design goal of this hash table is to maintain
     * concurrent readability (typically method get(), but also
     * iterators and related methods) while minimizing update
     * contention. Secondary goals are to keep space consumption about
     * the same or better than java.util.HashMap, and to support high
     * initial insertion rates on an empty table by many threads.
     *
     * This map usually acts as a binned (bucketed) hash table.  Each
     * key-value mapping is held in a Node.  Most nodes are instances
     * of the basic Node class with hash, key, value, and next
     * fields. However, various subclasses exist: TreeNodes are
     * arranged in balanced trees, not lists.  TreeBins hold the roots
     * of sets of TreeNodes. ForwardingNodes are placed at the heads
     * of bins during resizing. ReservationNodes are used as
     * placeholders while establishing values in computeIfAbsent and
     * related methods.  The types TreeBin, ForwardingNode, and
     * ReservationNode do not hold normal user keys, values, or
     * hashes, and are readily distinguishable during search etc
     * because they have negative hash fields and null key and value
     * fields. (These special nodes are either uncommon or transient,
     * so the impact of carrying around some unused fields is
     * insignificant.)
     *
     * The table is lazily initialized to a power-of-two size upon the
     * first insertion.  Each bin in the table normally contains a
     * list of Nodes (most often, the list has only zero or one Node).
     * Table accesses require volatile/atomic reads, writes, and
     * CASes.  Because there is no other way to arrange this without
     * adding further indirections, we use intrinsics
     * (jdk.internal.misc.Unsafe) operations.
     *
     * We use the top (sign) bit of Node hash fields for control
     * purposes -- it is available anyway because of addressing
     * constraints.  Nodes with negative hash fields are specially
     * handled or ignored in map methods.
     *
     * Insertion (via put or its variants) of the first node in an
     * empty bin is performed by just CASing it to the bin.  This is
     * by far the most common case for put operations under most
     * key/hash distributions.  Other update operations (insert,
     * delete, and replace) require locks.  We do not want to waste
     * the space required to associate a distinct lock object with
     * each bin, so instead use the first node of a bin list itself as
     * a lock. Locking support for these locks relies on builtin
     * "synchronized" monitors.
     *
     * Using the first node of a list as a lock does not by itself
     * suffice though: When a node is locked, any update must first
     * validate that it is still the first node after locking it, and
     * retry if not. Because new nodes are always appended to lists,
     * once a node is first in a bin, it remains first until deleted
     * or the bin becomes invalidated (upon resizing).
     *
     * The main disadvantage of per-bin locks is that other update
     * operations on other nodes in a bin list protected by the same
     * lock can stall, for example when user equals() or mapping
     * functions take a long time.  However, statistically, under
     * random hash codes, this is not a common problem.  Ideally, the
     * frequency of nodes in bins follows a Poisson distribution
     * (http://en.wikipedia.org/wiki/Poisson_distribution) with a
     * parameter of about 0.5 on average, given the resizing threshold
     * of 0.75, although with a large variance because of resizing
     * granularity. Ignoring variance, the expected occurrences of
     * list size k are (exp(-0.5) * pow(0.5, k) / factorial(k)). The
     * first values are:
     *
     * 0:    0.60653066
     * 1:    0.30326533
     * 2:    0.07581633
     * 3:    0.01263606
     * 4:    0.00157952
     * 5:    0.00015795
     * 6:    0.00001316
     * 7:    0.00000094
     * 8:    0.00000006
     * more: less than 1 in ten million
     *
     * Lock contention probability for two threads accessing distinct
     * elements is roughly 1 / (8 * #elements) under random hashes.
     *
     * Actual hash code distributions encountered in practice
     * sometimes deviate significantly from uniform randomness.  This
     * includes the case when N > (1<<30), so some keys MUST collide.
     * Similarly for dumb or hostile usages in which multiple keys are
     * designed to have identical hash codes or ones that differs only
     * in masked-out high bits. So we use a secondary strategy that
     * applies when the number of nodes in a bin exceeds a
     * threshold. These TreeBins use a balanced tree to hold nodes (a
     * specialized form of red-black trees), bounding search time to
     * O(log N).  Each search step in a TreeBin is at least twice as
     * slow as in a regular list, but given that N cannot exceed
     * (1<<64) (before running out of addresses) this bounds search
     * steps, lock hold times, etc, to reasonable constants (roughly
     * 100 nodes inspected per operation worst case) so long as keys
     * are Comparable (which is very common -- String, Long, etc).
     * TreeBin nodes (TreeNodes) also maintain the same "next"
     * traversal pointers as regular nodes, so can be traversed in
     * iterators in the same way.
     *
     * The table is resized when occupancy exceeds a percentage
     * threshold (nominally, 0.75, but see below).  Any thread
     * noticing an overfull bin may assist in resizing after the
     * initiating thread allocates and sets up the replacement array.
     * However, rather than stalling, these other threads may proceed
     * with insertions etc.  The use of TreeBins shields us from the
     * worst case effects of overfilling while resizes are in
     * progress.  Resizing proceeds by transferring bins, one by one,
     * from the table to the next table. However, threads claim small
     * blocks of indices to transfer (via field transferIndex) before
     * doing so, reducing contention.  A generation stamp in field
     * sizeCtl ensures that resizings do not overlap. Because we are
     * using power-of-two expansion, the elements from each bin must
     * either stay at same index, or move with a power of two
     * offset. We eliminate unnecessary node creation by catching
     * cases where old nodes can be reused because their next fields
     * won't change.  On average, only about one-sixth of them need
     * cloning when a table doubles. The nodes they replace will be
     * garbage collectable as soon as they are no longer referenced by
     * any reader thread that may be in the midst of concurrently
     * traversing table.  Upon transfer, the old table bin contains
     * only a special forwarding node (with hash field "MOVED") that
     * contains the next table as its key. On encountering a
     * forwarding node, access and update operations restart, using
     * the new table.
     *
     * Each bin transfer requires its bin lock, which can stall
     * waiting for locks while resizing. However, because other
     * threads can join in and help resize rather than contend for
     * locks, average aggregate waits become shorter as resizing
     * progresses.  The transfer operation must also ensure that all
     * accessible bins in both the old and new table are usable by any
     * traversal.  This is arranged in part by proceeding from the
     * last bin (table.length - 1) up towards the first.  Upon seeing
     * a forwarding node, traversals (see class Traverser) arrange to
     * move to the new table without revisiting nodes.  To ensure that
     * no intervening nodes are skipped even when moved out of order,
     * a stack (see class TableStack) is created on first encounter of
     * a forwarding node during a traversal, to maintain its place if
     * later processing the current table. The need for these
     * save/restore mechanics is relatively rare, but when one
     * forwarding node is encountered, typically many more will be.
     * So Traversers use a simple caching scheme to avoid creating so
     * many new TableStack nodes. (Thanks to Peter Levart for
     * suggesting use of a stack here.)
     *
     * The traversal scheme also applies to partial traversals of
     * ranges of bins (via an alternate Traverser constructor)
     * to support partitioned aggregate operations.  Also, read-only
     * operations give up if ever forwarded to a null table, which
     * provides support for shutdown-style clearing, which is also not
     * currently implemented.
     *
     * Lazy table initialization minimizes footprint until first use,
     * and also avoids resizings when the first operation is from a
     * putAll, constructor with map argument, or deserialization.
     * These cases attempt to override the initial capacity settings,
     * but harmlessly fail to take effect in cases of races.
     *
     * The element count is maintained using a specialization of
     * LongAdder. We need to incorporate a specialization rather than
     * just use a LongAdder in order to access implicit
     * contention-sensing that leads to creation of multiple
     * CounterCells.  The counter mechanics avoid contention on
     * updates but can encounter cache thrashing if read too
     * frequently during concurrent access. To avoid reading so often,
     * resizing under contention is attempted only upon adding to a
     * bin already holding two or more nodes. Under uniform hash
     * distributions, the probability of this occurring at threshold
     * is around 13%, meaning that only about 1 in 8 puts check
     * threshold (and after resizing, many fewer do so).
     *
     * TreeBins use a special form of comparison for search and
     * related operations (which is the main reason we cannot use
     * existing collections such as TreeMaps). TreeBins contain
     * Comparable elements, but may contain others, as well as
     * elements that are Comparable but not necessarily Comparable for
     * the same T, so we cannot invoke compareTo among them. To handle
     * this, the tree is ordered primarily by hash value, then by
     * Comparable.compareTo order if applicable.  On lookup at a node,
     * if elements are not comparable or compare as 0 then both left
     * and right children may need to be searched in the case of tied
     * hash values. (This corresponds to the full list search that
     * would be necessary if all elements were non-Comparable and had
     * tied hashes.) On insertion, to keep a total ordering (or as
     * close as is required here) across rebalancings, we compare
     * classes and identityHashCodes as tie-breakers. The red-black
     * balancing code is updated from pre-jdk-collections
     * (http://gee.cs.oswego.edu/dl/classes/collections/RBCell.java)
     * based in turn on Cormen, Leiserson, and Rivest "Introduction to
     * Algorithms" (CLR).
     *
     * TreeBins also require an additional locking mechanism.  While
     * list traversal is always possible by readers even during
     * updates, tree traversal is not, mainly because of tree-rotations
     * that may change the root node and/or its linkages.  TreeBins
     * include a simple read-write lock mechanism parasitic on the
     * main bin-synchronization strategy: Structural adjustments
     * associated with an insertion or removal are already bin-locked
     * (and so cannot conflict with other writers) but must wait for
     * ongoing readers to finish. Since there can be only one such
     * waiter, we use a simple scheme using a single "waiter" field to
     * block writers.  However, readers need never block.  If the root
     * lock is held, they proceed along the slow traversal path (via
     * next-pointers) until the lock becomes available or the list is
     * exhausted, whichever comes first. These cases are not fast, but
     * maximize aggregate expected throughput.
     *
     * Maintaining API and serialization compatibility with previous
     * versions of this class introduces several oddities. Mainly: We
     * leave untouched but unused constructor arguments referring to
     * concurrencyLevel. We accept a loadFactor constructor argument,
     * but apply it only to initial table capacity (which is the only
     * time that we can guarantee to honor it.) We also declare an
     * unused "Segment" class that is instantiated in minimal form
     * only when serializing.
     *
     * Also, solely for compatibility with previous versions of this
     * class, it extends AbstractMap, even though all of its methods
     * are overridden, so it is just useless baggage.
     *
     * This file is organized to make things a little easier to follow
     * while reading than they might otherwise: First the main static
     * declarations and utilities, then fields, then main public
     * methods (with a few factorings of multiple public methods into
     * internal ones), then sizing methods, trees, traversers, and
     * bulk operations.
     */
    
    
    /**
     * The default concurrency level for this table.
     * Unused but defined for compatibility with previous versions of this class.
     */
    private static final int DEFAULT_CONCURRENCY_LEVEL = 16;    // 默认并发级别，未使用，只是为与该类的先前版本兼容
    
    /**
     * The largest possible (non-power of two) array size.
     * Needed by toArray and related methods.
     */
    static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;    // 哈希数组最大容量(用于toArray中的阈值判断)
    
    /**
     * The largest possible table capacity.  This value must be
     * exactly 1<<30 to stay within Java array allocation and indexing
     * bounds for power of two table sizes, and is further required
     * because the top two bits of 32bit hash fields are used for
     * control purposes.
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;        // 哈希数组最大容量
    
    /**
     * The default initial table capacity.  Must be a power of 2
     * (i.e., at least 1) and at most MAXIMUM_CAPACITY.
     */
    private static final int DEFAULT_CAPACITY = 16;             // 哈希数组默认容量
    
    /**
     * The load factor for this table. Overrides of this value in
     * constructors affect only the initial table capacity.  The
     * actual floating point value isn't normally used -- it is
     * simpler to use expressions such as {@code n - (n >>> 2)} for
     * the associated resizing threshold.
     */
    private static final float LOAD_FACTOR = 0.75f;     // ConcurrentHashMap默认装载因子（负荷系数）
    
    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2, and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     */
    static final int TREEIFY_THRESHOLD = 8;     // 某个哈希槽（链）上的元素数量增加到此值后，这些元素进入波动期，即将从链表转换为红黑树
    
    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * The value should be at least 4 * TREEIFY_THRESHOLD to avoid
     * conflicts between resizing and treeification thresholds.
     */
    static final int MIN_TREEIFY_CAPACITY = 64; // 哈希数组的容量至少增加到此值，且满足TREEIFY_THRESHOLD的要求时，将链表转换为红黑树
    
    /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     */
    static final int UNTREEIFY_THRESHOLD = 6;   // 哈希槽（链）上的红黑树上的元素数量减少到此值时，将红黑树转换为链表
    
    /**
     * Minimum number of rebinnings per transfer step. Ranges are
     * subdivided to allow multiple resizer threads.  This value
     * serves as a lower bound to avoid resizers encountering
     * excessive memory contention.  The value should be at least
     * DEFAULT_CAPACITY.
     */
    private static final int MIN_TRANSFER_STRIDE = 16;
    
    /**
     * The number of bits used for generation stamp in sizeCtl.
     * Must be at least 6 for 32bit arrays.
     */
    private static final int RESIZE_STAMP_BITS = 16;
    
    /**
     * The maximum number of threads that can help resize.
     * Must fit in 32 - RESIZE_STAMP_BITS bits.
     */
    private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
    
    /**
     * The bit shift for recording size stamp in sizeCtl.
     */
    private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
    
    /*
     * Encodings for Node hash fields. See above for explanation.
     */
    static final int MOVED     = -1; // hash for forwarding nodes       // 前向结点
    static final int TREEBIN   = -2; // hash for roots of trees         // 红黑树(头结点)
    static final int RESERVED  = -3; // hash for transient reservations // 占位结点
    static final int HASH_BITS = 0x7fffffff; // usable bits of normal node hash
    
    /** Number of CPUS, to place bounds on some sizings */
    // 虚拟机可用的处理器数量
    static final int NCPU = Runtime.getRuntime().availableProcessors();
    
    
    /* ---------------- Fields -------------- */
    
    /**
     * The array of bins. Lazily initialized upon first insertion.
     * Size is always a power of two. Accessed directly by iterators.
     */
    transient volatile Node<K,V>[] table;   // 哈希数组（注：哈希数组的容量跟ConcurrentHashMap可以存储的元素数量不是一回事）
    
    /**
     * The next table to use; non-null only while resizing.
     */
    private transient volatile Node<K,V>[] nextTable;   // 哈希数组扩容时使用的新数组
    
    /**
     * Base counter value, used mainly when there is no contention,
     * but also as a fallback during table initialization
     * races. Updated via CAS.
     */
    private transient volatile long baseCount;
    
    /**
     * Table initialization and resizing control.
     * When negative, the table is being initialized or resized: -1 for initialization,
     * else -(1 + the number of active resizing threads).
     * Otherwise, when table is null, holds the initial table size to use upon creation, or 0 for default.
     * After initialization, holds the next element count value upon which to resize the table.
     */
    /*
     * 当哈希数组还未创建时，sizeCtl代表预设容量，可由初始容量计算而来，参见构造器中的计算方式
     * 如果sizeCtl的值为-1，则代表当前Map正在初始化哈希数组
     * 当哈希数组已创建后，sizeCtl代表扩容阈值，通常是0.75*容量
     */
    private transient volatile int sizeCtl;
    
    /**
     * The next table index (plus one) to split while resizing.
     */
    private transient volatile int transferIndex;
    
    /**
     * Spinlock (locked via CAS) used when resizing and/or creating CounterCells.
     */
    private transient volatile int cellsBusy;
    
    /**
     * Table of counter cells. When non-null, size is a power of 2.
     */
    private transient volatile CounterCell[] counterCells;
    
    // views
    private transient KeySetView<K,V> keySet;
    private transient ValuesView<K,V> values;
    private transient EntrySetView<K,V> entrySet;
    
    
    // Unsafe mechanics
    private static final Unsafe U = Unsafe.getUnsafe();
    private static final long SIZECTL;
    private static final long TRANSFERINDEX;
    private static final long BASECOUNT;
    private static final long CELLSBUSY;
    private static final long CELLVALUE;
    private static final int ABASE;
    private static final int ASHIFT;
    static {
        SIZECTL = U.objectFieldOffset(ConcurrentHashMap.class, "sizeCtl");
        TRANSFERINDEX = U.objectFieldOffset(ConcurrentHashMap.class, "transferIndex");
        BASECOUNT = U.objectFieldOffset(ConcurrentHashMap.class, "baseCount");
        CELLSBUSY = U.objectFieldOffset(ConcurrentHashMap.class, "cellsBusy");
        CELLVALUE = U.objectFieldOffset(CounterCell.class, "value");
        
        // 寻找Node数组中的元素时约定的起始偏移地址（更像是一个标记）
        ABASE = U.arrayBaseOffset(Node[].class);
        
        // Node数组每个元素所占字节数（必须为2的冪）
        int scale = U.arrayIndexScale(Node[].class);
        if((scale & (scale - 1)) != 0) {
            throw new ExceptionInInitializerError("array index scale not a power of two");
        }
        
        // 计算log2(scale)，并向下取整
        ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
        
        // Reduce the risk of rare disastrous classloading in first call to
        // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
        Class<?> ensureLoaded = LockSupport.class;
        
        // Eager class load observed to help JIT during startup
        ensureLoaded = ReservationNode.class;
    }
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a new, empty map with the default initial table size (16).
     */
    public ConcurrentHashMap() {
    }
    
    /**
     * Creates a new, empty map with an initial table size
     * accommodating the specified number of elements without the need
     * to dynamically resize.
     *
     * @param initialCapacity The implementation performs internal
     *                        sizing to accommodate this many elements.
     *
     * @throws IllegalArgumentException if the initial capacity of
     *                                  elements is negative
     */
    public ConcurrentHashMap(int initialCapacity) {
        this(initialCapacity, LOAD_FACTOR, 1);
    }
    
    /**
     * Creates a new, empty map with an initial table size based on
     * the given number of elements ({@code initialCapacity}) and
     * initial table density ({@code loadFactor}).
     *
     * @param initialCapacity the initial capacity. The implementation
     *                        performs internal sizing to accommodate this many elements,
     *                        given the specified load factor.
     * @param loadFactor      the load factor (table density) for
     *                        establishing the initial table size
     *
     * @throws IllegalArgumentException if the initial capacity of
     *                                  elements is negative or the load factor is nonpositive
     * @since 1.6
     */
    public ConcurrentHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, 1);
    }
    
    /**
     * Creates a new, empty map with an initial table size based on
     * the given number of elements ({@code initialCapacity}), initial
     * table density ({@code loadFactor}), and number of concurrently
     * updating threads ({@code concurrencyLevel}).
     *
     * @param initialCapacity  the initial capacity. The implementation
     *                         performs internal sizing to accommodate this many elements,
     *                         given the specified load factor.
     * @param loadFactor       the load factor (table density) for
     *                         establishing the initial table size
     * @param concurrencyLevel the estimated number of concurrently
     *                         updating threads. The implementation may use this value as
     *                         a sizing hint.
     *
     * @throws IllegalArgumentException if the initial capacity is
     *                                  negative or the load factor or concurrencyLevel are
     *                                  nonpositive
     */
    public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
        if(!(loadFactor>0.0f) || initialCapacity<0 || concurrencyLevel<=0) {
            throw new IllegalArgumentException();
        }
        
        if(initialCapacity<concurrencyLevel) {  // Use at least as many bins
            initialCapacity = concurrencyLevel;   // as estimated threads
        }
        
        long size = (long) (1.0 + (long) initialCapacity / loadFactor);
        
        this.sizeCtl = (size >= (long) MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : tableSizeFor((int) size);
    }
    
    /**
     * Creates a new map with the same mappings as the given map.
     *
     * @param m the map
     */
    public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
        this.sizeCtl = DEFAULT_CAPACITY;
        putAll(m);
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 存值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Maps the specified key to the specified value in this table.
     * Neither the key nor the value can be null.
     *
     * <p>The value can be retrieved by calling the {@code get} method
     * with a key that is equal to the original key.
     *
     * @param key   key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     *
     * @return the previous value associated with {@code key}, or
     * {@code null} if there was no mapping for {@code key}
     *
     * @throws NullPointerException if the specified key or value is null
     */
    // 将指定的元素（key-value）存入当前map，并返回旧值，允许覆盖
    public V put(K key, V value) {
        return putVal(key, value, false);
    }
    
    /**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     * or {@code null} if there was no mapping for the key
     *
     * @throws NullPointerException if the specified key or value is null
     */
    // 将指定的元素（key-value）存入当前map，并返回旧值，不允许覆盖
    public V putIfAbsent(K key, V value) {
        return putVal(key, value, true);
    }
    
    /**
     * Copies all of the mappings from the specified map to this one.
     * These mappings replace any mappings that this map had for any of the
     * keys currently in the specified map.
     *
     * @param m mappings to be stored in this map
     */
    // 将指定Map中的元素存入到当前Map（允许覆盖）
    public void putAll(Map<? extends K, ? extends V> m) {
        tryPresize(m.size());
        
        for(Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
            putVal(e.getKey(), e.getValue(), false);
        }
    }
    
    /*▲ 存值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 取值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code key.equals(k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * @throws NullPointerException if the specified key is null
     */
    // 根据指定的key获取对应的value，如果不存在，则返回null
    public V get(Object key) {
        Node<K, V>[] tab;
        Node<K, V> e, p;
        int n, eh;
        K ek;
        
        int h = spread(key.hashCode());
        
        if((tab = table) != null && (n = tab.length)>0 && (e = tabAt(tab, (n - 1) & h)) != null) {
            if((eh = e.hash) == h) {
                if((ek = e.key) == key || (ek != null && key.equals(ek))) {
                    return e.val;
                }
            } else if(eh<0) {
                return (p = e.find(h, key)) != null ? p.val : null;
            }
            
            while((e = e.next) != null) {
                if(e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek)))) {
                    return e.val;
                }
            }
        }
        
        return null;
    }
    
    /**
     * Returns the value to which the specified key is mapped, or the
     * given default value if this map contains no mapping for the
     * key.
     *
     * @param key          the key whose associated value is to be returned
     * @param defaultValue the value to return if this map contains
     *                     no mapping for the given key
     *
     * @return the mapping for the key, if present; else the default value
     *
     * @throws NullPointerException if the specified key is null
     */
    // 根据指定的key获取对应的value，如果不存在，则返回指定的默认值defaultValue
    public V getOrDefault(Object key, V defaultValue) {
        V v = get(key);
        return v == null ? defaultValue : v;
    }
    
    /*▲ 取值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 移除 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Removes the key (and its corresponding value) from this map.
     * This method does nothing if the key is not in the map.
     *
     * @param key the key that needs to be removed
     *
     * @return the previous value associated with {@code key}, or
     * {@code null} if there was no mapping for {@code key}
     *
     * @throws NullPointerException if the specified key is null
     */
    // 移除拥有指定key的元素，并返回刚刚移除的元素的值
    public V remove(Object key) {
        return replaceNode(key, null, null);
    }
    
    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if the specified key is null
     */
    // 移除拥有指定key和value的元素，返回值表示是否移除成功
    public boolean remove(Object key, Object value) {
        if(key == null) {
            throw new NullPointerException();
        }
        
        return value != null && replaceNode(key, null, value) != null;
    }
    
    
    /**
     * Removes all of the mappings from this map.
     */
    // 清空当前Map中所有元素
    public void clear() {
        long delta = 0L; // negative number of deletions
        int i = 0;
        
        Node<K, V>[] tab = table;
        
        while(tab != null && i<tab.length) {
            int fh;
            Node<K, V> f = tabAt(tab, i);
            
            if(f == null) {
                ++i;
            } else if((fh = f.hash) == MOVED) {
                tab = helpTransfer(tab, f);
                i = 0; // restart
            } else {
                synchronized(f) {
                    if(tabAt(tab, i) == f) {
                        Node<K, V> p = (fh >= 0 ? f : (f instanceof TreeBin) ? ((TreeBin<K, V>) f).first : null);
                        while(p != null) {
                            --delta;
                            p = p.next;
                        }
                        
                        // 设置tab[i]为null
                        setTabAt(tab, i++, null);
                    }
                }
            }
        }
        
        if(delta != 0L) {
            addCount(delta, -1);
        }
    }
    
    /*▲ 移除 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 替换 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     * or {@code null} if there was no mapping for the key
     *
     * @throws NullPointerException if the specified key or value is null
     */
    // 将拥有指定key的元素的值替换为newValue，并返回刚刚替换的元素的值（替换失败返回null）
    public V replace(K key, V newValue) {
        if(key == null || newValue == null) {
            throw new NullPointerException();
        }
        
        return replaceNode(key, newValue, null);
    }
    
    /**
     * {@inheritDoc}
     *
     * @throws NullPointerException if any of the arguments are null
     */
    // 将拥有指定key和oldValue的元素的值替换为newValue，返回值表示是否成功替换
    public boolean replace(K key, V oldValue, V newValue) {
        if(key == null || oldValue == null || newValue == null) {
            throw new NullPointerException();
        }
        
        return replaceNode(key, newValue, oldValue) != null;
    }
    
    // 替换当前Map中的所有元素，替换策略由function决定，function的入参是元素的key和value，出参作为新值
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        if(function == null) {
            throw new NullPointerException();
        }
        
        Node<K, V>[] t;
        
        if((t = table) != null) {
            Traverser<K, V> it = new Traverser<>(t, t.length, 0, t.length);
            
            for(Node<K, V> p; (p = it.advance()) != null; ) {
                V oldValue = p.val;
                
                for(K key = p.key; ; ) {
                    V newValue = function.apply(key, oldValue);
                    
                    if(newValue == null) {
                        throw new NullPointerException();
                    }
                    
                    if(replaceNode(key, newValue, oldValue) != null || (oldValue = get(key)) == null) {
                        break;
                    }
                }
            }
        }
    }
    
    /*▲ 替换 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 包含查询 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Tests if the specified object is a key in this table.
     *
     * @param key possible key
     *
     * @return {@code true} if and only if the specified object
     * is a key in this table, as determined by the
     * {@code equals} method; {@code false} otherwise
     *
     * @throws NullPointerException if the specified key is null
     */
    // 判断Map中是否存在指定key的元素
    public boolean containsKey(Object key) {
        return get(key) != null;
    }
    
    /**
     * Returns {@code true} if this map maps one or more keys to the
     * specified value. Note: This method may require a full traversal
     * of the map, and is much slower than method {@code containsKey}.
     *
     * @param value value whose presence in this map is to be tested
     *
     * @return {@code true} if this map maps one or more keys to the
     * specified value
     *
     * @throws NullPointerException if the specified value is null
     */
    // 判断Map中是否存在指定value的元素
    public boolean containsValue(Object value) {
        if(value == null) {
            throw new NullPointerException();
        }
        
        Node<K, V>[] t = table;
        if(t != null) {
            Traverser<K, V> it = new Traverser<>(t, t.length, 0, t.length);
            
            for(Node<K, V> p; (p = it.advance()) != null; ) {
                V v = p.val;
                if(v == value || (v != null && value.equals(v))) {
                    return true;
                }
            }
        }
        
        return false;
    }
    
    /**
     * Tests if some key maps into the specified value in this table.
     *
     * <p>Note that this method is identical in functionality to
     * {@link #containsValue(Object)}, and exists solely to ensure
     * full compatibility with class {@link java.util.Hashtable},
     * which supported this method prior to introduction of the
     * Java Collections Framework.
     *
     * @param value a value to search for
     *
     * @return {@code true} if and only if some key maps to the
     * {@code value} argument in this table as
     * determined by the {@code equals} method;
     * {@code false} otherwise
     *
     * @throws NullPointerException if the specified value is null
     */
    // 判断Map中是否存在指定value的元素
    public boolean contains(Object value) {
        return containsValue(value);
    }
    
    /*▲ 包含查询 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 视图 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a {@link Set} view of the keys contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa. The set supports element
     * removal, which removes the corresponding mapping from this map,
     * via the {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.  It does not support the {@code add} or
     * {@code addAll} operations.
     *
     * <p>The view's iterators and spliterators are
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * <p>The view's {@code spliterator} reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#DISTINCT}, and {@link Spliterator#NONNULL}.
     *
     * @return the set view
     */
    // 获取Map中key的集合
    public KeySetView<K, V> keySet() {
        KeySetView<K, V> ks;
        if((ks = keySet) != null) {
            return ks;
        }
        return keySet = new KeySetView<K, V>(this, null);
    }
    
    /**
     * Returns a {@link Collection} view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are
     * reflected in the collection, and vice-versa.  The collection
     * supports element removal, which removes the corresponding
     * mapping from this map, via the {@code Iterator.remove},
     * {@code Collection.remove}, {@code removeAll},
     * {@code retainAll}, and {@code clear} operations.  It does not
     * support the {@code add} or {@code addAll} operations.
     *
     * <p>The view's iterators and spliterators are
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * <p>The view's {@code spliterator} reports {@link Spliterator#CONCURRENT}
     * and {@link Spliterator#NONNULL}.
     *
     * @return the collection view
     */
    // 获取Map中value的集合
    public Collection<V> values() {
        ValuesView<K, V> vs;
        if((vs = values) != null) {
            return vs;
        }
        return values = new ValuesView<K, V>(this);
    }
    
    /**
     * Returns a {@link Set} view of the mappings contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  The set supports element
     * removal, which removes the corresponding mapping from the map,
     * via the {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.
     *
     * <p>The view's iterators and spliterators are
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * <p>The view's {@code spliterator} reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#DISTINCT}, and {@link Spliterator#NONNULL}.
     *
     * @return the set view
     */
    // 获取Map中key-value对的集合
    public Set<Map.Entry<K, V>> entrySet() {
        EntrySetView<K, V> es;
        if((es = entrySet) != null) {
            return es;
        }
        return entrySet = new EntrySetView<K, V>(this);
    }
    
    /*▲ 视图 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 遍历 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    // 遍历ConcurrentHashMap中的元素，并对其应用action操作，action的入参是元素的key和value
    public void forEach(BiConsumer<? super K, ? super V> action) {
        if (action == null) {
            throw new NullPointerException();
        }
        
        Node<K,V>[] t;
        if ((t = table) != null) {
            Traverser<K,V> it = new Traverser<K,V>(t, t.length, 0, t.length);
            for (Node<K,V> p; (p = it.advance()) != null; ) {
                action.accept(p.key, p.val);
            }
        }
    }
    
    /**
     * Returns an enumeration of the keys in this table.
     *
     * @return an enumeration of the keys in this table
     *
     * @see #keySet()
     */
    // 返回关于键集的枚举
    public Enumeration<K> keys() {
        Node<K, V>[] t;
        int f = (t = table) == null ? 0 : t.length;
        return new KeyIterator<K, V>(t, f, 0, f, this);
    }
    
    /**
     * Returns an enumeration of the values in this table.
     *
     * @return an enumeration of the values in this table
     *
     * @see #values()
     */
    // 返回关于值集的枚举
    public Enumeration<V> elements() {
        Node<K, V>[] t;
        int f = (t = table) == null ? 0 : t.length;
        return new ValueIterator<K, V>(t, f, 0, f, this);
    }
    
    /*▲ 遍历 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 重新映射 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * If the specified key is not already associated with a
     * (non-null) value, associates it with the given value.
     * Otherwise, replaces the value with the results of the given
     * remapping function, or removes if {@code null}. The entire
     * method invocation is performed atomically.  Some attempted
     * update operations on this map by other threads may be blocked
     * while computation is in progress, so the computation should be
     * short and simple, and must not attempt to update any other
     * mappings of this Map.
     *
     * @param key               key with which the specified value is to be associated
     * @param value             the value to use if absent
     * @param remappingFunction the function to recompute a value if present
     *
     * @return the new value associated with the specified key, or null if none
     *
     * @throws NullPointerException if the specified key or the
     *                              remappingFunction is null
     * @throws RuntimeException     or Error if the remappingFunction does so,
     *                              in which case the mapping is unchanged
     */
    /*
     * 插入/删除/替换操作，返回新值（可能为null）
     * 此方法的主要意图：使用备用value和旧value创造的新value来更新旧value
     *
     * 注：以下流程图中，涉及到判断(◇)时，纵向代表【是】，横向代表【否】。此外，使用★代表计算。
     *
     *  ●查找同位元素●
     *      |
     *      ↓
     * ◇存在同位元素◇    --→ ★新value=备用value★ --→ ■如果新value不为null，【插入】新value■
     *      | 是         否
     *      ↓
     * ★新value=(旧value, 备用value)★
     *      |
     *      ↓
     * ◇新value不为null◇ --→ ■【删除】同位元素■
     *      | 是          否
     *      ↓
     * ■新value【替换】旧value■
     */
    public V merge(K key, V bakValue, BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        if(key == null || bakValue == null || remappingFunction == null) {
            throw new NullPointerException();
        }
        
        int h = spread(key.hashCode());
        
        V newValue = null;
        int delta = 0;
        int binCount = 0;
        Node<K, V>[] tab = table;
        for(; ; ) {
            Node<K, V> f;
            int n, i, fh;
            if(tab == null || (n = tab.length) == 0) {
                tab = initTable();
            } else if((f = tabAt(tab, i = (n - 1) & h)) == null) {
                // 当做新元素插入
                Node<K, V> x = new Node<>(h, key, bakValue);
                // 原子地更新tab[i]为x
                if(casTabAt(tab, i, null, x)) {
                    delta = 1;
                    newValue = bakValue;
                    break;
                }
            } else if((fh = f.hash) == MOVED) {
                tab = helpTransfer(tab, f);
            } else {
                synchronized(f) {
                    if(tabAt(tab, i) == f) {
                        if(fh >= 0) {
                            binCount = 1;
                            
                            for(Node<K, V> e = f, pred = null; ; ++binCount) {
                                K ek;
                                
                                // 找到了同位元素
                                if(e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek)))) {
                                    // 生成新值
                                    newValue = remappingFunction.apply(e.val, bakValue);
                                    
                                    // 新值不为空，则替换
                                    if(newValue != null) {
                                        e.val = newValue;
                                        
                                        // 新值为空，则移除旧元素
                                    } else {
                                        delta = -1;
                                        Node<K, V> en = e.next;
                                        
                                        if(pred != null) {
                                            pred.next = en;
                                        } else {
                                            // 设置tab[i]为en
                                            setTabAt(tab, i, en);
                                        }
                                    }
                                    
                                    break;
                                }
                                
                                pred = e;
                                
                                // 当做新元素插入
                                if((e = e.next) == null) {
                                    delta = 1;
                                    newValue = bakValue;
                                    pred.next = new Node<K, V>(h, key, newValue);
                                    break;
                                }
                            }
                        } else if(f instanceof TreeBin) {
                            binCount = 2;
                            TreeBin<K, V> t = (TreeBin<K, V>) f;
                            TreeNode<K, V> r = t.root;
                            TreeNode<K, V> p = (r == null) ? null : r.findTreeNode(h, key, null);
                            newValue = (p == null) ? bakValue : remappingFunction.apply(p.val, bakValue);
                            if(newValue != null) {
                                if(p != null) {
                                    p.val = newValue;
                                } else {
                                    delta = 1;
                                    t.putTreeVal(h, key, newValue);
                                }
                            } else if(p != null) {
                                delta = -1;
                                
                                if(t.removeTreeNode(p)) {
                                    Node<K, V> x = untreeify(t.first);
                                    // 设置tab[i]为x
                                    setTabAt(tab, i, x);
                                }
                            }
                        } else if(f instanceof ReservationNode) {
                            throw new IllegalStateException("Recursive update");
                        }
                    }
                }
                
                if(binCount != 0) {
                    if(binCount >= TREEIFY_THRESHOLD) {
                        treeifyBin(tab, i);
                    }
                    break;
                }
            }
        }// for(; ; )
        
        if(delta != 0) {
            addCount(delta, binCount);
        }
        
        return newValue;
    }
    
    /**
     * Attempts to compute a mapping for the specified key and its
     * current mapped value (or {@code null} if there is no current
     * mapping). The entire method invocation is performed atomically.
     * Some attempted update operations on this map by other threads
     * may be blocked while computation is in progress, so the
     * computation should be short and simple, and must not attempt to
     * update any other mappings of this Map.
     *
     * @param key               key with which the specified value is to be associated
     * @param remappingFunction the function to compute a value
     *
     * @return the new value associated with the specified key, or null if none
     *
     * @throws NullPointerException  if the specified key or remappingFunction
     *                               is null
     * @throws IllegalStateException if the computation detectably
     *                               attempts a recursive update to this map that would
     *                               otherwise never complete
     * @throws RuntimeException      or Error if the remappingFunction does so,
     *                               in which case the mapping is unchanged
     */
    /*
     * 插入/删除/替换操作，返回新值（可能为null）
     * 此方法的主要意图：使用key和旧value创造的新value来更新旧value
     *
     * 注：以下流程图中，涉及到判断(◇)时，纵向代表【是】，横向代表【否】。此外，使用★代表计算。
     *
     *  ●查找同位元素●
     *      |
     *      ↓
     * ◇存在同位元素◇     --→ ★新value=(key, null)★
     *      | 是          否　           |
     *      ↓ 　　　                     |
     * ★新value=(key, 旧value)★        |
     *      ├---------------------------┘
     *      ↓
     * ◇新value不为null◇ --→ ■如果存在同位元素，则【删除】同位元素■
     *      | 是          否
     *      ↓
     * ■  存在同位元素，则新value【替换】旧value■
     * ■不存在同位元素，则【插入】新value       ■
     */
    public V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        if(key == null || remappingFunction == null) {
            throw new NullPointerException();
        }
        
        int h = spread(key.hashCode());
        
        V newValue = null;
        int delta = 0;
        int binCount = 0;
        
        for(Node<K, V>[] tab = table; ; ) {
            Node<K, V> f;
            int n, i, fh;
            
            if(tab == null || (n = tab.length) == 0) {
                tab = initTable();
                
            } else if((f = tabAt(tab, i = (n - 1) & h)) == null) {
                // 设置一个占位结点
                Node<K, V> x = new ReservationNode<>();
                
                synchronized(x) {
                    // 原子地更新tab[i]为x
                    if(casTabAt(tab, i, null, x)) {
                        binCount = 1;
                        Node<K, V> node = null;
                        try {
                            newValue = remappingFunction.apply(key, null);
                            
                            if(newValue != null) {
                                delta = 1;
                                node = new Node<K, V>(h, key, newValue);
                            }
                        } finally {
                            // 设置tab[i]为node
                            setTabAt(tab, i, node);
                        }
                    }
                }
                
                if(binCount != 0) {
                    break;
                }
            } else if((fh = f.hash) == MOVED) {
                tab = helpTransfer(tab, f);
            } else {
                synchronized(f) {
                    if(tabAt(tab, i) == f) {
                        if(fh >= 0) {
                            binCount = 1;
                            
                            for(Node<K, V> e = f, pred = null; ; ++binCount) {
                                K ek;
                                
                                // 找到了同位元素
                                if(e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek)))) {
                                    newValue = remappingFunction.apply(key, e.val);
                                    
                                    if(newValue != null) {
                                        e.val = newValue;
                                    } else {
                                        delta = -1;
                                        Node<K, V> en = e.next;
                                        if(pred != null) {
                                            pred.next = en;
                                        } else {
                                            // 设置tab[i]为en
                                            setTabAt(tab, i, en);
                                        }
                                    }
                                    break;
                                }
                                
                                pred = e;
                                
                                if((e = e.next) == null) {
                                    newValue = remappingFunction.apply(key, null);
                                    
                                    if(newValue != null) {
                                        if(pred.next != null) {
                                            throw new IllegalStateException("Recursive update");
                                        }
                                        delta = 1;
                                        pred.next = new Node<K, V>(h, key, newValue);
                                    }
                                    
                                    break;
                                }
                            }
                        } else if(f instanceof TreeBin) {
                            binCount = 1;
                            
                            TreeBin<K, V> t = (TreeBin<K, V>) f;
                            TreeNode<K, V> r, p;
                            
                            if((r = t.root) != null) {
                                p = r.findTreeNode(h, key, null);
                            } else {
                                p = null;
                            }
                            
                            V pv = (p == null) ? null : p.val;
                            
                            newValue = remappingFunction.apply(key, pv);
                            
                            if(newValue != null) {
                                if(p != null) {
                                    p.val = newValue;
                                } else {
                                    delta = 1;
                                    t.putTreeVal(h, key, newValue);
                                }
                            } else if(p != null) {
                                delta = -1;
                                if(t.removeTreeNode(p)) {
                                    Node<K, V> x = untreeify(t.first);
                                    // 设置tab[i]为x
                                    setTabAt(tab, i, x);
                                }
                            }
                        } else if(f instanceof ReservationNode) {
                            throw new IllegalStateException("Recursive update");
                        }
                    }
                }
                
                if(binCount != 0) {
                    if(binCount >= TREEIFY_THRESHOLD) {
                        treeifyBin(tab, i);
                    }
                    
                    break;
                }
            }
        }
        
        if(delta != 0) {
            addCount(delta, binCount);
        }
        
        return newValue;
    }
    
    /**
     * If the value for the specified key is present, attempts to
     * compute a new mapping given the key and its current mapped
     * value.  The entire method invocation is performed atomically.
     * Some attempted update operations on this map by other threads
     * may be blocked while computation is in progress, so the
     * computation should be short and simple, and must not attempt to
     * update any other mappings of this map.
     *
     * @param key               key with which a value may be associated
     * @param remappingFunction the function to compute a value
     *
     * @return the new value associated with the specified key, or null if none
     *
     * @throws NullPointerException  if the specified key or remappingFunction
     *                               is null
     * @throws IllegalStateException if the computation detectably
     *                               attempts a recursive update to this map that would
     *                               otherwise never complete
     * @throws RuntimeException      or Error if the remappingFunction does so,
     *                               in which case the mapping is unchanged
     */
    /*
     * 删除/替换操作，返回新值（可能为null）
     * 此方法的主要意图：存在同位元素时，使用key和旧value创造的新value来更新旧value
     *
     * 注：以下流程图中，涉及到判断(◇)时，纵向代表【是】，横向代表【否】。此外，使用★代表计算。
     *
     *  ●查找同位元素●
     *      |
     *      ↓
     * ◇存在同位元素◇
     *      | 是
     *      ↓
     * ★新value=(key, 旧value)★
     *      |
     *      ↓
     * ◇新value不为null◇ --→ ■【删除】同位元素■
     *      | 是          否
     *      ↓
     * ■新value【替换】旧value■
     */
    public V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        if(key == null || remappingFunction == null) {
            throw new NullPointerException();
        }
        
        int h = spread(key.hashCode());
        
        V newValue = null;
        int delta = 0;
        int binCount = 0;
        for(Node<K, V>[] tab = table; ; ) {
            Node<K, V> f;
            int n, i, fh;
            
            if(tab == null || (n = tab.length) == 0) {
                tab = initTable();
            } else if((f = tabAt(tab, i = (n - 1) & h)) == null) {
                break;
            } else if((fh = f.hash) == MOVED) {
                tab = helpTransfer(tab, f);
            } else {
                synchronized(f) {
                    if(tabAt(tab, i) == f) {
                        if(fh >= 0) {
                            binCount = 1;
                            for(Node<K, V> e = f, pred = null; ; ++binCount) {
                                K ek;
                                
                                // 找到了同位元素
                                if(e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek)))) {
                                    newValue = remappingFunction.apply(key, e.val);
                                    
                                    if(newValue != null) {
                                        e.val = newValue;
                                    } else {
                                        delta = -1;
                                        Node<K, V> en = e.next;
                                        if(pred != null) {
                                            pred.next = en;
                                        } else {
                                            // 设置tab[i]为en
                                            setTabAt(tab, i, en);
                                        }
                                    }
                                    break;
                                }
                                
                                pred = e;
                                
                                if((e = e.next) == null) {
                                    break;
                                }
                            }
                        } else if(f instanceof TreeBin) {
                            binCount = 2;
                            TreeBin<K, V> t = (TreeBin<K, V>) f;
                            TreeNode<K, V> r, p;
                            if((r = t.root) != null && (p = r.findTreeNode(h, key, null)) != null) {
                                newValue = remappingFunction.apply(key, p.val);
                                
                                if(newValue != null) {
                                    p.val = newValue;
                                } else {
                                    delta = -1;
                                    if(t.removeTreeNode(p)) {
                                        Node<K, V> x = untreeify(t.first);
                                        // 设置tab[i]为x
                                        setTabAt(tab, i, x);
                                    }
                                }
                            }
                        } else if(f instanceof ReservationNode) {
                            throw new IllegalStateException("Recursive update");
                        }
                    }
                }
                
                if(binCount != 0) {
                    break;
                }
            }
        }
        
        if(delta != 0) {
            addCount(delta, binCount);
        }
        
        return newValue;
    }
    
    /**
     * If the specified key is not already associated with a value,
     * attempts to compute its value using the given mapping function
     * and enters it into this map unless {@code null}.  The entire
     * method invocation is performed atomically, so the function is
     * applied at most once per key.  Some attempted update operations
     * on this map by other threads may be blocked while computation
     * is in progress, so the computation should be short and simple,
     * and must not attempt to update any other mappings of this map.
     *
     * @param key             key with which the specified value is to be associated
     * @param mappingFunction the function to compute a value
     *
     * @return the current (existing or computed) value associated with
     * the specified key, or null if the computed value is null
     *
     * @throws NullPointerException  if the specified key or mappingFunction
     *                               is null
     * @throws IllegalStateException if the computation detectably
     *                               attempts a recursive update to this map that would
     *                               otherwise never complete
     * @throws RuntimeException      or Error if the mappingFunction does so,
     *                               in which case the mapping is left unestablished
     */
    /*
     * 插入操作，返回新值（可能为null）
     * 此方法的主要意图：不存在同位元素时，使用key创造的新value来更新旧value。如果同位元素存在，则直接返回旧值。
     *
     * 注：以下流程图中，涉及到判断(◇)时，纵向代表【是】，横向代表【否】。此外，使用★代表计算。
     *
     *  ●查找同位元素●
     *      |
     *      ↓
     * ◇存在同位元素◇ --→ ★新value=(key)★
     *                否         |
     *                　         ↓
     *                　  ◇新value不为null◇
     *                　         |
     *                　         ↓
     *                　  ■不存在同位元素，则【插入】新value■
     */
    public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
        if(key == null || mappingFunction == null) {
            throw new NullPointerException();
        }
        
        int h = spread(key.hashCode());
        
        V newValue = null;
        int binCount = 0;
        for(Node<K, V>[] tab = table; ; ) {
            Node<K, V> f;
            int n, i, fh;
            K fk;
            V fv;
            
            if(tab == null || (n = tab.length) == 0) {
                tab = initTable();
            } else if((f = tabAt(tab, i = (n - 1) & h)) == null) {
                Node<K, V> x = new ReservationNode<>();
                
                synchronized(x) {
                    // 原子地更新tab[i]为x
                    if(casTabAt(tab, i, null, x)) {
                        binCount = 1;
                        Node<K, V> node = null;
                        try {
                            newValue = mappingFunction.apply(key);
                            
                            if(newValue != null) {
                                node = new Node<K, V>(h, key, newValue);
                            }
                        } finally {
                            // 设置tab[i]为node
                            setTabAt(tab, i, node);
                        }
                    }
                }
                if(binCount != 0) {
                    break;
                }
            } else if((fh = f.hash) == MOVED) {
                tab = helpTransfer(tab, f);
            } else if(fh == h    // check first node without acquiring lock
                && ((fk = f.key) == key || (fk != null && key.equals(fk))) && (fv = f.val) != null) {
                return fv;
            } else {
                boolean added = false;
                synchronized(f) {
                    if(tabAt(tab, i) == f) {
                        if(fh >= 0) {
                            binCount = 1;
                            for(Node<K, V> e = f; ; ++binCount) {
                                K ek;
                                if(e.hash == h && ((ek = e.key) == key || (ek != null && key.equals(ek)))) {
                                    newValue = e.val;
                                    break;
                                }
                                
                                Node<K, V> pred = e;
                                
                                if((e = e.next) == null) {
                                    newValue = mappingFunction.apply(key);
                                    
                                    if(newValue != null) {
                                        if(pred.next != null) {
                                            throw new IllegalStateException("Recursive update");
                                        }
                                        added = true;
                                        pred.next = new Node<K, V>(h, key, newValue);
                                    }
                                    break;
                                }
                            }
                        } else if(f instanceof TreeBin) {
                            binCount = 2;
                            TreeBin<K, V> t = (TreeBin<K, V>) f;
                            TreeNode<K, V> r, p;
                            if((r = t.root) != null && (p = r.findTreeNode(h, key, null)) != null) {
                                newValue = p.val;
                            } else if((newValue = mappingFunction.apply(key)) != null) {
                                added = true;
                                t.putTreeVal(h, key, newValue);
                            }
                        } else if(f instanceof ReservationNode) {
                            throw new IllegalStateException("Recursive update");
                        }
                    }
                }
                
                if(binCount != 0) {
                    if(binCount >= TREEIFY_THRESHOLD) {
                        treeifyBin(tab, i);
                    }
                    
                    if(!added) {
                        return newValue;
                    }
                    
                    break;
                }
            }
        }
        
        if(newValue != null) {
            addCount(1L, binCount);
        }
        
        return newValue;
    }
    
    /*▲ 重新映射 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     */
    // 获取当前Map中的元素数量
    public int size() {
        long n = sumCount();
        return ((n<0L) ? 0 : (n>(long) Integer.MAX_VALUE) ? Integer.MAX_VALUE : (int) n);
    }
    
    /**
     * {@inheritDoc}
     */
    // 判断当前Map是否为空集
    public boolean isEmpty() {
        return sumCount()<=0L; // ignore transient negative values
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 并行操作 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Performs the given action for each key.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param action               the action
     *
     * @since 1.8
     */
    public void forEachKey(long parallelismThreshold, Consumer<? super K> action) {
        if(action == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Void> task = new ForEachKeyTask<K, V>(null, batch, 0, 0, table, action);
        
        task.invoke();
    }
    
    /**
     * Performs the given action for each value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param action               the action
     *
     * @since 1.8
     */
    public void forEachValue(long parallelismThreshold, Consumer<? super V> action) {
        if(action == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Void> task = new ForEachValueTask<K, V>(null, batch, 0, 0, table, action);
        
        task.invoke();
    }
    
    /**
     * Performs the given action for each entry.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param action               the action
     *
     * @since 1.8
     */
    public void forEachEntry(long parallelismThreshold, Consumer<? super Map.Entry<K, V>> action) {
        if(action == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Void> task = new ForEachEntryTask<K, V>(null, batch, 0, 0, table, action);
        
        task.invoke();
    }
    
    /**
     * Performs the given action for each (key, value).
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param action               the action
     *
     * @since 1.8
     */
    public void forEach(long parallelismThreshold, BiConsumer<? super K, ? super V> action) {
        if(action == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Void> task = new ForEachMappingTask<>(null, batch, 0, 0, table, action);
        
        task.invoke();
    }
    
    
    
    /**
     * Performs the given action for each non-null transformation
     * of each key.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element, or null if there is no transformation (in
     *                             which case the action is not applied)
     * @param action               the action
     * @param <U>                  the return type of the transformer
     *
     * @since 1.8
     */
    public <U> void forEachKey(long parallelismThreshold, Function<? super K, ? extends U> transformer, Consumer<? super U> action) {
        if(transformer == null || action == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Void> task = new ForEachTransformedKeyTask<K, V, U>(null, batch, 0, 0, table, transformer, action);
        
        task.invoke();
    }
    
    /**
     * Performs the given action for each non-null transformation
     * of each value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element, or null if there is no transformation (in
     *                             which case the action is not applied)
     * @param action               the action
     * @param <U>                  the return type of the transformer
     *
     * @since 1.8
     */
    public <U> void forEachValue(long parallelismThreshold, Function<? super V, ? extends U> transformer, Consumer<? super U> action) {
        if(transformer == null || action == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Void> task = new ForEachTransformedValueTask<K, V, U>(null, batch, 0, 0, table, transformer, action);
        
        task.invoke();
    }
    
    /**
     * Performs the given action for each non-null transformation
     * of each entry.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element, or null if there is no transformation (in
     *                             which case the action is not applied)
     * @param action               the action
     * @param <U>                  the return type of the transformer
     *
     * @since 1.8
     */
    public <U> void forEachEntry(long parallelismThreshold, Function<Map.Entry<K, V>, ? extends U> transformer, Consumer<? super U> action) {
        if(transformer == null || action == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Void> task = new ForEachTransformedEntryTask<K, V, U>(null, batch, 0, 0, table, transformer, action);
        
        task.invoke();
    }
    
    /**
     * Performs the given action for each non-null transformation
     * of each (key, value).
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element, or null if there is no transformation (in
     *                             which case the action is not applied)
     * @param action               the action
     * @param <U>                  the return type of the transformer
     *
     * @since 1.8
     */
    public <U> void forEach(long parallelismThreshold, BiFunction<? super K, ? super V, ? extends U> transformer, Consumer<? super U> action) {
        if(transformer == null || action == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Void> task = new ForEachTransformedMappingTask<K, V, U>(null, batch, 0, 0, table, transformer, action);
        
        task.invoke();
    }
    
    
    
    /**
     * Returns a non-null result from applying the given search
     * function on each key, or null if none. Upon success,
     * further element processing is suppressed and the results of
     * any other parallel invocations of the search function are
     * ignored.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param searchFunction       a function returning a non-null
     *                             result on success, else null
     * @param <U>                  the return type of the search function
     *
     * @return a non-null result from applying the given search
     * function on each key, or null if none
     *
     * @since 1.8
     */
    public <U> U searchKeys(long parallelismThreshold, Function<? super K, ? extends U> searchFunction) {
        if(searchFunction == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<U> task = new SearchKeysTask<K, V, U>(null, batch, 0, 0, table, searchFunction, new AtomicReference<U>());
        
        return task.invoke();
    }
    
    /**
     * Returns a non-null result from applying the given search
     * function on each value, or null if none.  Upon success,
     * further element processing is suppressed and the results of
     * any other parallel invocations of the search function are
     * ignored.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param searchFunction       a function returning a non-null
     *                             result on success, else null
     * @param <U>                  the return type of the search function
     *
     * @return a non-null result from applying the given search
     * function on each value, or null if none
     *
     * @since 1.8
     */
    public <U> U searchValues(long parallelismThreshold, Function<? super V, ? extends U> searchFunction) {
        if(searchFunction == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<U> task = new SearchValuesTask<K, V, U>(null, batch, 0, 0, table, searchFunction, new AtomicReference<U>());
        
        return task.invoke();
    }
    
    /**
     * Returns a non-null result from applying the given search
     * function on each entry, or null if none.  Upon success,
     * further element processing is suppressed and the results of
     * any other parallel invocations of the search function are
     * ignored.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param searchFunction       a function returning a non-null
     *                             result on success, else null
     * @param <U>                  the return type of the search function
     *
     * @return a non-null result from applying the given search
     * function on each entry, or null if none
     *
     * @since 1.8
     */
    public <U> U searchEntries(long parallelismThreshold, Function<Map.Entry<K, V>, ? extends U> searchFunction) {
        if(searchFunction == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<U> task = new SearchEntriesTask<K, V, U>(null, batch, 0, 0, table, searchFunction, new AtomicReference<U>());
        
        return task.invoke();
    }
    
    /**
     * Returns a non-null result from applying the given search
     * function on each (key, value), or null if none.  Upon
     * success, further element processing is suppressed and the
     * results of any other parallel invocations of the search
     * function are ignored.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param searchFunction       a function returning a non-null
     *                             result on success, else null
     * @param <U>                  the return type of the search function
     *
     * @return a non-null result from applying the given search
     * function on each (key, value), or null if none
     *
     * @since 1.8
     */
    public <U> U search(long parallelismThreshold, BiFunction<? super K, ? super V, ? extends U> searchFunction) {
        if(searchFunction == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<U> task = new SearchMappingsTask<K, V, U>(null, batch, 0, 0, table, searchFunction, new AtomicReference<U>());
        
        return task.invoke();
    }
    
    
    
    /**
     * Returns the result of accumulating all keys using the given
     * reducer to combine values, or null if none.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating all keys using the given
     * reducer to combine values, or null if none
     *
     * @since 1.8
     */
    public K reduceKeys(long parallelismThreshold, BiFunction<? super K, ? super K, ? extends K> reducer) {
        if(reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<K> task = new ReduceKeysTask<K, V>(null, batch, 0, 0, table, null, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating all values using the
     * given reducer to combine values, or null if none.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating all values
     *
     * @since 1.8
     */
    public V reduceValues(long parallelismThreshold, BiFunction<? super V, ? super V, ? extends V> reducer) {
        if(reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<V> task = new ReduceValuesTask<K, V>(null, batch, 0, 0, table, null, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating all entries using the
     * given reducer to combine values, or null if none.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating all entries
     *
     * @since 1.8
     */
    public Map.Entry<K, V> reduceEntries(long parallelismThreshold, BiFunction<Map.Entry<K, V>, Map.Entry<K, V>, ? extends Map.Entry<K, V>> reducer) {
        if(reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Map.Entry<K, V>> task = new ReduceEntriesTask<K, V>(null, batch, 0, 0, table, null, reducer);
        
        return task.invoke();
    }
    
    
    
    /**
     * Returns the result of accumulating the given transformation
     * of all keys using the given reducer to combine values, or
     * null if none.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element, or null if there is no transformation (in
     *                             which case it is not combined)
     * @param reducer              a commutative associative combining function
     * @param <U>                  the return type of the transformer
     *
     * @return the result of accumulating the given transformation
     * of all keys
     *
     * @since 1.8
     */
    public <U> U reduceKeys(long parallelismThreshold, Function<? super K, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<U> task = new MapReduceKeysTask<K, V, U>(null, batch, 0, 0, table, null, transformer, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all values using the given reducer to combine values, or
     * null if none.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element, or null if there is no transformation (in
     *                             which case it is not combined)
     * @param reducer              a commutative associative combining function
     * @param <U>                  the return type of the transformer
     *
     * @return the result of accumulating the given transformation
     * of all values
     *
     * @since 1.8
     */
    public <U> U reduceValues(long parallelismThreshold, Function<? super V, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<U> task = new MapReduceValuesTask<K, V, U>(null, batch, 0, 0, table, null, transformer, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all entries using the given reducer to combine values,
     * or null if none.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element, or null if there is no transformation (in
     *                             which case it is not combined)
     * @param reducer              a commutative associative combining function
     * @param <U>                  the return type of the transformer
     *
     * @return the result of accumulating the given transformation
     * of all entries
     *
     * @since 1.8
     */
    public <U> U reduceEntries(long parallelismThreshold, Function<Map.Entry<K, V>, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<U> task = new MapReduceEntriesTask<K, V, U>(null, batch, 0, 0, table, null, transformer, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all (key, value) pairs using the given reducer to
     * combine values, or null if none.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element, or null if there is no transformation (in
     *                             which case it is not combined)
     * @param reducer              a commutative associative combining function
     * @param <U>                  the return type of the transformer
     *
     * @return the result of accumulating the given transformation
     * of all (key, value) pairs
     *
     * @since 1.8
     */
    public <U> U reduce(long parallelismThreshold, BiFunction<? super K, ? super V, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<U> task = new MapReduceMappingsTask<K, V, U>(null, batch, 0, 0, table, null, transformer, reducer);
        
        return task.invoke();
    }
    
    
    
    /**
     * Returns the result of accumulating the given transformation
     * of all keys using the given reducer to combine values, and
     * the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all keys
     *
     * @since 1.8
     */
    public int reduceKeysToInt(long parallelismThreshold, ToIntFunction<? super K> transformer, int basis, IntBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Integer> task = new MapReduceKeysToIntTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all values using the given reducer to combine values,
     * and the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all values
     *
     * @since 1.8
     */
    public int reduceValuesToInt(long parallelismThreshold, ToIntFunction<? super V> transformer, int basis, IntBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Integer> task = new MapReduceValuesToIntTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all entries using the given reducer to combine values,
     * and the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all entries
     *
     * @since 1.8
     */
    public int reduceEntriesToInt(long parallelismThreshold, ToIntFunction<Map.Entry<K, V>> transformer, int basis, IntBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Integer> task = new MapReduceEntriesToIntTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all (key, value) pairs using the given reducer to
     * combine values, and the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all (key, value) pairs
     *
     * @since 1.8
     */
    public int reduceToInt(long parallelismThreshold, ToIntBiFunction<? super K, ? super V> transformer, int basis, IntBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Integer> task = new MapReduceMappingsToIntTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    
    
    /**
     * Returns the result of accumulating the given transformation
     * of all keys using the given reducer to combine values, and
     * the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all keys
     *
     * @since 1.8
     */
    public long reduceKeysToLong(long parallelismThreshold, ToLongFunction<? super K> transformer, long basis, LongBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Long> task = new MapReduceKeysToLongTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all values using the given reducer to combine values,
     * and the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all values
     *
     * @since 1.8
     */
    public long reduceValuesToLong(long parallelismThreshold, ToLongFunction<? super V> transformer, long basis, LongBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Long> task = new MapReduceValuesToLongTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all entries using the given reducer to combine values,
     * and the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all entries
     *
     * @since 1.8
     */
    public long reduceEntriesToLong(long parallelismThreshold, ToLongFunction<Map.Entry<K, V>> transformer, long basis, LongBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Long> task = new MapReduceEntriesToLongTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all (key, value) pairs using the given reducer to
     * combine values, and the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all (key, value) pairs
     *
     * @since 1.8
     */
    public long reduceToLong(long parallelismThreshold, ToLongBiFunction<? super K, ? super V> transformer, long basis, LongBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Long> task = new MapReduceMappingsToLongTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    
    
    /**
     * Returns the result of accumulating the given transformation
     * of all keys using the given reducer to combine values, and
     * the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all keys
     *
     * @since 1.8
     */
    public double reduceKeysToDouble(long parallelismThreshold, ToDoubleFunction<? super K> transformer, double basis, DoubleBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Double> task = new MapReduceKeysToDoubleTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all values using the given reducer to combine values,
     * and the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all values
     *
     * @since 1.8
     */
    public double reduceValuesToDouble(long parallelismThreshold, ToDoubleFunction<? super V> transformer, double basis, DoubleBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Double> task = new MapReduceValuesToDoubleTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all entries using the given reducer to combine values,
     * and the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all entries
     *
     * @since 1.8
     */
    public double reduceEntriesToDouble(long parallelismThreshold, ToDoubleFunction<Map.Entry<K, V>> transformer, double basis, DoubleBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Double> task = new MapReduceEntriesToDoubleTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /**
     * Returns the result of accumulating the given transformation
     * of all (key, value) pairs using the given reducer to
     * combine values, and the given basis as an identity value.
     *
     * @param parallelismThreshold the (estimated) number of elements
     *                             needed for this operation to be executed in parallel
     * @param transformer          a function returning the transformation
     *                             for an element
     * @param basis                the identity (initial default value) for the reduction
     * @param reducer              a commutative associative combining function
     *
     * @return the result of accumulating the given transformation
     * of all (key, value) pairs
     *
     * @since 1.8
     */
    public double reduceToDouble(long parallelismThreshold, ToDoubleBiFunction<? super K, ? super V> transformer, double basis, DoubleBinaryOperator reducer) {
        if(transformer == null || reducer == null) {
            throw new NullPointerException();
        }
        
        int batch = batchFor(parallelismThreshold);
        
        ForkJoinTask<Double> task = new MapReduceMappingsToDoubleTask<K, V>(null, batch, 0, 0, table, null, transformer, basis, reducer);
        
        return task.invoke();
    }
    
    /*▲ 并行操作 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 序列化 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    private static final long serialVersionUID = 7249069246763182397L;
    
    /**
     * Serialized pseudo-fields, provided only for jdk7 compatibility.
     *
     * @serialField segments Segment[]
     * The segments, each of which is a specialized hash table.
     * @serialField segmentMask int
     * Mask value for indexing into segments. The upper bits of a
     * key's hash code are used to choose the segment.
     * @serialField segmentShift int
     * Shift value for indexing within segments.
     */
    private static final ObjectStreamField[] serialPersistentFields = {new ObjectStreamField("segments", Segment[].class), new ObjectStreamField("segmentMask", Integer.TYPE), new ObjectStreamField("segmentShift", Integer.TYPE),};
    
    /**
     * Saves this map to a stream (that is, serializes it).
     *
     * @param s the stream
     *
     * @throws java.io.IOException if an I/O error occurs
     * @serialData the serialized fields, followed by the key (Object) and value
     * (Object) for each key-value mapping, followed by a null pair.
     * The key-value mappings are emitted in no particular order.
     */
    private void writeObject(ObjectOutputStream s) throws IOException {
        // For serialization compatibility
        // Emulate segment calculation from previous version of this class
        int sshift = 0;
        int ssize = 1;
        while(ssize<DEFAULT_CONCURRENCY_LEVEL) {
            ++sshift;
            ssize <<= 1;
        }
        int segmentShift = 32 - sshift;
        int segmentMask = ssize - 1;
        @SuppressWarnings("unchecked")
        Segment<K, V>[] segments = (Segment<K, V>[]) new Segment<?, ?>[DEFAULT_CONCURRENCY_LEVEL];
        for(int i = 0; i<segments.length; ++i) {
            segments[i] = new Segment<K, V>(LOAD_FACTOR);
        }
        ObjectOutputStream.PutField streamFields = s.putFields();
        streamFields.put("segments", segments);
        streamFields.put("segmentShift", segmentShift);
        streamFields.put("segmentMask", segmentMask);
        s.writeFields();
        
        Node<K, V>[] t;
        if((t = table) != null) {
            Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
            for(Node<K, V> p; (p = it.advance()) != null; ) {
                s.writeObject(p.key);
                s.writeObject(p.val);
            }
        }
        s.writeObject(null);
        s.writeObject(null);
    }
    
    /**
     * Reconstitutes this map from a stream (that is, deserializes it).
     *
     * @param s the stream
     *
     * @throws ClassNotFoundException if the class of a serialized object
     *                                could not be found
     * @throws java.io.IOException    if an I/O error occurs
     */
    private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
        /*
         * To improve performance in typical cases, we create nodes
         * while reading, then place in table once size is known.
         * However, we must also validate uniqueness and deal with
         * overpopulated bins while doing so, which requires
         * specialized versions of putVal mechanics.
         */
        sizeCtl = -1; // force exclusion for table construction
        s.defaultReadObject();
        long size = 0L;
        Node<K, V> p = null;
        for(; ; ) {
            @SuppressWarnings("unchecked")
            K k = (K) s.readObject();
            @SuppressWarnings("unchecked")
            V v = (V) s.readObject();
            if(k != null && v != null) {
                p = new Node<K, V>(spread(k.hashCode()), k, v, p);
                ++size;
            } else {
                break;
            }
        }
        
        if(size == 0L) {
            sizeCtl = 0;
        } else {
            long ts = (long) (1.0 + size / LOAD_FACTOR);
            int n = (ts >= (long) MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : tableSizeFor((int) ts);
            @SuppressWarnings("unchecked")
            Node<K, V>[] tab = (Node<K, V>[]) new Node<?, ?>[n];
            int mask = n - 1;
            long added = 0L;
            while(p != null) {
                boolean insertAtFront;
                Node<K, V> next = p.next, first;
                int h = p.hash, j = h & mask;
                if((first = tabAt(tab, j)) == null) {
                    insertAtFront = true;
                } else {
                    K k = p.key;
                    if(first.hash<0) {
                        TreeBin<K, V> t = (TreeBin<K, V>) first;
                        if(t.putTreeVal(h, k, p.val) == null) {
                            ++added;
                        }
                        insertAtFront = false;
                    } else {
                        int binCount = 0;
                        insertAtFront = true;
                        Node<K, V> q;
                        K qk;
                        for(q = first; q != null; q = q.next) {
                            if(q.hash == h && ((qk = q.key) == k || (qk != null && k.equals(qk)))) {
                                insertAtFront = false;
                                break;
                            }
                            ++binCount;
                        }
                        if(insertAtFront && binCount >= TREEIFY_THRESHOLD) {
                            insertAtFront = false;
                            ++added;
                            p.next = first;
                            TreeNode<K, V> hd = null, tl = null;
                            for(q = p; q != null; q = q.next) {
                                TreeNode<K, V> t = new TreeNode<K, V>(q.hash, q.key, q.val, null, null);
                                if((t.prev = tl) == null) {
                                    hd = t;
                                } else {
                                    tl.next = t;
                                }
                                tl = t;
                            }
                            setTabAt(tab, j, new TreeBin<K, V>(hd));
                        }
                    }
                }
                if(insertAtFront) {
                    ++added;
                    p.next = first;
                    // 设置tab[j]为p
                    setTabAt(tab, j, p);
                }
                p = next;
            }
            table = tab;
            sizeCtl = n - (n >>> 2);
            baseCount = added;
        }
    }
    
    /*▲ 序列化 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Returns the hash code value for this {@link Map}, i.e.,
     * the sum of, for each key-value pair in the map,
     * {@code key.hashCode() ^ value.hashCode()}.
     *
     * @return the hash code value for this map
     */
    public int hashCode() {
        int h = 0;
        Node<K, V>[] t;
        if((t = table) != null) {
            Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
            for(Node<K, V> p; (p = it.advance()) != null; )
                h += p.key.hashCode() ^ p.val.hashCode();
        }
        return h;
    }
    
    /**
     * Compares the specified object with this map for equality.
     * Returns {@code true} if the given object is a map with the same
     * mappings as this map.  This operation may return misleading
     * results if either map is concurrently modified during execution
     * of this method.
     *
     * @param o object to be compared for equality with this map
     *
     * @return {@code true} if the specified object is equal to this map
     */
    public boolean equals(Object o) {
        if(o != this) {
            if(!(o instanceof Map)) {
                return false;
            }
            
            Map<?, ?> m = (Map<?, ?>) o;
            Node<K, V>[] t;
            int f = (t = table) == null ? 0 : t.length;
            Traverser<K, V> it = new Traverser<K, V>(t, f, 0, f);
            for(Node<K, V> p; (p = it.advance()) != null; ) {
                V val = p.val;
                Object v = m.get(p.key);
                if(v == null || (v != val && !v.equals(val))) {
                    return false;
                }
            }
            
            for(Map.Entry<?, ?> e : m.entrySet()) {
                Object mk, mv, v;
                if((mk = e.getKey()) == null || (mv = e.getValue()) == null || (v = get(mk)) == null || (mv != v && !mv.equals(v))) {
                    return false;
                }
            }
        }
        return true;
    }
    
    /**
     * Returns a string representation of this map.  The string
     * representation consists of a list of key-value mappings (in no
     * particular order) enclosed in braces ("{@code {}}").  Adjacent
     * mappings are separated by the characters {@code ", "} (comma
     * and space).  Each key-value mapping is rendered as the key
     * followed by an equals sign ("{@code =}") followed by the
     * associated value.
     *
     * @return a string representation of this map
     */
    public String toString() {
        Node<K, V>[] t;
        int f = (t = table) == null ? 0 : t.length;
        Traverser<K, V> it = new Traverser<K, V>(t, f, 0, f);
        StringBuilder sb = new StringBuilder();
        sb.append('{');
        Node<K, V> p;
        if((p = it.advance()) != null) {
            for(; ; ) {
                K k = p.key;
                V v = p.val;
                sb.append(k == this ? "(this Map)" : k);
                sb.append('=');
                sb.append(v == this ? "(this Map)" : v);
                if((p = it.advance()) == null)
                    break;
                sb.append(',').append(' ');
            }
        }
        return sb.append('}').toString();
    }
    
    
    
    /* ---------------- Static utilities -------------- */
    
    /**
     * Spreads (XORs) higher bits of hash to lower and also forces top
     * bit to 0. Because the table uses power-of-two masking, sets of
     * hashes that vary only in bits above the current mask will
     * always collide. (Among known examples are sets of Float keys
     * holding consecutive whole numbers in small tables.)  So we
     * apply a transform that spreads the impact of higher bits
     * downward. There is a tradeoff between speed, utility, and
     * quality of bit-spreading. Because many common sets of hashes
     * are already reasonably distributed (so don't benefit from
     * spreading), and because we use trees to handle large sets of
     * collisions in bins, we just XOR some shifted bits in the
     * cheapest possible way to reduce systematic lossage, as well as
     * to incorporate impact of the highest bits that would otherwise
     * never be used in index calculations because of table bounds.
     */
    // 哈希算法
    static final int spread(int h) {
        return (h ^ (h >>> 16)) & HASH_BITS;
    }
    
    /**
     * Returns a power of two table size for the given desired capacity.
     * See Hackers Delight, sec 3.2
     */
    /*
     * 根据预期的容量cap计算出ConcurrentHashMap中的哈希数组实际需要分配的容量
     * 如果输入值是2的冪，则原样返回，如果不是2的冪，则向上取就近的冪
     * 比如输入13，返回16，输入17，返回32
     */
    private static final int tableSizeFor(int cap) {
        int n = -1 >>> Integer.numberOfLeadingZeros(cap - 1);
        return (n<0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }
    
    /**
     * Returns x's Class if it is of the form "class C implements
     * Comparable<C>", else null.
     */
    // 用于红黑树元素比较
    static Class<?> comparableClassFor(Object x) {
        if (x instanceof Comparable) {
            Class<?> c; Type[] ts, as; ParameterizedType p;
            
            // bypass checks
            if ((c = x.getClass()) == String.class) {
                return c;
            }
            
            if ((ts = c.getGenericInterfaces()) != null) {
                for (Type t : ts) {
                    if ((t instanceof ParameterizedType)
                        && ((p = (ParameterizedType)t).getRawType() == Comparable.class)
                        && (as = p.getActualTypeArguments()) != null
                        && as.length == 1 && as[0] == c) // type arg is c
                        return c;
                }
            }
        }
        
        return null;
    }
    
    /**
     * Returns k.compareTo(x) if x matches kc (k's screened comparable
     * class), else 0.
     */
    // 用于红黑树元素比较
    @SuppressWarnings({"rawtypes","unchecked"}) // for cast to Comparable
    static int compareComparables(Class<?> kc, Object k, Object x) {
        return (x == null || x.getClass() != kc ? 0 : ((Comparable)k).compareTo(x));
    }
    
    
    /* ---------------- Table element access -------------- */
    
    /*
     * Atomic access methods are used for table elements as well as
     * elements of in-progress next table while resizing.  All uses of
     * the tab arguments must be null checked by callers.  All callers
     * also paranoically precheck that tab's length is not zero (or an
     * equivalent check), thus ensuring that any index argument taking
     * the form of a hash value anded with (length - 1) is a valid
     * index.  Note that, to be correct wrt arbitrary concurrency
     * errors by users, these checks must operate on local variables,
     * which accounts for some odd-looking inline assignments below.
     * Note that calls to setTabAt always occur within locked regions,
     * and so require only release ordering.
     */
    
    // 获取tab[i]
    @SuppressWarnings("unchecked")
    static final <K, V> Node<K, V> tabAt(Node<K, V>[] tab, int i) {
        // 计算下标i的元素在底层的偏移地址
        long offset = ((long) i << ASHIFT) + ABASE;
        
        // 获取对象tab中offset地址处对应的引用类型字段的值
        return (Node<K, V>) U.getObjectAcquire(tab, offset);
    }
    
    // 原子地更新tab[i]为node
    static final <K, V> boolean casTabAt(Node<K, V>[] tab, int i, Node<K, V> expected, Node<K, V> node) {
        // 计算下标i的元素在底层的偏移地址
        long offset = ((long) i << ASHIFT) + ABASE;
        
        return U.compareAndSetObject(tab, offset, expected, node);
    }
    
    // 设置tab[i]为x
    static final <K, V> void setTabAt(Node<K, V>[] tab, int i, Node<K, V> x) {
        // 计算下标i的元素在底层的偏移地址
        long offset = ((long) i << ASHIFT) + ABASE;
        
        U.putObjectRelease(tab, offset, x);
    }
    
    
    // Original (since JDK1.2) Map methods
    
    /*
     * 向当前Map中存入新的元素，并返回旧元素
     *
     * onlyIfAbsent 是否需要维持原状（不覆盖旧值）
     */
    final V putVal(K key, V value, boolean onlyIfAbsent) {
        if(key == null || value == null) {
            throw new NullPointerException();
        }
        
        /*
         * 计算key的哈希值，在这个过程中会调用key的hashCode()方法
         *
         * key是一个对象的引用（可以看成地址）
         * 理论上讲，key的值是否相等，跟计算出的哈希值是否相等，没有必然联系，一切都取决于hashCode()这个方法
         */
        int hash = spread(key.hashCode());
        
        int binCount = 0;
        
        Node<K, V>[] tab = table;
        
        while(true) {
            Node<K, V> f;   // 指向待插入元素应当插入的位置
            int fh;         // 元素f对应的哈希值
            K fk;           // 元素f中的key
            V fv;           // 元素f中的value
            
            int len;        // 哈希数组容量
            int i;          // 当前key在哈希数组上的索引
            
            // 如果哈希数组还未初始化，或者容量无效，则需要初始化一个哈希数组
            if(tab == null || (len = tab.length) == 0) {
                // 初始化哈希数组
                tab = initTable();
                
                /*
                 * 如果哈希数组已经初始化，则需要确定待插入元素所在的哈希槽
                 *
                 * f指向hash所在的哈希槽（链）上的首个元素
                 * f==null意味着需要在哈希槽的相应位置插入首个元素
                 *
                 * 如果当前哈希槽处没有旧元素，说明当前的node将作为首个元素插入
                 */
            } else if((f = tabAt(tab, i = (len - 1) & hash)) == null) {
                // node是相应哈希槽处的首个元素
                Node<K, V> node = new Node<>(hash, key, value);
                
                // 原子地更新tab[i]为node
                if(casTabAt(tab, i, null, node)) {
                    // 跳出外循环
                    break;                   // no lock when adding to empty bin
                }
                
                /*
                 * 如果待插入元素所在的哈希槽上已经有别的结点存在，且该结点类型为MOVED
                 * 说明当前哈希数组正在扩容中，此时，可以尝试加速扩容过程
                 */
            } else if((fh = f.hash) == MOVED) {
                tab = helpTransfer(tab, f);
                
                /*
                 * 如果待插入元素所在的哈希槽上已经有别的结点存在，且当前状态不是在扩容当中，那么首先判断该结点是否为同位元素
                 * 如果遇到了同位元素，但不允许覆盖存储，则直接返回待插入的值
                 */
            } else if(onlyIfAbsent // 如果不允许覆盖存储
                && fh == hash && ((fk = f.key) == key || (fk != null && key.equals(fk))) && (fv = f.val) != null) { // 如果遇到了同位元素
                
                return fv;
                
                /*
                 * 如果不是同位元素，则需要搜寻合适的插入位置
                 * 如果是同位元素，且允许覆盖，则直接覆盖旧元素
                 */
            } else {
                V oldVal = null;
                
                // 对已有元素进行搜索，尝试插入新结点(尾插)
                synchronized(f) {
                    // 如果tab[i]==f，则代表当前待插入状态仍然可信
                    if(tabAt(tab, i) == f) {
                        // 如果当前哈希槽首个元素是普通结点
                        if(fh >= 0) {
                            binCount = 1;
                            
                            // 遍历哈希槽(链)
                            for(Node<K, V> e = f; ; ++binCount) {
                                K ek;
                                
                                // 找到了同位元素，替换旧元素或者跳出循环
                                if(e.hash == hash && ((ek = e.key) == key || (ek != null && key.equals(ek)))) {
                                    // 记录同位元素旧值
                                    oldVal = e.val;
                                    
                                    // 如果允许覆盖，则存入新值
                                    if(!onlyIfAbsent) {
                                        e.val = value;
                                    }
                                    
                                    // 跳出内循环
                                    break;
                                }
                                
                                Node<K, V> pred = e;
                                e = e.next;
                                
                                // 无法找到同位元素的话，说明需要在哈希槽(链)末尾新增元素
                                if(e==null) {
                                    pred.next = new Node<K, V>(hash, key, value);
                                    
                                    // 跳出内循环
                                    break;
                                }
                            } // for
                            
                            // 如果当前哈希槽首个元素是红黑树(头结点)
                        } else if(f instanceof TreeBin) {
                            Node<K, V> p;
                            binCount = 2;
                            if((p = ((TreeBin<K, V>) f).putTreeVal(hash, key, value)) != null) {
                                oldVal = p.val;
                                if(!onlyIfAbsent) {
                                    p.val = value;
                                }
                            }
                        } else if(f instanceof ReservationNode) {
                            throw new IllegalStateException("Recursive update");
                        }
                    }
                } // synchronized
                
                // 如果对已有元素搜索过，则计数会发生变动，这里需要进一步观察
                if(binCount != 0) {
                    // 哈希槽（链）上的元素数量增加到TREEIFY_THRESHOLD后，这些元素进入波动期，即将从链表转换为红黑树
                    if(binCount >= TREEIFY_THRESHOLD) {
                        treeifyBin(tab, i);
                    }
                    
                    if(oldVal != null) {
                        return oldVal;
                    }
                    
                    break;
                }
            }
        } // while
        
        // 增加计数
        addCount(1L, binCount);
        
        return null;
    }
    
    /**
     * Implementation for the four public remove/replace methods:
     * Replaces node value with v, conditional upon match of cv if non-null.
     * If resulting value is null, delete.
     */
    /*
     * 根据key查找同位元素，如果找不到，直接返回null
     * 如果存在同位元素，则：
     * 　　如果cv不为空，且不等于旧值，仍然返回null
     * 　　如果cv为null，或者cv等于旧值，则：
     * 　　　　如果newValue不为空，用新值覆盖旧值
     * 　　　　如果newValue为null，移除旧值
     */
    final V replaceNode(Object key, V newValue, Object cv) {
        int hash = spread(key.hashCode());
        
        Node<K,V>[] tab = table;
        
        for(; ; ) {
            Node<K,V> f; int n, i, fh;
            
            // 如果指定的哈希槽处没有结点可以处理
            if (tab == null || (n = tab.length) == 0 || (f = tabAt(tab, i = (n - 1) & hash)) == null) {
                break;
                
                // 正在扩容中
            } else if ((fh = f.hash) == MOVED) {
                tab = helpTransfer(tab, f);
            } else {
                V oldVal = null;
                
                // 是否处理过结点
                boolean validated = false;
                
                synchronized (f) {
                    // 双重检查
                    if (tabAt(tab, i) == f) {
                        // 处理普通结点
                        if (fh >= 0) {
                            validated = true;
                            
                            // 初始化为哈希槽上首个结点
                            Node<K,V> e = f;
                            
                            Node<K,V> pred = null;
                            
                            while(true) {
                                K ek;
                                
                                // 如果找到了同位元素
                                if (e.hash == hash && ((ek = e.key) == key || (ek != null && key.equals(ek)))) {
                                    // 旧值
                                    V ev = e.val;
                                    
                                    if (cv == null || cv == ev || (ev != null && cv.equals(ev))) {
                                        oldVal = ev;
                                        
                                        if (newValue != null) {
                                            // 覆盖旧值
                                            e.val = newValue;
                                            
                                            // 如果newValue为null，需要进一步判断
                                        } else {
                                            // 如果pred为null，说明需要移除哈希槽上首个结点
                                            if(pred == null) {
                                                // 设置tab[i]为e.next
                                                setTabAt(tab, i, e.next);
                                            } else {
                                                // 移除旧结点
                                                pred.next = e.next;
                                            }
                                        }
                                    }
                                    
                                    break;
                                }
                                
                                pred = e;
                                
                                if ((e = e.next) == null) {
                                    break;
                                }
                            }// while(true)
                            
                            // 处理红黑树
                        } else if (f instanceof TreeBin) {
                            validated = true;
                            
                            TreeBin<K,V> t = (TreeBin<K,V>)f;
                            TreeNode<K,V> r, p;
                            
                            if ((r = t.root) != null &&
                                (p = r.findTreeNode(hash, key, null)) != null) {
                                V pv = p.val;
                                if (cv == null || cv == pv || (pv != null && cv.equals(pv))) { oldVal = pv;
                                    if (newValue != null) {
                                        p.val = newValue;
                                    } else if (t.removeTreeNode(p)) {
                                        Node<K, V> x = untreeify(t.first);
                                        // 设置tab[i]为x
                                        setTabAt(tab, i, x);
                                    }
                                }
                            }
                        } else if (f instanceof ReservationNode) {
                            throw new IllegalStateException("Recursive update");
                        }
                    }
                }// synchronized
                
                // 如果处理过结点，则需要进一步验证
                if (validated) {
                    if (oldVal != null) {
                        if (newValue == null) {
                            addCount(-1L, -1);
                        }
                        
                        return oldVal;
                    }
                    
                    break;
                }
            }
        }// for(; ; )
        
        return null;
    }
    
    
    // Overrides of JDK8+ Map extension method defaults
    
    /**
     * Helper method for EntrySetView.removeIf.
     */
    boolean removeEntryIf(Predicate<? super Entry<K,V>> function) {
        if (function == null) throw new NullPointerException();
        Node<K,V>[] t;
        boolean removed = false;
        if ((t = table) != null) {
            Traverser<K,V> it = new Traverser<K,V>(t, t.length, 0, t.length);
            for (Node<K,V> p; (p = it.advance()) != null; ) {
                K k = p.key;
                V v = p.val;
                Map.Entry<K,V> e = new AbstractMap.SimpleImmutableEntry<>(k, v);
                if (function.test(e) && replaceNode(k, null, v) != null) {
                    removed = true;
                }
            }
        }
        return removed;
    }
    
    /**
     * Helper method for ValuesView.removeIf.
     */
    boolean removeValueIf(Predicate<? super V> function) {
        if (function == null) throw new NullPointerException();
        Node<K,V>[] t;
        boolean removed = false;
        if ((t = table) != null) {
            Traverser<K,V> it = new Traverser<K,V>(t, t.length, 0, t.length);
            for (Node<K,V> p; (p = it.advance()) != null; ) {
                K k = p.key;
                V v = p.val;
                if (function.test(v) && replaceNode(k, null, v) != null) {
                    removed = true;
                }
            }
        }
        return removed;
    }
    
    
    // ConcurrentHashMap-only methods
    
    /**
     * Returns the number of mappings. This method should be used
     * instead of {@link #size} because a ConcurrentHashMap may
     * contain more mappings than can be represented as an int. The
     * value returned is an estimate; the actual count may differ if
     * there are concurrent insertions or removals.
     *
     * @return the number of mappings
     * @since 1.8
     */
    public long mappingCount() {
        long n = sumCount();
        return (n < 0L) ? 0L : n; // ignore transient negative values
    }
    
    /**
     * Creates a new {@link Set} backed by a ConcurrentHashMap
     * from the given type to {@code Boolean.TRUE}.
     *
     * @param <K> the element type of the returned set
     * @return the new set
     * @since 1.8
     */
    public static <K> KeySetView<K,Boolean> newKeySet() {
        return new KeySetView<K,Boolean>(new ConcurrentHashMap<K,Boolean>(), Boolean.TRUE);
    }
    
    /**
     * Creates a new {@link Set} backed by a ConcurrentHashMap
     * from the given type to {@code Boolean.TRUE}.
     *
     * @param initialCapacity The implementation performs internal
     * sizing to accommodate this many elements.
     * @param <K> the element type of the returned set
     * @return the new set
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative
     * @since 1.8
     */
    public static <K> KeySetView<K,Boolean> newKeySet(int initialCapacity) {
        return new KeySetView<K,Boolean>(new ConcurrentHashMap<K,Boolean>(initialCapacity), Boolean.TRUE);
    }
    
    /**
     * Returns a {@link Set} view of the keys in this map, using the
     * given common mapped value for any additions (i.e., {@link
     * Collection#add} and {@link Collection#addAll(Collection)}).
     * This is of course only appropriate if it is acceptable to use
     * the same value for all additions from this view.
     *
     * @param mappedValue the mapped value to use for any additions
     * @return the set view
     * @throws NullPointerException if the mappedValue is null
     */
    public KeySetView<K,V> keySet(V mappedValue) {
        if (mappedValue == null) {
            throw new NullPointerException();
        }
        
        return new KeySetView<K,V>(this, mappedValue);
    }
    
    
    /* ---------------- Table Initialization and Resizing -------------- */
    
    /**
     * Returns the stamp bits for resizing a table of size n.
     * Must be negative when shifted left by RESIZE_STAMP_SHIFT.
     */
    static final int resizeStamp(int n) {
        return Integer.numberOfLeadingZeros(n) | (1 << (RESIZE_STAMP_BITS - 1));
    }
    
    /**
     * Helps transfer if a resize is in progress.
     */
    // 加速扩容过程
    final Node<K, V>[] helpTransfer(Node<K, V>[] tab, Node<K, V> f) {
        Node<K, V>[] nextTab;
        
        int sc;
        
        if(tab != null && (f instanceof ForwardingNode) && (nextTab = ((ForwardingNode<K, V>) f).nextTable) != null) {
            int rs = resizeStamp(tab.length);
            
            while(nextTab == nextTable && table == tab && (sc = sizeCtl)<0) {
                if((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 || sc == rs + MAX_RESIZERS || transferIndex<=0) {
                    break;
                }
                
                if(U.compareAndSetInt(this, SIZECTL, sc, sc + 1)) {
                    transfer(tab, nextTab);
                    break;
                }
            }
            
            return nextTab;
        }
        
        return table;
    }
    
    /**
     * Initializes table, using the size recorded in sizeCtl.
     */
    // 初始化哈希数组
    private final Node<K, V>[] initTable() {
        Node<K, V>[] tab;
        
        // 直到哈希数组初始化完成后，循环结束
        while((tab = table) == null || tab.length == 0) {
            
            int sc = sizeCtl;
            
            // 如果当前Map已经处于初始化阶段，则当前线程让出CPU时间片
            if(sc<0) {
                Thread.yield(); // lost initialization race; just spin
                
                // 原子地将sizeCtl字段更新为-1，代表当前Map进入初始化阶段
            } else if(U.compareAndSetInt(this, SIZECTL, sc, -1)) {
                try {
                    if((tab = table) == null || tab.length == 0) {
                        /*
                         * 如果指定了初始容量，则使用预设容量，否则使用默认的初始容量
                         * 参见：构造器中初始容量与预设容量的换算方式
                         */
                        int len = (sc>0) ? sc : DEFAULT_CAPACITY;
                        
                        // 创建新的哈希数组
                        @SuppressWarnings("unchecked")
                        Node<K, V>[] newTable = (Node<K, V>[]) new Node<?, ?>[len];
                        table = tab = newTable;
                        
                        // 更新为扩容阈值：0.75*容量
                        sc = len - (len >>> 2);
                    }
                } finally {
                    // 恢复sizeCtl标记为非负数
                    sizeCtl = sc;
                }
                
                break;
            }
        }
        
        return tab;
    }
    
    /**
     * Adds to count, and if table is too small and not already resizing, initiates transfer.
     * If already resizing, helps perform transfer if work is available.
     * Rechecks occupancy after a transfer to see if another resize is already needed because resizings are lagging additions.
     *
     * @param x     the count to add
     * @param check if <0, don't check resize, if <= 1 only check if uncontended
     */
    private final void addCount(long x, int check) {
        CounterCell[] cs;
        long b, s;
        
        if((cs = counterCells) != null || !U.compareAndSetLong(this, BASECOUNT, b = baseCount, s = b + x)) {
            CounterCell c;
            long v;
            int m;
            boolean uncontended = true;
            
            if(cs == null || (m = cs.length - 1)<0 || (c = cs[ThreadLocalRandom.getProbe() & m]) == null || !(uncontended = U.compareAndSetLong(c, CELLVALUE, v = c.value, v + x))) {
                fullAddCount(x, uncontended);
                return;
            }
            
            if(check<=1) {
                return;
            }
            
            s = sumCount();
        }
        
        if(check >= 0) {
            Node<K, V>[] tab, nt;
            
            int n, sc;
            
            while(s >= (long) (sc = sizeCtl) && (tab = table) != null && (n = tab.length)<MAXIMUM_CAPACITY) {
                int rs = resizeStamp(n);
                
                if(sc<0) {
                    if((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 || sc == rs + MAX_RESIZERS || (nt = nextTable) == null || transferIndex<=0) {
                        break;
                    }
                    
                    if(U.compareAndSetInt(this, SIZECTL, sc, sc + 1)) {
                        transfer(tab, nt);
                    }
                } else if(U.compareAndSetInt(this, SIZECTL, sc, (rs << RESIZE_STAMP_SHIFT) + 2)) {
                    transfer(tab, null);
                }
                
                s = sumCount();
            }
        }
    }
    
    /**
     * Tries to presize table to accommodate the given number of elements.
     *
     * @param size number of elements (doesn't need to be perfectly accurate)
     */
    // 尝试调整哈希数组的大小，以容纳指定数量的元素
    private final void tryPresize(int size) {
        // 预备扩容到的目标容量
        int cap = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY                          // 如果哈希数组容量已经为最大容量的一半，则直接使用最大容量
            : tableSizeFor(size + (size >>> 1) + 1);    // 理想情形下，哈希数组容量增加0.5倍
        
        int sc;
        
        while((sc = sizeCtl) >= 0) {
            Node<K, V>[] tab = table;
            
            int len;
            
            // 如果哈希数组还未初始化，或者容量无效，则需要初始化一个哈希数组
            if(tab == null || (len = tab.length) == 0) {
                // 确定待扩容的新容量
                len = Math.max(sc, cap);
                
                // 原子地将sizeCtl字段更新为-1，代表当前Map进入了初始化哈希数组的阶段
                if(U.compareAndSetInt(this, SIZECTL, sc, -1)) {
                    try {
                        if(table == tab) {
                            @SuppressWarnings("unchecked")
                            Node<K, V>[] nt = (Node<K, V>[]) new Node<?, ?>[len];
                            table = nt;
                            
                            // 确定容量阙值为0.75*容量
                            sc = len - (len >>> 2);
                        }
                    } finally {
                        // 恢复sizeCtl标记为非负数
                        sizeCtl = sc;
                    }
                }
                
                // 在哈希数组已经初始化的情形下：如果预备扩容到的目标容量小于阙值(无需扩容)，或者哈希数组当前容量已达上限(无法扩容)，直接退出
            } else if(cap<=sc || len >= MAXIMUM_CAPACITY) {
                break;
                
                // 如果需要扩容
            } else if(tab == table) {
                int stamp = resizeStamp(len) << RESIZE_STAMP_SHIFT;
                
                // 原子地更新sizeCtl为
                if(U.compareAndSetInt(this, SIZECTL, sc, stamp + 2)) {
                    transfer(tab, null);
                }
            }
        }
    }
    
    /**
     * Moves and/or copies the nodes in each bin to new table. See above for explanation.
     */
    // 在哈希数组的扩容过程中进行数据迁移
    private final void transfer(Node<K, V>[] tab, Node<K, V>[] nextTab) {
        int n = tab.length;
        
        int stride = (NCPU>1) ? (n >>> 3) / NCPU : n;
        
        if(stride<MIN_TRANSFER_STRIDE) {
            stride = MIN_TRANSFER_STRIDE; // subdivide range
        }
        
        if(nextTab == null) {            // initiating
            try {
                @SuppressWarnings("unchecked")
                Node<K, V>[] nt = (Node<K, V>[]) new Node<?, ?>[n << 1];
                nextTab = nt;
            } catch(Throwable ex) {      // try to cope with OOME
                sizeCtl = Integer.MAX_VALUE;
                return;
            }
            
            nextTable = nextTab;
            transferIndex = n;
        }
        
        ForwardingNode<K, V> fwd = new ForwardingNode<K, V>(nextTab);
        
        int nextn = nextTab.length;
        
        boolean advance = true;
        
        boolean finishing = false; // to ensure sweep before committing nextTab
        
        int i = 0;
        int bound = 0;
        
        while(true) {
            Node<K, V> f;
            int fh;
            
            while(advance) {
                int nextIndex;
                int nextBound;
                
                if(--i >= bound || finishing) {
                    advance = false;
                } else if((nextIndex = transferIndex)<=0) {
                    i = -1;
                    advance = false;
                } else if(U.compareAndSetInt(this, TRANSFERINDEX, nextIndex, nextBound = (nextIndex>stride ? nextIndex - stride : 0))) {
                    bound = nextBound;
                    i = nextIndex - 1;
                    advance = false;
                }
            }
            
            if(i<0 || i >= n || i + n >= nextn) {
                int sc;
                
                if(finishing) {
                    nextTable = null;
                    table = nextTab;
                    sizeCtl = (n << 1) - (n >>> 1);
                    return;
                }
                
                if(U.compareAndSetInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
                    int stamp = resizeStamp(n) << RESIZE_STAMP_SHIFT;
                    
                    if((sc - 2) != stamp) {
                        return;
                    }
                    
                    finishing = advance = true;
                    
                    i = n; // recheck before commit
                }
                
                // 如果tab[i]==null
            } else if((f = tabAt(tab, i)) == null) {
                // 原子地更新tab[i]为fwd
                advance = casTabAt(tab, i, null, fwd);
            } else if((fh = f.hash) == MOVED) {
                advance = true; // already processed
            } else {
                synchronized(f) {
                    if(tabAt(tab, i) == f) {
                        Node<K, V> ln, hn;
                        
                        if(fh >= 0) {
                            int runBit = fh & n;
                            
                            Node<K, V> lastRun = f;
                            
                            for(Node<K, V> p = f.next; p != null; p = p.next) {
                                int b = p.hash & n;
                                if(b != runBit) {
                                    runBit = b;
                                    lastRun = p;
                                }
                            }
                            
                            if(runBit == 0) {
                                ln = lastRun;
                                hn = null;
                            } else {
                                hn = lastRun;
                                ln = null;
                            }
                            
                            for(Node<K, V> p = f; p != lastRun; p = p.next) {
                                int ph = p.hash;
                                K pk = p.key;
                                V pv = p.val;
                                if((ph & n) == 0) {
                                    ln = new Node<K, V>(ph, pk, pv, ln);
                                } else {
                                    hn = new Node<K, V>(ph, pk, pv, hn);
                                }
                            }
                            
                            // nextTab[i]=ln
                            setTabAt(nextTab, i, ln);
                            
                            // nextTab[i+n]=hn
                            setTabAt(nextTab, i + n, hn);
                            
                            // tab[i]=fwd
                            setTabAt(tab, i, fwd);
                            
                            advance = true;
                        } else if(f instanceof TreeBin) {
                            TreeBin<K, V> t = (TreeBin<K, V>) f;
                            TreeNode<K, V> lo = null, loTail = null;
                            TreeNode<K, V> hi = null, hiTail = null;
                            int lc = 0, hc = 0;
                            for(Node<K, V> e = t.first; e != null; e = e.next) {
                                int h = e.hash;
                                TreeNode<K, V> p = new TreeNode<K, V>(h, e.key, e.val, null, null);
                                if((h & n) == 0) {
                                    if((p.prev = loTail) == null) {
                                        lo = p;
                                    } else {
                                        loTail.next = p;
                                    }
                                    loTail = p;
                                    ++lc;
                                } else {
                                    if((p.prev = hiTail) == null) {
                                        hi = p;
                                    } else {
                                        hiTail.next = p;
                                    }
                                    hiTail = p;
                                    ++hc;
                                }
                            }
                            
                            ln = (lc<=UNTREEIFY_THRESHOLD) ? untreeify(lo) : (hc != 0) ? new TreeBin<K, V>(lo) : t;
                            
                            hn = (hc<=UNTREEIFY_THRESHOLD) ? untreeify(hi) : (lc != 0) ? new TreeBin<K, V>(hi) : t;
                            
                            // nextTab[i]=ln
                            setTabAt(nextTab, i, ln);
                            
                            // nextTab[i+n]=hn
                            setTabAt(nextTab, i + n, hn);
                            
                            // tab[i]=fwd
                            setTabAt(tab, i, fwd);
                            
                            advance = true;
                        }
                    }
                }
            }
        }
    }
    
    
    /* ---------------- Counter support -------------- */
    
    final long sumCount() {
        CounterCell[] cs = counterCells;
        
        long sum = baseCount;
        
        if(cs != null) {
            for(CounterCell c : cs) {
                if(c != null) {
                    sum += c.value;
                }
            }
        }
        
        return sum;
    }
    
    // See LongAdder version for explanation
    private final void fullAddCount(long x, boolean wasUncontended) {
        int h;
        
        if((h = ThreadLocalRandom.getProbe()) == 0) {
            ThreadLocalRandom.localInit();      // force initialization
            h = ThreadLocalRandom.getProbe();
            wasUncontended = true;
        }
        
        boolean collide = false;                // True if last slot nonempty
        
        while(true) {
            CounterCell[] cs;
            CounterCell c;
            int n;
            long v;
            
            if((cs = counterCells) != null && (n = cs.length)>0) {
                if((c = cs[(n - 1) & h]) == null) {
                    if(cellsBusy == 0) {            // Try to attach new Cell
                        CounterCell r = new CounterCell(x); // Optimistic create
                        if(cellsBusy == 0 && U.compareAndSetInt(this, CELLSBUSY, 0, 1)) {
                            boolean created = false;
                            try {               // Recheck under lock
                                CounterCell[] rs;
                                int m, j;
                                if((rs = counterCells) != null && (m = rs.length)>0 && rs[j = (m - 1) & h] == null) {
                                    rs[j] = r;
                                    created = true;
                                }
                            } finally {
                                cellsBusy = 0;
                            }
                            
                            if(created) {
                                break;
                            }
                            
                            continue;           // Slot is now non-empty
                        }
                    }
                    
                    collide = false;
                    
                    // CAS already known to fail
                } else if(!wasUncontended) {
                    wasUncontended = true;      // Continue after rehash
                } else if(U.compareAndSetLong(c, CELLVALUE, v = c.value, v + x)) {
                    break;
                } else if(counterCells != cs || n >= NCPU) {
                    collide = false;            // At max size or stale
                } else if(!collide) {
                    collide = true;
                } else if(cellsBusy == 0 && U.compareAndSetInt(this, CELLSBUSY, 0, 1)) {
                    try {
                        // Expand table unless stale
                        if(counterCells == cs) {
                            counterCells = Arrays.copyOf(cs, n << 1);
                        }
                    } finally {
                        cellsBusy = 0;
                    }
                    collide = false;
                    continue;                   // Retry with expanded table
                }
                
                h = ThreadLocalRandom.advanceProbe(h);
            } else if(cellsBusy == 0 && counterCells == cs && U.compareAndSetInt(this, CELLSBUSY, 0, 1)) {
                boolean init = false;
                
                try {                           // Initialize table
                    if(counterCells == cs) {
                        CounterCell[] rs = new CounterCell[2];
                        rs[h & 1] = new CounterCell(x);
                        counterCells = rs;
                        init = true;
                    }
                } finally {
                    cellsBusy = 0;
                }
                
                if(init) {
                    break;
                }
            } else if(U.compareAndSetLong(this, BASECOUNT, v = baseCount, v + x)) {
                break;                          // Fall back on using base
            }
        }
    }
    
    
    /* ---------------- Conversion from/to TreeBins -------------- */
    
    /**
     * Replaces all linked nodes in bin at given index unless table is too small, in which case resizes instead.
     */
    /*
     * 观察哈希槽（链）上处于波动期的元素，以决定下一步是扩容还是将链表转换为红黑树
     *
     * tab：待转换的链表
     * hash：某元素的哈希值
     */
    private final void treeifyBin(Node<K, V>[] tab, int index) {
        if(tab == null) {
            return;
        }
        
        Node<K, V> b;
        int n;
        
        // 哈希数组的容量还未达到形成一棵红黑树的最低要求
        if((n = tab.length)<MIN_TREEIFY_CAPACITY) {
            // 尝试调整哈希数组的大小，以容纳指定数量的元素（默认增加一倍）
            tryPresize(n << 1);
            
            // 满足从链表转换为红黑树的要求，则将链表转换为红黑树
        } else {
            // 获取tab[index]
            b = tabAt(tab, index);
            if(b == null || b.hash<0) {
                return;
            }
            
            synchronized(b) {
                // 双重检查机制
                if(tabAt(tab, index) == b) {
                    TreeNode<K, V> hd = null, tl = null;
                    
                    // 1.首先，将所有普通结点“转换”为红黑树结点并串联起来
                    for(Node<K, V> e = b; e != null; e = e.next) {
                        
                        TreeNode<K, V> p = new TreeNode<>(e.hash, e.key, e.val, null, null);
                        
                        if((p.prev = tl) == null) {
                            hd = p;
                        } else {
                            tl.next = p;
                        }
                        
                        tl = p;
                    }
                    
                    // 2.其次，创建红黑树，即把所有红黑树结点安置在树中合适的位置上
                    TreeBin<K, V> treeBin = new TreeBin<>(hd);
                    
                    // 3.最后，将红黑树设置到哈希槽上
                    setTabAt(tab, index, treeBin);
                }
            }
        }
    }
    
    /**
     * Returns a list of non-TreeNodes replacing those in given list.
     */
    static <K, V> Node<K, V> untreeify(Node<K, V> b) {
        Node<K, V> hd = null, tl = null;
        for(Node<K, V> q = b; q != null; q = q.next) {
            Node<K, V> p = new Node<>(q.hash, q.key, q.val);
            if(tl == null) {
                hd = p;
            } else {
                tl.next = p;
            }
            
            tl = p;
        }
        return hd;
    }
    
    /**
     * Computes initial batch value for bulk tasks.
     * The returned value is approximately exp2 of the number of times (minus one) to split task by two before executing leaf action.
     * This value is faster to compute and more convenient to use as a guide to splitting than is the depth,
     * since it is used while dividing by two anyway.
     */
    final int batchFor(long b) {
        long n;
        
        if(b == Long.MAX_VALUE || (n = sumCount())<=1L || n<b) {
            return 0;
        }
        
        int sp = ForkJoinPool.getCommonPoolParallelism() << 2; // slack of 4
        
        return (b>0L && (n /= b)<sp) ? (int) n : sp;
    }
    
    
    
    
    
    
    /**
     * Key-value entry.  This class is never exported out as a
     * user-mutable Map.Entry (i.e., one supporting setValue; see
     * MapEntry below), but can be used for read-only traversals used
     * in bulk tasks.  Subclasses of Node with a negative hash field
     * are special, and contain null keys and values (but are never
     * exported).  Otherwise, keys and vals are never null.
     */
    // ConcurrentHashMap中的普通结点信息，每个Node代表一个元素，里面包含了key和value的信息
    static class Node<K, V> implements Map.Entry<K, V> {
        final int hash;
        final K key;
        volatile V val;
        volatile Node<K, V> next;
        
        Node(int hash, K key, V val) {
            this.hash = hash;
            this.key = key;
            this.val = val;
        }
        
        Node(int hash, K key, V val, Node<K, V> next) {
            this(hash, key, val);
            this.next = next;
        }
        
        public final K getKey() {
            return key;
        }
        
        public final V getValue() {
            return val;
        }
        
        public final int hashCode() {
            return key.hashCode() ^ val.hashCode();
        }
        
        public final String toString() {
            return Helpers.mapEntryToString(key, val);
        }
        
        public final V setValue(V value) {
            throw new UnsupportedOperationException();
        }
        
        public final boolean equals(Object o) {
            Object k, v, u;
            Map.Entry<?, ?> e;
            return ((o instanceof Map.Entry) && (k = (e = (Map.Entry<?, ?>) o).getKey()) != null && (v = e.getValue()) != null && (k == key || k.equals(key)) && (v == (u = val) || v.equals(u)));
        }
        
        /**
         * Virtualized support for map.get(); overridden in subclasses.
         */
        Node<K, V> find(int h, Object k) {
            Node<K, V> e = this;
            if(k != null) {
                do {
                    K ek;
                    if(e.hash == h && ((ek = e.key) == k || (ek != null && k.equals(ek)))) {
                        return e;
                    }
                } while((e = e.next) != null);
            }
            return null;
        }
    }
    
    /**
     * Nodes for use in TreeBins.
     */
    // ConcurrentHashMap中的红黑树结点
    static final class TreeNode<K, V> extends Node<K, V> {
        TreeNode<K, V> parent;  // red-black tree links
        TreeNode<K, V> left;
        TreeNode<K, V> right;
        
        boolean red;
        
        TreeNode<K, V> prev;    // needed to unlink next upon deletion
        
        
        // 仍然需要维护next链接
        TreeNode(int hash, K key, V val, Node<K, V> next, TreeNode<K, V> parent) {
            super(hash, key, val, next);
            this.parent = parent;
        }
        
        Node<K, V> find(int h, Object k) {
            return findTreeNode(h, k, null);
        }
        
        /**
         * Returns the TreeNode (or null if not found) for the given key
         * starting at given root.
         */
        // 根据给定的key和hash（由key计算而来）查找对应的（同位）元素，如果找不到，则返回null
        final TreeNode<K, V> findTreeNode(int h, Object k, Class<?> kc) {
            if(k != null) {
                TreeNode<K, V> p = this;
                
                do {
                    int ph, dir;
                    K pk;
                    TreeNode<K, V> q;
                    TreeNode<K, V> pl = p.left, pr = p.right;
                    if((ph = p.hash)>h) {
                        p = pl;
                    } else if(ph<h) {
                        p = pr;
                    } else if((pk = p.key) == k || (pk != null && k.equals(pk))) {
                        return p;
                    } else if(pl == null) {
                        p = pr;
                    } else if(pr == null) {
                        p = pl;
                    } else if((kc != null || (kc = comparableClassFor(k)) != null) && (dir = compareComparables(kc, k, pk)) != 0) {
                        p = (dir<0) ? pl : pr;
                    } else if((q = pr.findTreeNode(h, k, kc)) != null) {
                        return q;
                    } else {
                        p = pl;
                    }
                } while(p != null);
            }
            
            return null;
        }
        
    }
    
    /**
     * TreeNodes used at the heads of bins. TreeBins do not hold user
     * keys or values, but instead point to list of TreeNodes and
     * their root. They also maintain a parasitic read-write lock
     * forcing writers (who hold bin lock) to wait for readers (who do
     * not) to complete before tree restructuring operations.
     */
    // 红黑树(头结点)
    static final class TreeBin<K, V> extends Node<K, V> {
        // values for lockState
        static final int WRITER = 1; // set while holding write lock
        static final int WAITER = 2; // set when waiting for write lock
        static final int READER = 4; // increment value for setting read lock
        
        volatile TreeNode<K, V> first;
        
        TreeNode<K, V> root;
        
        volatile Thread waiter;
        
        volatile int lockState;
        
        private static final Unsafe U = Unsafe.getUnsafe();
        private static final long LOCKSTATE = U.objectFieldOffset(TreeBin.class, "lockState");
        
        
        
        /**
         * Creates bin with initial set of nodes headed by b.
         */
        // 创建红黑树，即把所有红黑树结点安置在树中合适的位置上
        TreeBin(TreeNode<K, V> n) {
            super(TREEBIN, null, null);
            
            this.first = n;
            
            TreeNode<K, V> r = null;
            TreeNode<K, V> x = n;
            TreeNode<K, V> next;
            
            while(x != null) {
                next = (TreeNode<K, V>) x.next;
                x.left = x.right = null;
                if(r == null) {
                    x.parent = null;
                    x.red = false;
                    r = x;
                } else {
                    K k = x.key;
                    int h = x.hash;
                    Class<?> kc = null;
                    TreeNode<K, V> p = r;
                    
                    while(true) {
                        int dir, ph;
                        
                        K pk = p.key;
                        
                        if((ph = p.hash)>h) {
                            dir = -1;
                        } else if(ph<h) {
                            dir = 1;
                        } else if((kc == null && (kc = comparableClassFor(k)) == null) || (dir = compareComparables(kc, k, pk)) == 0) {
                            dir = tieBreakOrder(k, pk);
                        }
                        
                        TreeNode<K, V> xp = p;
                        
                        if((p = (dir<=0) ? p.left : p.right) == null) {
                            x.parent = xp;
                            
                            if(dir<=0) {
                                xp.left = x;
                            } else {
                                xp.right = x;
                            }
                            
                            r = balanceInsertion(r, x);
                            
                            break;
                        }
                    }// while(true)
                }
                
                x = next;
            }
            
            this.root = r;
            
            assert checkInvariants(root);
        }
        
        
        
        /*▼ 插入/删除 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
        
        /**
         * Finds or adds a node.
         *
         * @return null if added
         */
        // 向当前红黑树中插入元素
        final TreeNode<K, V> putTreeVal(int h, K k, V v) {
            Class<?> kc = null;
            
            boolean searched = false;
            
            for(TreeNode<K, V> p = root; ; ) {
                int dir, ph;
                K pk;
                if(p == null) {
                    first = root = new TreeNode<K, V>(h, k, v, null, null);
                    break;
                } else if((ph = p.hash)>h) {
                    dir = -1;
                } else if(ph<h) {
                    dir = 1;
                } else if((pk = p.key) == k || (pk != null && k.equals(pk))) {
                    return p;
                } else if((kc == null && (kc = comparableClassFor(k)) == null) || (dir = compareComparables(kc, k, pk)) == 0) {
                    
                    if(!searched) {
                        TreeNode<K, V> q, ch;
                        searched = true;
                        if(((ch = p.left) != null && (q = ch.findTreeNode(h, k, kc)) != null) || ((ch = p.right) != null && (q = ch.findTreeNode(h, k, kc)) != null)) {
                            return q;
                        }
                    }
                    
                    dir = tieBreakOrder(k, pk);
                }
                
                TreeNode<K, V> xp = p;
                
                if((p = (dir<=0) ? p.left : p.right) == null) {
                    TreeNode<K, V> x, f = first;
                    
                    first = x = new TreeNode<K, V>(h, k, v, f, xp);
                    
                    if(f != null) {
                        f.prev = x;
                    }
                    
                    if(dir<=0) {
                        xp.left = x;
                    } else {
                        xp.right = x;
                    }
                    
                    if(!xp.red) {
                        x.red = true;
                    } else {
                        lockRoot();
                        try {
                            root = balanceInsertion(root, x);
                        } finally {
                            unlockRoot();
                        }
                    }
                    break;
                }
            }
            
            assert checkInvariants(root);
            
            return null;
        }
        
        /**
         * Removes the given node, that must be present before this
         * call.  This is messier than typical red-black deletion code
         * because we cannot swap the contents of an interior node
         * with a leaf successor that is pinned by "next" pointers
         * that are accessible independently of lock. So instead we
         * swap the tree linkages.
         *
         * @return true if now too small, so should be untreeified
         */
        // 将元素从红黑树中移除
        final boolean removeTreeNode(TreeNode<K, V> p) {
            TreeNode<K, V> next = (TreeNode<K, V>) p.next;
            TreeNode<K, V> pred = p.prev;  // unlink traversal pointers
            TreeNode<K, V> r, rl;
            
            if(pred == null) {
                first = next;
            } else {
                pred.next = next;
            }
            
            if(next != null) {
                next.prev = pred;
            }
            
            if(first == null) {
                root = null;
                return true;
            }
            if((r = root) == null || r.right == null || // too small
                (rl = r.left) == null || rl.left == null) {
                return true;
            }
            
            lockRoot();
            
            try {
                TreeNode<K, V> replacement;
                TreeNode<K, V> pl = p.left;
                TreeNode<K, V> pr = p.right;
                
                if(pl != null && pr != null) {
                    TreeNode<K, V> s = pr, sl;
                    
                    // find successor
                    while((sl = s.left) != null) {
                        s = sl;
                    }
                    
                    boolean c = s.red;
                    s.red = p.red;
                    p.red = c; // swap colors
                    TreeNode<K, V> sr = s.right;
                    TreeNode<K, V> pp = p.parent;
                    
                    if(s == pr) { // p was s's direct parent
                        p.parent = s;
                        s.right = p;
                    } else {
                        TreeNode<K, V> sp = s.parent;
                        if((p.parent = sp) != null) {
                            if(s == sp.left) {
                                sp.left = p;
                            } else {
                                sp.right = p;
                            }
                        }
                        
                        if((s.right = pr) != null) {
                            pr.parent = s;
                        }
                    }
                    
                    p.left = null;
                    if((p.right = sr) != null) {
                        sr.parent = p;
                    }
                    
                    if((s.left = pl) != null) {
                        pl.parent = s;
                    }
                    
                    if((s.parent = pp) == null) {
                        r = s;
                    } else if(p == pp.left) {
                        pp.left = s;
                    } else {
                        pp.right = s;
                    }
                    
                    if(sr != null) {
                        replacement = sr;
                    } else {
                        replacement = p;
                    }
                } else if(pl != null) {
                    replacement = pl;
                } else if(pr != null) {
                    replacement = pr;
                } else {
                    replacement = p;
                }
                
                if(replacement != p) {
                    TreeNode<K, V> pp = replacement.parent = p.parent;
                    if(pp == null) {
                        r = replacement;
                    } else if(p == pp.left) {
                        pp.left = replacement;
                    } else {
                        pp.right = replacement;
                    }
                    
                    p.left = p.right = p.parent = null;
                }
                
                root = (p.red) ? r : balanceDeletion(r, replacement);
                
                // detach pointers
                if(p == replacement) {
                    TreeNode<K, V> pp;
                    if((pp = p.parent) != null) {
                        if(p == pp.left) {
                            pp.left = null;
                        } else if(p == pp.right) {
                            pp.right = null;
                        }
                        
                        p.parent = null;
                    }
                }
            } finally {
                unlockRoot();
            }
            
            assert checkInvariants(root);
            
            return false;
        }
        
        // 将元素x插入到红黑树root后，可能会破坏其平衡性，所以这里需要做出调整，保持红黑树的平衡
        static <K, V> TreeNode<K, V> balanceInsertion(TreeNode<K, V> root, TreeNode<K, V> x) {
            x.red = true;
            
            TreeNode<K, V> xp, xpp, xppl, xppr;
            
            while(true) {
                if((xp = x.parent) == null) {
                    x.red = false;
                    return x;
                } else if(!xp.red || (xpp = xp.parent) == null) {
                    return root;
                }
                
                if(xp == (xppl = xpp.left)) {
                    if((xppr = xpp.right) != null && xppr.red) {
                        xppr.red = false;
                        xp.red = false;
                        xpp.red = true;
                        x = xpp;
                    } else {
                        if(x == xp.right) {
                            root = rotateLeft(root, x = xp);
                            xpp = (xp = x.parent) == null ? null : xp.parent;
                        }
                        
                        if(xp != null) {
                            xp.red = false;
                            if(xpp != null) {
                                xpp.red = true;
                                root = rotateRight(root, xpp);
                            }
                        }
                    }
                } else {
                    if(xppl != null && xppl.red) {
                        xppl.red = false;
                        xp.red = false;
                        xpp.red = true;
                        x = xpp;
                    } else {
                        if(x == xp.left) {
                            root = rotateRight(root, x = xp);
                            xpp = (xp = x.parent) == null ? null : xp.parent;
                        }
                        
                        if(xp != null) {
                            xp.red = false;
                            if(xpp != null) {
                                xpp.red = true;
                                root = rotateLeft(root, xpp);
                            }
                        }
                    }
                }
            }
        }
        
        // 将元素x从红黑树root移除后，可能会破坏其平衡性，所以这里需要做出调整，保持红黑树的平衡
        static <K, V> TreeNode<K, V> balanceDeletion(TreeNode<K, V> root, TreeNode<K, V> x) {
            TreeNode<K, V> xp, xpl, xpr;
            
            while(true) {
                if(x == null || x == root) {
                    return root;
                } else if((xp = x.parent) == null) {
                    x.red = false;
                    return x;
                } else if(x.red) {
                    x.red = false;
                    return root;
                } else if((xpl = xp.left) == x) {
                    if((xpr = xp.right) != null && xpr.red) {
                        xpr.red = false;
                        xp.red = true;
                        root = rotateLeft(root, xp);
                        xpr = (xp = x.parent) == null ? null : xp.right;
                    }
                    
                    if(xpr == null) {
                        x = xp;
                    } else {
                        TreeNode<K, V> sl = xpr.left, sr = xpr.right;
                        if((sr == null || !sr.red) && (sl == null || !sl.red)) {
                            xpr.red = true;
                            x = xp;
                        } else {
                            if(sr == null || !sr.red) {
                                if(sl != null) {
                                    sl.red = false;
                                }
                                xpr.red = true;
                                root = rotateRight(root, xpr);
                                xpr = (xp = x.parent) == null ? null : xp.right;
                            }
                            
                            if(xpr != null) {
                                xpr.red = (xp != null) && xp.red;
                                if((sr = xpr.right) != null) {
                                    sr.red = false;
                                }
                            }
                            
                            if(xp != null) {
                                xp.red = false;
                                root = rotateLeft(root, xp);
                            }
                            
                            x = root;
                        }
                    }
                } else { // symmetric
                    if(xpl != null && xpl.red) {
                        xpl.red = false;
                        xp.red = true;
                        root = rotateRight(root, xp);
                        xpl = (xp = x.parent) == null ? null : xp.left;
                    }
                    
                    if(xpl == null) {
                        x = xp;
                    } else {
                        TreeNode<K, V> sl = xpl.left, sr = xpl.right;
                        if((sl == null || !sl.red) && (sr == null || !sr.red)) {
                            xpl.red = true;
                            x = xp;
                        } else {
                            if(sl == null || !sl.red) {
                                if(sr != null) {
                                    sr.red = false;
                                }
                                xpl.red = true;
                                root = rotateLeft(root, xpl);
                                xpl = (xp = x.parent) == null ? null : xp.left;
                            }
                            
                            if(xpl != null) {
                                xpl.red = (xp != null) && xp.red;
                                if((sl = xpl.left) != null) {
                                    sl.red = false;
                                }
                            }
                            
                            if(xp != null) {
                                xp.red = false;
                                root = rotateRight(root, xp);
                            }
                            
                            x = root;
                        }
                    }
                }
            }
        }
        
        // 左旋
        static <K, V> TreeNode<K, V> rotateLeft(TreeNode<K, V> root, TreeNode<K, V> p) {
            TreeNode<K, V> r, pp, rl;
            
            if(p != null && (r = p.right) != null) {
                if((rl = p.right = r.left) != null) {
                    rl.parent = p;
                }
                
                if((pp = r.parent = p.parent) == null) {
                    (root = r).red = false;
                } else if(pp.left == p) {
                    pp.left = r;
                } else {
                    pp.right = r;
                }
                r.left = p;
                p.parent = r;
            }
            
            return root;
        }
        
        // 右旋
        static <K, V> TreeNode<K, V> rotateRight(TreeNode<K, V> root, TreeNode<K, V> p) {
            TreeNode<K, V> l, pp, lr;
            
            if(p != null && (l = p.left) != null) {
                if((lr = p.left = l.right) != null) {
                    lr.parent = p;
                }
                
                if((pp = l.parent = p.parent) == null) {
                    (root = l).red = false;
                } else if(pp.right == p) {
                    pp.right = l;
                } else {
                    pp.left = l;
                }
                
                l.right = p;
                p.parent = l;
            }
            
            return root;
        }
        
        /*▲ 插入/删除 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
        
        
        
        /**
         * Tie-breaking utility for ordering insertions when equal
         * hashCodes and non-comparable. We don't require a total
         * order, just a consistent insertion rule to maintain
         * equivalence across rebalancings. Tie-breaking further than
         * necessary simplifies testing a bit.
         */
        // 用于红黑树元素比较
        static int tieBreakOrder(Object a, Object b) {
            int d;
            
            if(a == null
                || b == null
                || (d = a.getClass().getName().compareTo(b.getClass().getName())) == 0) {
                d = (System.identityHashCode(a)<=System.identityHashCode(b) ? -1 : 1);
            }
            
            return d;
        }
        
        /**
         * Returns matching node or null if none. Tries to search
         * using tree comparisons from root, but continues linear
         * search when lock not available.
         */
        // 根据给定的key和hash（由key计算而来）查找对应的（同位）元素，如果找不到，则返回null
        final Node<K, V> find(int hash, Object k) {
            if(k != null) {
                for(Node<K, V> e = first; e != null; ) {
                    int s;
                    K ek;
                    
                    if(((s = lockState) & (WAITER | WRITER)) != 0) {
                        if(e.hash == hash && ((ek = e.key) == k || (ek != null && k.equals(ek)))) {
                            return e;
                        }
                        e = e.next;
                    } else if(U.compareAndSetInt(this, LOCKSTATE, s, s + READER)) {
                        TreeNode<K, V> r, p;
                        try {
                            p = ((r = root) == null ? null : r.findTreeNode(hash, k, null));
                        } finally {
                            Thread w;
                            if(U.getAndAddInt(this, LOCKSTATE, -READER) == (READER | WAITER) && (w = waiter) != null) {
                                LockSupport.unpark(w);
                            }
                        }
                        return p;
                    }
                }
            }
            return null;
        }
        
        /**
         * Acquires write lock for tree restructuring.
         */
        private final void lockRoot() {
            if(!U.compareAndSetInt(this, LOCKSTATE, 0, WRITER)) {
                contendedLock(); // offload to separate method
            }
        }
        
        /**
         * Releases write lock for tree restructuring.
         */
        private final void unlockRoot() {
            lockState = 0;
        }
        
        /**
         * Possibly blocks awaiting root lock.
         */
        private final void contendedLock() {
            boolean waiting = false;
            for(int s; ; ) {
                if(((s = lockState) & ~WAITER) == 0) {
                    if(U.compareAndSetInt(this, LOCKSTATE, s, WRITER)) {
                        if(waiting) {
                            waiter = null;
                        }
                        return;
                    }
                } else if((s & WAITER) == 0) {
                    if(U.compareAndSetInt(this, LOCKSTATE, s, s | WAITER)) {
                        waiting = true;
                        waiter = Thread.currentThread();
                    }
                } else if(waiting) {
                    LockSupport.park(this);
                }
            }
        }
        
        /**
         * Checks invariants recursively for the tree of Nodes rooted at t.
         */
        static <K, V> boolean checkInvariants(TreeNode<K, V> t) {
            TreeNode<K, V> tp = t.parent, tl = t.left, tr = t.right, tb = t.prev, tn = (TreeNode<K, V>) t.next;
            
            if(tb != null && tb.next != t) {
                return false;
            }
            
            if(tn != null && tn.prev != t) {
                return false;
            }
            
            if(tp != null && t != tp.left && t != tp.right) {
                return false;
            }
            
            if(tl != null && (tl.parent != t || tl.hash>t.hash)) {
                return false;
            }
            
            if(tr != null && (tr.parent != t || tr.hash<t.hash)) {
                return false;
            }
            
            if(t.red && tl != null && tl.red && tr != null && tr.red) {
                return false;
            }
            
            if(tl != null && !checkInvariants(tl)) {
                return false;
            }
            
            return tr == null || checkInvariants(tr);
        }
        
    }
    
    
    
    /**
     * A node inserted at head of bins during transfer operations.
     */
    // 前向结点，应用在数据迁移中。比如Map在扩容时，会将新容器包裹在前向结点中，扩容完毕之后，则撤销该前向结点
    static final class ForwardingNode<K, V> extends Node<K, V> {
        final Node<K, V>[] nextTable;
        
        ForwardingNode(Node<K, V>[] tab) {
            super(MOVED, null, null);
            this.nextTable = tab;
        }
        
        Node<K, V> find(int h, Object k) {
            // loop to avoid arbitrarily deep recursion on forwarding nodes
outer:
            for(Node<K, V>[] tab = nextTable; ; ) {
                Node<K, V> e;
                int n;
                
                if(k == null || tab == null || (n = tab.length) == 0 || (e = tabAt(tab, (n - 1) & h)) == null) {
                    return null;
                }
                
                for(; ; ) {
                    int eh;
                    K ek;
                    
                    if((eh = e.hash) == h && ((ek = e.key) == k || (ek != null && k.equals(ek)))) {
                        return e;
                    }
                    
                    if(eh<0) {
                        if(e instanceof ForwardingNode) {
                            tab = ((ForwardingNode<K, V>) e).nextTable;
                            continue outer;
                        } else {
                            return e.find(h, k);
                        }
                    }
                    
                    if((e = e.next) == null) {
                        return null;
                    }
                }
            }
        }
    }
    
    /**
     * A place-holder node used in computeIfAbsent and compute.
     */
    // 占位结点
    static final class ReservationNode<K, V> extends Node<K, V> {
        ReservationNode() {
            super(RESERVED, null, null);
        }
        
        Node<K, V> find(int h, Object k) {
            return null;
        }
    }
    
    
    
    /**
     * Base class for views.
     */
    abstract static class CollectionView<K, V, E> implements Collection<E>, Serializable {
        private static final long serialVersionUID = 7249069246763182397L;
        
        final ConcurrentHashMap<K, V> map;
        
        private static final String OOME_MSG = "Required array size too large";
        
        CollectionView(ConcurrentHashMap<K, V> map) {
            this.map = map;
        }
        
        /**
         * Returns an iterator over the elements in this collection.
         *
         * <p>The returned iterator is
         * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
         *
         * @return an iterator over the elements in this collection
         */
        public abstract Iterator<E> iterator();
        
        public abstract boolean contains(Object o);
        
        public abstract boolean remove(Object o);
        
        // implementations below rely on concrete classes supplying these abstract methods
        
        /**
         * Removes all of the elements from this view, by removing all
         * the mappings from the map backing this view.
         */
        public final void clear() {
            map.clear();
        }
        
        public final int size() {
            return map.size();
        }
        
        public final boolean isEmpty() {
            return map.isEmpty();
        }
        
        public final Object[] toArray() {
            long sz = map.mappingCount();
            if(sz>MAX_ARRAY_SIZE)
                throw new OutOfMemoryError(OOME_MSG);
            int n = (int) sz;
            Object[] r = new Object[n];
            int i = 0;
            for(E e : this) {
                if(i == n) {
                    if(n >= MAX_ARRAY_SIZE)
                        throw new OutOfMemoryError(OOME_MSG);
                    if(n >= MAX_ARRAY_SIZE - (MAX_ARRAY_SIZE >>> 1) - 1)
                        n = MAX_ARRAY_SIZE;
                    else
                        n += (n >>> 1) + 1;
                    r = Arrays.copyOf(r, n);
                }
                r[i++] = e;
            }
            return (i == n) ? r : Arrays.copyOf(r, i);
        }
        
        @SuppressWarnings("unchecked")
        public final <T> T[] toArray(T[] a) {
            long sz = map.mappingCount();
            if(sz>MAX_ARRAY_SIZE)
                throw new OutOfMemoryError(OOME_MSG);
            int m = (int) sz;
            T[] r = (a.length >= m) ? a : (T[]) java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), m);
            int n = r.length;
            int i = 0;
            for(E e : this) {
                if(i == n) {
                    if(n >= MAX_ARRAY_SIZE)
                        throw new OutOfMemoryError(OOME_MSG);
                    if(n >= MAX_ARRAY_SIZE - (MAX_ARRAY_SIZE >>> 1) - 1)
                        n = MAX_ARRAY_SIZE;
                    else
                        n += (n >>> 1) + 1;
                    r = Arrays.copyOf(r, n);
                }
                r[i++] = (T) e;
            }
            if(a == r && i<n) {
                r[i] = null; // null-terminate
                return r;
            }
            return (i == n) ? r : Arrays.copyOf(r, i);
        }
        
        /**
         * Returns a string representation of this collection.
         * The string representation consists of the string representations
         * of the collection's elements in the order they are returned by
         * its iterator, enclosed in square brackets ({@code "[]"}).
         * Adjacent elements are separated by the characters {@code ", "}
         * (comma and space).  Elements are converted to strings as by
         * {@link String#valueOf(Object)}.
         *
         * @return a string representation of this collection
         */
        public final String toString() {
            StringBuilder sb = new StringBuilder();
            sb.append('[');
            Iterator<E> it = iterator();
            if(it.hasNext()) {
                for(; ; ) {
                    Object e = it.next();
                    sb.append(e == this ? "(this Collection)" : e);
                    if(!it.hasNext())
                        break;
                    sb.append(',').append(' ');
                }
            }
            return sb.append(']').toString();
        }
        
        public final boolean containsAll(Collection<?> c) {
            if(c != this) {
                for(Object e : c) {
                    if(e == null || !contains(e))
                        return false;
                }
            }
            return true;
        }
        
        public final boolean retainAll(Collection<?> c) {
            if(c == null)
                throw new NullPointerException();
            boolean modified = false;
            for(Iterator<E> it = iterator(); it.hasNext(); ) {
                if(!c.contains(it.next())) {
                    it.remove();
                    modified = true;
                }
            }
            return modified;
        }
        
        /**
         * Returns the map backing this view.
         *
         * @return the map backing this view
         */
        public ConcurrentHashMap<K, V> getMap() {
            return map;
        }
        
        public boolean removeAll(Collection<?> c) {
            if(c == null)
                throw new NullPointerException();
            boolean modified = false;
            // Use (c instanceof Set) as a hint that lookup in c is as
            // efficient as this view
            Node<K, V>[] t;
            if((t = map.table) == null) {
                return false;
            } else if(c instanceof Set<?> && c.size()>t.length) {
                for(Iterator<?> it = iterator(); it.hasNext(); ) {
                    if(c.contains(it.next())) {
                        it.remove();
                        modified = true;
                    }
                }
            } else {
                for(Object e : c)
                    modified |= remove(e);
            }
            return modified;
        }
        
    }
    
    /**
     * A view of a ConcurrentHashMap as a {@link Set} of keys, in
     * which additions may optionally be enabled by mapping to a
     * common value.  This class cannot be directly instantiated.
     * See {@link #keySet() keySet()},
     * {@link #keySet(Object) keySet(V)},
     * {@link #newKeySet() newKeySet()},
     * {@link #newKeySet(int) newKeySet(int)}.
     *
     * @since 1.8
     */
    public static class KeySetView<K,V> extends CollectionView<K,V,K> implements Set<K>, Serializable {
        
        private static final long serialVersionUID = 7249069246763182397L;
        
        private final V value;
        
        KeySetView(ConcurrentHashMap<K, V> map, V value) {  // non-public
            super(map);
            this.value = value;
        }
        
        /**
         * Returns the default mapped value for additions,
         * or {@code null} if additions are not supported.
         *
         * @return the default mapped value for additions, or {@code null}
         * if not supported
         */
        public V getMappedValue() {
            return value;
        }
        
        /**
         * {@inheritDoc}
         *
         * @throws NullPointerException if the specified key is null
         */
        public boolean contains(Object o) {
            return map.containsKey(o);
        }
        
        /**
         * Removes the key from this map view, by removing the key (and its
         * corresponding value) from the backing map.  This method does
         * nothing if the key is not in the map.
         *
         * @param o the key to be removed from the backing map
         *
         * @return {@code true} if the backing map contained the specified key
         *
         * @throws NullPointerException if the specified key is null
         */
        public boolean remove(Object o) {
            return map.remove(o) != null;
        }
        
        /**
         * @return an iterator over the keys of the backing map
         */
        public Iterator<K> iterator() {
            Node<K, V>[] t;
            ConcurrentHashMap<K, V> m = map;
            int f = (t = m.table) == null ? 0 : t.length;
            return new KeyIterator<K, V>(t, f, 0, f, m);
        }
        
        /**
         * Adds the specified key to this set view by mapping the key to
         * the default mapped value in the backing map, if defined.
         *
         * @param e key to be added
         *
         * @return {@code true} if this set changed as a result of the call
         *
         * @throws NullPointerException          if the specified key is null
         * @throws UnsupportedOperationException if no default mapped value
         *                                       for additions was provided
         */
        public boolean add(K e) {
            V v = value;
            if(v == null) {
                throw new UnsupportedOperationException();
            }
            
            return map.putVal(e, v, true) == null;
        }
        
        /**
         * Adds all of the elements in the specified collection to this set,
         * as if by calling {@link #add} on each one.
         *
         * @param c the elements to be inserted into this set
         *
         * @return {@code true} if this set changed as a result of the call
         *
         * @throws NullPointerException          if the collection or any of its
         *                                       elements are {@code null}
         * @throws UnsupportedOperationException if no default mapped value
         *                                       for additions was provided
         */
        public boolean addAll(Collection<? extends K> c) {
            boolean added = false;
            V v = value;
            
            if(v == null) {
                throw new UnsupportedOperationException();
            }
            
            for(K e : c) {
                if(map.putVal(e, v, true) == null) {
                    added = true;
                }
            }
            
            return added;
        }
        
        public int hashCode() {
            int h = 0;
            for(K e : this) {
                h += e.hashCode();
            }
            return h;
        }
        
        public boolean equals(Object o) {
            Set<?> c;
            return (o instanceof Set) && ((c = (Set<?>) o) == this || (containsAll(c) && c.containsAll(this)));
        }
        
        public Spliterator<K> spliterator() {
            Node<K, V>[] t;
            ConcurrentHashMap<K, V> m = map;
            long n = m.sumCount();
            int f = (t = m.table) == null ? 0 : t.length;
            return new KeySpliterator<K, V>(t, f, 0, f, n<0L ? 0L : n);
        }
        
        public void forEach(Consumer<? super K> action) {
            if(action == null) {
                throw new NullPointerException();
            }
            
            Node<K, V>[] t;
            if((t = map.table) != null) {
                Traverser<K, V> it = new Traverser<>(t, t.length, 0, t.length);
                for(Node<K, V> p; (p = it.advance()) != null; ) {
                    action.accept(p.key);
                }
            }
        }
    }
    
    /**
     * A view of a ConcurrentHashMap as a {@link Collection} of
     * values, in which additions are disabled. This class cannot be
     * directly instantiated. See {@link #values()}.
     */
    static final class ValuesView<K,V> extends CollectionView<K,V,V> implements Collection<V>, Serializable {
        
        private static final long serialVersionUID = 2249069246763182397L;
        
        ValuesView(ConcurrentHashMap<K, V> map) {
            super(map);
        }
        
        public final boolean contains(Object o) {
            return map.containsValue(o);
        }
        
        public final boolean remove(Object o) {
            if(o != null) {
                for(Iterator<V> it = iterator(); it.hasNext(); ) {
                    if(o.equals(it.next())) {
                        it.remove();
                        return true;
                    }
                }
            }
            return false;
        }
        
        public final Iterator<V> iterator() {
            ConcurrentHashMap<K, V> m = map;
            Node<K, V>[] t;
            int f = (t = m.table) == null ? 0 : t.length;
            return new ValueIterator<K, V>(t, f, 0, f, m);
        }
        
        public final boolean add(V e) {
            throw new UnsupportedOperationException();
        }
        
        public final boolean addAll(Collection<? extends V> c) {
            throw new UnsupportedOperationException();
        }
        
        @Override
        public boolean removeAll(Collection<?> c) {
            if(c == null) {
                throw new NullPointerException();
            }
            
            boolean modified = false;
            for(Iterator<V> it = iterator(); it.hasNext(); ) {
                if(c.contains(it.next())) {
                    it.remove();
                    modified = true;
                }
            }
            
            return modified;
        }
        
        public boolean removeIf(Predicate<? super V> filter) {
            return map.removeValueIf(filter);
        }
        
        public Spliterator<V> spliterator() {
            Node<K, V>[] t;
            ConcurrentHashMap<K, V> m = map;
            long n = m.sumCount();
            int f = (t = m.table) == null ? 0 : t.length;
            return new ValueSpliterator<K, V>(t, f, 0, f, n<0L ? 0L : n);
        }
        
        public void forEach(Consumer<? super V> action) {
            if(action == null) {
                throw new NullPointerException();
            }
            
            Node<K, V>[] t;
            if((t = map.table) != null) {
                Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
                for(Node<K, V> p; (p = it.advance()) != null; ) {
                    action.accept(p.val);
                }
            }
        }
    }
    
    /**
     * A view of a ConcurrentHashMap as a {@link Set} of (key, value)
     * entries.  This class cannot be directly instantiated. See
     * {@link #entrySet()}.
     */
    static final class EntrySetView<K, V> extends CollectionView<K, V, Map.Entry<K, V>> implements Set<Map.Entry<K, V>>, java.io.Serializable {
        private static final long serialVersionUID = 2249069246763182397L;
        
        EntrySetView(ConcurrentHashMap<K, V> map) {
            super(map);
        }
        
        public final int hashCode() {
            int h = 0;
            Node<K, V>[] t;
            if((t = map.table) != null) {
                Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
                for(Node<K, V> p; (p = it.advance()) != null; ) {
                    h += p.hashCode();
                }
            }
            return h;
        }
        
        public final boolean equals(Object o) {
            Set<?> c;
            return ((o instanceof Set) && ((c = (Set<?>) o) == this || (containsAll(c) && c.containsAll(this))));
        }
        
        public boolean contains(Object o) {
            Object k, v, r;
            Map.Entry<?, ?> e;
            return ((o instanceof Map.Entry)
                && (k = (e = (Map.Entry<?, ?>) o).getKey()) != null
                && (r = map.get(k)) != null
                && (v = e.getValue()) != null
                && (v == r || v.equals(r)));
        }
        
        public boolean remove(Object o) {
            Object k, v;
            Map.Entry<?, ?> e;
            return ((o instanceof Map.Entry)
                && (k = (e = (Map.Entry<?, ?>) o).getKey()) != null
                && (v = e.getValue()) != null
                && map.remove(k, v));
        }
        
        /**
         * @return an iterator over the entries of the backing map
         */
        public Iterator<Map.Entry<K, V>> iterator() {
            ConcurrentHashMap<K, V> m = map;
            Node<K, V>[] t;
            int f = (t = m.table) == null ? 0 : t.length;
            return new EntryIterator<K, V>(t, f, 0, f, m);
        }
        
        public boolean add(Entry<K, V> e) {
            return map.putVal(e.getKey(), e.getValue(), false) == null;
        }
        
        public boolean addAll(Collection<? extends Entry<K, V>> c) {
            boolean added = false;
            for(Entry<K, V> e : c) {
                if(add(e))
                    added = true;
            }
            return added;
        }
        
        public boolean removeIf(Predicate<? super Entry<K, V>> filter) {
            return map.removeEntryIf(filter);
        }
        
        public Spliterator<Map.Entry<K, V>> spliterator() {
            Node<K, V>[] t;
            ConcurrentHashMap<K, V> m = map;
            long n = m.sumCount();
            int f = (t = m.table) == null ? 0 : t.length;
            return new EntrySpliterator<K, V>(t, f, 0, f, n<0L ? 0L : n, m);
        }
        
        public void forEach(Consumer<? super Map.Entry<K, V>> action) {
            if(action == null) {
                throw new NullPointerException();
            }
            
            Node<K, V>[] t;
            
            if((t = map.table) != null) {
                Traverser<K, V> it = new Traverser<K, V>(t, t.length, 0, t.length);
                for(Node<K, V> p; (p = it.advance()) != null; ) {
                    action.accept(new MapEntry<K, V>(p.key, p.val, map));
                }
            }
        }
    }
    
    
    
    /**
     * Exported Entry for EntryIterator.
     */
    static final class MapEntry<K, V> implements Map.Entry<K, V> {
        final K key; // non-null
        final ConcurrentHashMap<K, V> map;
        V val;       // non-null
        
        MapEntry(K key, V val, ConcurrentHashMap<K, V> map) {
            this.key = key;
            this.val = val;
            this.map = map;
        }
        
        public K getKey() {
            return key;
        }
        
        public V getValue() {
            return val;
        }
        
        public int hashCode() {
            return key.hashCode() ^ val.hashCode();
        }
        
        public String toString() {
            return Helpers.mapEntryToString(key, val);
        }
        
        public boolean equals(Object o) {
            Object k, v;
            Map.Entry<?, ?> e;
            return ((o instanceof Map.Entry) && (k = (e = (Map.Entry<?, ?>) o).getKey()) != null && (v = e.getValue()) != null && (k == key || k.equals(key)) && (v == val || v.equals(val)));
        }
        
        /**
         * Sets our entry's value and writes through to the map. The
         * value to return is somewhat arbitrary here. Since we do not
         * necessarily track asynchronous changes, the most recent
         * "previous" value could be different from what we return (or
         * could even have been removed, in which case the put will
         * re-establish). We do not and cannot guarantee more.
         */
        public V setValue(V value) {
            if(value == null) {
                throw new NullPointerException();
            }
            V v = val;
            val = value;
            map.put(key, value);
            return v;
        }
    }
    
    /**
     * Base of key, value, and entry Iterators. Adds fields to
     * Traverser to support iterator.remove.
     */
    static class BaseIterator<K, V> extends Traverser<K, V> {
        final ConcurrentHashMap<K, V> map;
        Node<K, V> lastReturned;
        
        BaseIterator(Node<K, V>[] tab, int size, int index, int limit, ConcurrentHashMap<K, V> map) {
            super(tab, size, index, limit);
            this.map = map;
            advance();
        }
        
        public final boolean hasNext() {
            return next != null;
        }
        
        public final boolean hasMoreElements() {
            return next != null;
        }
        
        public final void remove() {
            Node<K, V> p;
            if((p = lastReturned) == null) {
                throw new IllegalStateException();
            }
            lastReturned = null;
            map.replaceNode(p.key, null, null);
        }
    }
    
    static final class KeyIterator<K, V> extends BaseIterator<K, V> implements Iterator<K>, Enumeration<K> {
        KeyIterator(Node<K, V>[] tab, int size, int index, int limit, ConcurrentHashMap<K, V> map) {
            super(tab, size, index, limit, map);
        }
        
        public final K next() {
            Node<K, V> p;
            if((p = next) == null) {
                throw new NoSuchElementException();
            }
            K k = p.key;
            lastReturned = p;
            advance();
            return k;
        }
        
        public final K nextElement() {
            return next();
        }
    }
    
    static final class ValueIterator<K, V> extends BaseIterator<K, V> implements Iterator<V>, Enumeration<V> {
        ValueIterator(Node<K, V>[] tab, int size, int index, int limit, ConcurrentHashMap<K, V> map) {
            super(tab, size, index, limit, map);
        }
        
        public final V next() {
            Node<K, V> p;
            if((p = next) == null) {
                throw new NoSuchElementException();
            }
            V v = p.val;
            lastReturned = p;
            advance();
            return v;
        }
        
        public final V nextElement() {
            return next();
        }
    }
    
    static final class EntryIterator<K, V> extends BaseIterator<K, V> implements Iterator<Map.Entry<K, V>> {
        EntryIterator(Node<K, V>[] tab, int size, int index, int limit, ConcurrentHashMap<K, V> map) {
            super(tab, size, index, limit, map);
        }
        
        public final Map.Entry<K, V> next() {
            Node<K, V> p;
            if((p = next) == null) {
                throw new NoSuchElementException();
            }
            K k = p.key;
            V v = p.val;
            lastReturned = p;
            advance();
            return new MapEntry<K, V>(k, v, map);
        }
    }
    
    
    
    /**
     * Records the table, its length, and current traversal index for a
     * traverser that must process a region of a forwarded table before
     * proceeding with current table.
     */
    static final class TableStack<K, V> {
        int length;
        int index;
        Node<K, V>[] tab;
        TableStack<K, V> next;
    }
    
    /**
     * Encapsulates traversal for methods such as containsValue; also
     * serves as a base class for other iterators and spliterators.
     *
     * Method advance visits once each still-valid node that was
     * reachable upon iterator construction. It might miss some that
     * were added to a bin after the bin was visited, which is OK wrt
     * consistency guarantees. Maintaining this property in the face
     * of possible ongoing resizes requires a fair amount of
     * bookkeeping state that is difficult to optimize away amidst
     * volatile accesses.  Even so, traversal maintains reasonable
     * throughput.
     *
     * Normally, iteration proceeds bin-by-bin traversing lists.
     * However, if the table has been resized, then all future steps
     * must traverse both the bin at the current index as well as at
     * (index + baseSize); and so on for further resizings. To
     * paranoically cope with potential sharing by users of iterators
     * across threads, iteration terminates if a bounds checks fails
     * for a table read.
     */
    static class Traverser<K, V> {
        final int baseSize;     // initial table size
        Node<K, V>[] tab;        // current table; updated if resized
        Node<K, V> next;         // the next entry to use
        TableStack<K, V> stack, spare; // to save/restore on ForwardingNodes
        int index;              // index of bin to use next
        int baseIndex;          // current index of initial table
        int baseLimit;          // index bound for initial table
        
        Traverser(Node<K, V>[] tab, int size, int index, int limit) {
            this.tab = tab;
            this.baseSize = size;
            this.baseIndex = this.index = index;
            this.baseLimit = limit;
            this.next = null;
        }
        
        /**
         * Advances if possible, returning next valid node, or null if none.
         */
        final Node<K, V> advance() {
            Node<K, V> e;
            
            if((e = next) != null) {
                e = e.next;
            }
            
            for(; ; ) {
                Node<K, V>[] t;
                int i, n;  // must use locals in checks
                
                if(e != null) {
                    return next = e;
                }
                
                if(baseIndex >= baseLimit || (t = tab) == null || (n = t.length)<=(i = index) || i<0) {
                    return next = null;
                }
                
                if((e = tabAt(t, i)) != null && e.hash<0) {
                    if(e instanceof ForwardingNode) {
                        tab = ((ForwardingNode<K, V>) e).nextTable;
                        e = null;
                        pushState(t, i, n);
                        continue;
                    } else if(e instanceof TreeBin) {
                        e = ((TreeBin<K, V>) e).first;
                    } else {
                        e = null;
                    }
                }
                
                if(stack != null) {
                    recoverState(n);
                } else if((index = i + baseSize) >= n) {
                    index = ++baseIndex; // visit upper slots if present
                }
            }
        }
        
        /**
         * Saves traversal state upon encountering a forwarding node.
         */
        private void pushState(Node<K, V>[] t, int i, int n) {
            TableStack<K, V> s = spare;  // reuse if possible
            if(s != null) {
                spare = s.next;
            } else {
                s = new TableStack<K, V>();
            }
            s.tab = t;
            s.length = n;
            s.index = i;
            s.next = stack;
            stack = s;
        }
        
        /**
         * Possibly pops traversal state.
         *
         * @param n length of current table
         */
        private void recoverState(int n) {
            TableStack<K, V> s;
            int len;
            
            while((s = stack) != null && (index += (len = s.length)) >= n) {
                n = len;
                index = s.index;
                tab = s.tab;
                s.tab = null;
                TableStack<K, V> next = s.next;
                s.next = spare; // save for reuse
                stack = next;
                spare = s;
            }
            
            if(s == null && (index += baseSize) >= n) {
                index = ++baseIndex;
            }
        }
    }
    
    static final class KeySpliterator<K, V> extends Traverser<K, V> implements Spliterator<K> {
        long est;               // size estimate
        
        KeySpliterator(Node<K, V>[] tab, int size, int index, int limit, long est) {
            super(tab, size, index, limit);
            this.est = est;
        }
        
        public KeySpliterator<K, V> trySplit() {
            int i, f, h;
            return (h = ((i = baseIndex) + (f = baseLimit)) >>> 1)<=i ? null : new KeySpliterator<K, V>(tab, baseSize, baseLimit = h, f, est >>>= 1);
        }
        
        public void forEachRemaining(Consumer<? super K> action) {
            if(action == null) {
                throw new NullPointerException();
            }
            
            for(Node<K, V> p; (p = advance()) != null; ) {
                action.accept(p.key);
            }
        }
        
        public boolean tryAdvance(Consumer<? super K> action) {
            if(action == null) {
                throw new NullPointerException();
            }
            
            Node<K, V> p;
            if((p = advance()) == null) {
                return false;
            }
            
            action.accept(p.key);
            
            return true;
        }
        
        public long estimateSize() {
            return est;
        }
        
        public int characteristics() {
            return Spliterator.DISTINCT | Spliterator.CONCURRENT | Spliterator.NONNULL;
        }
    }
    
    static final class ValueSpliterator<K, V> extends Traverser<K, V> implements Spliterator<V> {
        long est;               // size estimate
        
        ValueSpliterator(Node<K, V>[] tab, int size, int index, int limit, long est) {
            super(tab, size, index, limit);
            this.est = est;
        }
        
        public ValueSpliterator<K, V> trySplit() {
            int i, f, h;
            return (h = ((i = baseIndex) + (f = baseLimit)) >>> 1)<=i ? null : new ValueSpliterator<K, V>(tab, baseSize, baseLimit = h, f, est >>>= 1);
        }
        
        public void forEachRemaining(Consumer<? super V> action) {
            if(action == null) {
                throw new NullPointerException();
            }
            
            for(Node<K, V> p; (p = advance()) != null; ) {
                action.accept(p.val);
            }
        }
        
        public boolean tryAdvance(Consumer<? super V> action) {
            if(action == null) {
                throw new NullPointerException();
            }
            
            Node<K, V> p;
            if((p = advance()) == null) {
                return false;
            }
            
            action.accept(p.val);
            
            return true;
        }
        
        public long estimateSize() {
            return est;
        }
        
        public int characteristics() {
            return Spliterator.CONCURRENT | Spliterator.NONNULL;
        }
    }
    
    static final class EntrySpliterator<K, V> extends Traverser<K, V> implements Spliterator<Map.Entry<K, V>> {
        final ConcurrentHashMap<K, V> map; // To export MapEntry
        long est;               // size estimate
        
        EntrySpliterator(Node<K, V>[] tab, int size, int index, int limit, long est, ConcurrentHashMap<K, V> map) {
            super(tab, size, index, limit);
            this.map = map;
            this.est = est;
        }
        
        public EntrySpliterator<K, V> trySplit() {
            int i, f, h;
            return (h = ((i = baseIndex) + (f = baseLimit)) >>> 1)<=i ? null : new EntrySpliterator<K, V>(tab, baseSize, baseLimit = h, f, est >>>= 1, map);
        }
        
        public void forEachRemaining(Consumer<? super Map.Entry<K, V>> action) {
            if(action == null)
                throw new NullPointerException();
            for(Node<K, V> p; (p = advance()) != null; )
                action.accept(new MapEntry<K, V>(p.key, p.val, map));
        }
        
        public boolean tryAdvance(Consumer<? super Map.Entry<K, V>> action) {
            if(action == null)
                throw new NullPointerException();
            Node<K, V> p;
            if((p = advance()) == null)
                return false;
            action.accept(new MapEntry<K, V>(p.key, p.val, map));
            return true;
        }
        
        public long estimateSize() {
            return est;
        }
        
        public int characteristics() {
            return Spliterator.DISTINCT | Spliterator.CONCURRENT | Spliterator.NONNULL;
        }
    }
    
    
    
    /**
     * Base class for bulk tasks. Repeats some fields and code from
     * class Traverser, because we need to subclass CountedCompleter.
     */
    @SuppressWarnings("serial")
    abstract static class BulkTask<K,V,R> extends CountedCompleter<R> {
        Node<K,V>[] tab;        // same as Traverser
        Node<K,V> next;
        TableStack<K,V> stack, spare;
        int index;
        int baseIndex;
        int baseLimit;
        final int baseSize;
        int batch;              // split control
        
        BulkTask(BulkTask<K,V,?> par, int b, int i, int f, Node<K,V>[] t) {
            super(par);
            this.batch = b;
            this.index = this.baseIndex = i;
            if ((this.tab = t) == null) {
                this.baseSize = this.baseLimit = 0;
            } else if (par == null) {
                this.baseSize = this.baseLimit = t.length;
            } else {
                this.baseLimit = f;
                this.baseSize = par.baseSize;
            }
        }
        
        /**
         * Same as Traverser version.
         */
        final Node<K,V> advance() {
            Node<K,V> e;
            
            if ((e = next) != null) {
                e = e.next;
            }
            
            for (;;) {
                Node<K,V>[] t; int i, n;
                if (e != null) {
                    return next = e;
                }
                
                if (baseIndex >= baseLimit || (t = tab) == null ||
                    (n = t.length) <= (i = index) || i < 0) {
                    return next = null;
                }
                
                if ((e = tabAt(t, i)) != null && e.hash < 0) {
                    if (e instanceof ForwardingNode) {
                        tab = ((ForwardingNode<K,V>)e).nextTable;
                        e = null;
                        pushState(t, i, n);
                        continue;
                    } else if (e instanceof TreeBin) {
                        e = ((TreeBin<K,V>)e).first;
                    } else {
                        e = null;
                    }
                }
                
                if (stack != null) {
                    recoverState(n);
                } else if ((index = i + baseSize) >= n) {
                    index = ++baseIndex;
                }
            }
        }
        
        private void pushState(Node<K,V>[] t, int i, int n) {
            TableStack<K,V> s = spare;
            if (s != null) {
                spare = s.next;
            } else {
                s = new TableStack<K,V>();
            }
            s.tab = t;
            s.length = n;
            s.index = i;
            s.next = stack;
            stack = s;
        }
        
        private void recoverState(int n) {
            TableStack<K,V> s; int len;
            while ((s = stack) != null && (index += (len = s.length)) >= n) {
                n = len;
                index = s.index;
                tab = s.tab;
                s.tab = null;
                TableStack<K,V> next = s.next;
                s.next = spare; // save for reuse
                stack = next;
                spare = s;
            }
            
            if (s == null && (index += baseSize) >= n) {
                index = ++baseIndex;
            }
        }
    }
    
    
    
    /*
     * Task classes. Coded in a regular but ugly format/style to
     * simplify checks that each variant differs in the right way from
     * others. The null screenings exist because compilers cannot tell
     * that we've already null-checked task arguments, so we force
     * simplest hoisted bypass to help avoid convoluted traps.
     */
    @SuppressWarnings("serial")
    static final class ForEachKeyTask<K, V>     extends BulkTask<K, V, Void> {
        final Consumer<? super K> action;
        
        ForEachKeyTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, Consumer<? super K> action) {
            super(p, b, i, f, t);
            this.action = action;
        }
        
        public final void compute() {
            final Consumer<? super K> action;
            
            if((action = this.action) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    new ForEachKeyTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, action).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    action.accept(p.key);
                }
                
                propagateCompletion();
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class ForEachValueTask<K, V>   extends BulkTask<K, V, Void> {
        final Consumer<? super V> action;
        
        ForEachValueTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, Consumer<? super V> action) {
            super(p, b, i, f, t);
            this.action = action;
        }
        
        public final void compute() {
            final Consumer<? super V> action;
            
            if((action = this.action) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    new ForEachValueTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, action).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    action.accept(p.val);
                }
                
                propagateCompletion();
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class ForEachEntryTask<K, V>   extends BulkTask<K, V, Void> {
        final Consumer<? super Entry<K, V>> action;
        
        ForEachEntryTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, Consumer<? super Entry<K, V>> action) {
            super(p, b, i, f, t);
            this.action = action;
        }
        
        public final void compute() {
            final Consumer<? super Entry<K, V>> action;
            
            if((action = this.action) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    new ForEachEntryTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, action).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    action.accept(p);
                }
                
                propagateCompletion();
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class ForEachMappingTask<K, V> extends BulkTask<K, V, Void> {
        final BiConsumer<? super K, ? super V> action;
        
        ForEachMappingTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, BiConsumer<? super K, ? super V> action) {
            super(p, b, i, f, t);
            this.action = action;
        }
        
        public final void compute() {
            final BiConsumer<? super K, ? super V> action;
            
            if((action = this.action) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    new ForEachMappingTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, action).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    action.accept(p.key, p.val);
                }
                
                propagateCompletion();
            }
        }
    }
    
    
    
    @SuppressWarnings("serial")
    static final class ForEachTransformedKeyTask<K, V, U>     extends BulkTask<K, V, Void> {
        final Function<? super K, ? extends U> transformer;
        final Consumer<? super U> action;
        
        ForEachTransformedKeyTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, Function<? super K, ? extends U> transformer, Consumer<? super U> action) {
            super(p, b, i, f, t);
            this.transformer = transformer;
            this.action = action;
        }
        
        public final void compute() {
            final Function<? super K, ? extends U> transformer;
            final Consumer<? super U> action;
            
            if((transformer = this.transformer) != null && (action = this.action) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    new ForEachTransformedKeyTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, transformer, action).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    U u = transformer.apply(p.key);
                    if(u != null) {
                        action.accept(u);
                    }
                }
                
                propagateCompletion();
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class ForEachTransformedValueTask<K, V, U>   extends BulkTask<K, V, Void> {
        final Function<? super V, ? extends U> transformer;
        final Consumer<? super U> action;
        
        ForEachTransformedValueTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, Function<? super V, ? extends U> transformer, Consumer<? super U> action) {
            super(p, b, i, f, t);
            this.transformer = transformer;
            this.action = action;
        }
        
        public final void compute() {
            final Function<? super V, ? extends U> transformer;
            final Consumer<? super U> action;
            
            if((transformer = this.transformer) != null && (action = this.action) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    new ForEachTransformedValueTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, transformer, action).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    U u = transformer.apply(p.val);
                    if(u != null) {
                        action.accept(u);
                    }
                }
                
                propagateCompletion();
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class ForEachTransformedEntryTask<K, V, U>   extends BulkTask<K, V, Void> {
        final Function<Map.Entry<K, V>, ? extends U> transformer;
        final Consumer<? super U> action;
        
        ForEachTransformedEntryTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, Function<Map.Entry<K, V>, ? extends U> transformer, Consumer<? super U> action) {
            super(p, b, i, f, t);
            this.transformer = transformer;
            this.action = action;
        }
        
        public final void compute() {
            final Function<Map.Entry<K, V>, ? extends U> transformer;
            final Consumer<? super U> action;
            
            if((transformer = this.transformer) != null && (action = this.action) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    new ForEachTransformedEntryTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, transformer, action).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    U u = transformer.apply(p);
                    if(u != null) {
                        action.accept(u);
                    }
                }
                
                propagateCompletion();
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class ForEachTransformedMappingTask<K, V, U> extends BulkTask<K, V, Void> {
        final BiFunction<? super K, ? super V, ? extends U> transformer;
        final Consumer<? super U> action;
        
        ForEachTransformedMappingTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, BiFunction<? super K, ? super V, ? extends U> transformer, Consumer<? super U> action) {
            super(p, b, i, f, t);
            this.transformer = transformer;
            this.action = action;
        }
        
        public final void compute() {
            final BiFunction<? super K, ? super V, ? extends U> transformer;
            final Consumer<? super U> action;
            if((transformer = this.transformer) != null && (action = this.action) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    new ForEachTransformedMappingTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, transformer, action).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    U u = transformer.apply(p.key, p.val);
                    if(u != null) {
                        action.accept(u);
                    }
                }
                
                propagateCompletion();
            }
        }
    }
    
    
    
    @SuppressWarnings("serial")
    static final class SearchKeysTask<K, V, U>     extends BulkTask<K, V, U> {
        final Function<? super K, ? extends U> searchFunction;
        final AtomicReference<U> result;
        
        SearchKeysTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, Function<? super K, ? extends U> searchFunction, AtomicReference<U> result) {
            super(p, b, i, f, t);
            this.searchFunction = searchFunction;
            this.result = result;
        }
        
        public final U getRawResult() {
            return result.get();
        }
        
        public final void compute() {
            final Function<? super K, ? extends U> searchFunction;
            final AtomicReference<U> result;
            
            if((searchFunction = this.searchFunction) != null && (result = this.result) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    if(result.get() != null) {
                        return;
                    }
                    addToPendingCount(1);
                    new SearchKeysTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, searchFunction, result).fork();
                }
                
                while(result.get() == null) {
                    U u;
                    Node<K, V> p;
                    
                    if((p = advance()) == null) {
                        propagateCompletion();
                        break;
                    }
                    
                    if((u = searchFunction.apply(p.key)) != null) {
                        if(result.compareAndSet(null, u)) {
                            quietlyCompleteRoot();
                        }
                        break;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class SearchValuesTask<K, V, U>   extends BulkTask<K, V, U> {
        final Function<? super V, ? extends U> searchFunction;
        final AtomicReference<U> result;
        
        SearchValuesTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, Function<? super V, ? extends U> searchFunction, AtomicReference<U> result) {
            super(p, b, i, f, t);
            this.searchFunction = searchFunction;
            this.result = result;
        }
        
        public final U getRawResult() {
            return result.get();
        }
        
        public final void compute() {
            final Function<? super V, ? extends U> searchFunction;
            final AtomicReference<U> result;
            
            if((searchFunction = this.searchFunction) != null && (result = this.result) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    if(result.get() != null) {
                        return;
                    }
                    addToPendingCount(1);
                    new SearchValuesTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, searchFunction, result).fork();
                }
                
                while(result.get() == null) {
                    U u;
                    Node<K, V> p;
                    
                    if((p = advance()) == null) {
                        propagateCompletion();
                        break;
                    }
                    
                    if((u = searchFunction.apply(p.val)) != null) {
                        if(result.compareAndSet(null, u)) {
                            quietlyCompleteRoot();
                        }
                        break;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class SearchEntriesTask<K, V, U>  extends BulkTask<K, V, U> {
        final Function<Entry<K, V>, ? extends U> searchFunction;
        final AtomicReference<U> result;
        
        SearchEntriesTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, Function<Entry<K, V>, ? extends U> searchFunction, AtomicReference<U> result) {
            super(p, b, i, f, t);
            this.searchFunction = searchFunction;
            this.result = result;
        }
        
        public final U getRawResult() {
            return result.get();
        }
        
        public final void compute() {
            final Function<Entry<K, V>, ? extends U> searchFunction;
            final AtomicReference<U> result;
            
            if((searchFunction = this.searchFunction) != null && (result = this.result) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    if(result.get() != null) {
                        return;
                    }
                    addToPendingCount(1);
                    new SearchEntriesTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, searchFunction, result).fork();
                }
                
                while(result.get() == null) {
                    U u;
                    Node<K, V> p;
                    
                    if((p = advance()) == null) {
                        propagateCompletion();
                        break;
                    }
                    
                    if((u = searchFunction.apply(p)) != null) {
                        if(result.compareAndSet(null, u)) {
                            quietlyCompleteRoot();
                        }
                        return;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class SearchMappingsTask<K, V, U> extends BulkTask<K, V, U> {
        final BiFunction<? super K, ? super V, ? extends U> searchFunction;
        final AtomicReference<U> result;
        
        SearchMappingsTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, BiFunction<? super K, ? super V, ? extends U> searchFunction, AtomicReference<U> result) {
            super(p, b, i, f, t);
            this.searchFunction = searchFunction;
            this.result = result;
        }
        
        public final U getRawResult() {
            return result.get();
        }
        
        public final void compute() {
            final BiFunction<? super K, ? super V, ? extends U> searchFunction;
            final AtomicReference<U> result;
            
            if((searchFunction = this.searchFunction) != null && (result = this.result) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    if(result.get() != null) {
                        return;
                    }
                    addToPendingCount(1);
                    new SearchMappingsTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, searchFunction, result).fork();
                }
                
                while(result.get() == null) {
                    U u;
                    Node<K, V> p;
                    
                    if((p = advance()) == null) {
                        propagateCompletion();
                        break;
                    }
                    
                    if((u = searchFunction.apply(p.key, p.val)) != null) {
                        if(result.compareAndSet(null, u)) {
                            quietlyCompleteRoot();
                        }
                        break;
                    }
                }
            }
        }
    }
    
    
    
    @SuppressWarnings("serial")
    static final class ReduceKeysTask<K, V>    extends BulkTask<K, V, K> {
        final BiFunction<? super K, ? super K, ? extends K> reducer;
        K result;
        ReduceKeysTask<K, V> rights, nextRight;
        
        ReduceKeysTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, ReduceKeysTask<K, V> nextRight, BiFunction<? super K, ? super K, ? extends K> reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.reducer = reducer;
        }
        
        public final K getRawResult() {
            return result;
        }
        
        public final void compute() {
            final BiFunction<? super K, ? super K, ? extends K> reducer;
            if((reducer = this.reducer) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new ReduceKeysTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, reducer)).fork();
                }
                
                K r = null;
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    K u = p.key;
                    r = (r == null) ? u : u == null ? r : reducer.apply(r, u);
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    ReduceKeysTask<K, V> t = (ReduceKeysTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        K tr, sr;
                        if((sr = s.result) != null) {
                            t.result = (((tr = t.result) == null) ? sr : reducer.apply(tr, sr));
                        }
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class ReduceValuesTask<K, V>  extends BulkTask<K, V, V> {
        final BiFunction<? super V, ? super V, ? extends V> reducer;
        V result;
        ReduceValuesTask<K, V> rights, nextRight;
        
        ReduceValuesTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, ReduceValuesTask<K, V> nextRight, BiFunction<? super V, ? super V, ? extends V> reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.reducer = reducer;
        }
        
        public final V getRawResult() {
            return result;
        }
        
        public final void compute() {
            final BiFunction<? super V, ? super V, ? extends V> reducer;
            if((reducer = this.reducer) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new ReduceValuesTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, reducer)).fork();
                }
                
                V r = null;
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    V v = p.val;
                    r = (r == null) ? v : reducer.apply(r, v);
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    ReduceValuesTask<K, V> t = (ReduceValuesTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        V tr, sr;
                        if((sr = s.result) != null) {
                            t.result = (((tr = t.result) == null) ? sr : reducer.apply(tr, sr));
                        }
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class ReduceEntriesTask<K, V> extends BulkTask<K, V, Map.Entry<K, V>> {
        final BiFunction<Map.Entry<K, V>, Map.Entry<K, V>, ? extends Map.Entry<K, V>> reducer;
        Map.Entry<K, V> result;
        ReduceEntriesTask<K, V> rights, nextRight;
        
        ReduceEntriesTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, ReduceEntriesTask<K, V> nextRight, BiFunction<Entry<K, V>, Map.Entry<K, V>, ? extends Map.Entry<K, V>> reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.reducer = reducer;
        }
        
        public final Map.Entry<K, V> getRawResult() {
            return result;
        }
        
        public final void compute() {
            final BiFunction<Map.Entry<K, V>, Map.Entry<K, V>, ? extends Map.Entry<K, V>> reducer;
            if((reducer = this.reducer) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new ReduceEntriesTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, reducer)).fork();
                }
                
                Map.Entry<K, V> r = null;
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = (r == null) ? p : reducer.apply(r, p);
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    ReduceEntriesTask<K, V> t = (ReduceEntriesTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        Map.Entry<K, V> tr, sr;
                        if((sr = s.result) != null) {
                            t.result = (((tr = t.result) == null) ? sr : reducer.apply(tr, sr));
                        }
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    
    
    @SuppressWarnings("serial")
    static final class MapReduceKeysTask<K, V, U> extends BulkTask<K, V, U> {
        final Function<? super K, ? extends U> transformer;
        final BiFunction<? super U, ? super U, ? extends U> reducer;
        U result;
        MapReduceKeysTask<K, V, U> rights, nextRight;
        
        MapReduceKeysTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceKeysTask<K, V, U> nextRight, Function<? super K, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.reducer = reducer;
        }
        
        public final U getRawResult() {
            return result;
        }
        
        public final void compute() {
            final Function<? super K, ? extends U> transformer;
            final BiFunction<? super U, ? super U, ? extends U> reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceKeysTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, reducer)).fork();
                }
                
                U r = null;
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    U u;
                    if((u = transformer.apply(p.key)) != null) {
                        r = (r == null) ? u : reducer.apply(r, u);
                    }
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceKeysTask<K, V, U> t = (MapReduceKeysTask<K, V, U>) c, s = t.rights;
                    while(s != null) {
                        U tr, sr;
                        if((sr = s.result) != null) {
                            t.result = (((tr = t.result) == null) ? sr : reducer.apply(tr, sr));
                        }
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceValuesTask<K, V, U> extends BulkTask<K, V, U> {
        final Function<? super V, ? extends U> transformer;
        final BiFunction<? super U, ? super U, ? extends U> reducer;
        U result;
        MapReduceValuesTask<K, V, U> rights, nextRight;
        
        MapReduceValuesTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceValuesTask<K, V, U> nextRight, Function<? super V, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.reducer = reducer;
        }
        
        public final U getRawResult() {
            return result;
        }
        
        public final void compute() {
            final Function<? super V, ? extends U> transformer;
            final BiFunction<? super U, ? super U, ? extends U> reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceValuesTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, reducer)).fork();
                }
                
                U r = null;
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    U u;
                    if((u = transformer.apply(p.val)) != null) {
                        r = (r == null) ? u : reducer.apply(r, u);
                    }
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceValuesTask<K, V, U> t = (MapReduceValuesTask<K, V, U>) c, s = t.rights;
                    while(s != null) {
                        U tr, sr;
                        if((sr = s.result) != null) {
                            t.result = (((tr = t.result) == null) ? sr : reducer.apply(tr, sr));
                        }
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceEntriesTask<K, V, U> extends BulkTask<K, V, U> {
        final Function<Map.Entry<K, V>, ? extends U> transformer;
        final BiFunction<? super U, ? super U, ? extends U> reducer;
        U result;
        MapReduceEntriesTask<K, V, U> rights, nextRight;
        
        MapReduceEntriesTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceEntriesTask<K, V, U> nextRight, Function<Map.Entry<K, V>, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.reducer = reducer;
        }
        
        public final U getRawResult() {
            return result;
        }
        
        public final void compute() {
            final Function<Map.Entry<K, V>, ? extends U> transformer;
            final BiFunction<? super U, ? super U, ? extends U> reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceEntriesTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, reducer)).fork();
                }
                
                U r = null;
                for(Node<K, V> p; (p = advance()) != null; ) {
                    U u;
                    if((u = transformer.apply(p)) != null)
                        r = (r == null) ? u : reducer.apply(r, u);
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceEntriesTask<K, V, U> t = (MapReduceEntriesTask<K, V, U>) c, s = t.rights;
                    while(s != null) {
                        U tr, sr;
                        if((sr = s.result) != null)
                            t.result = (((tr = t.result) == null) ? sr : reducer.apply(tr, sr));
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceMappingsTask<K, V, U> extends BulkTask<K, V, U> {
        final BiFunction<? super K, ? super V, ? extends U> transformer;
        final BiFunction<? super U, ? super U, ? extends U> reducer;
        U result;
        MapReduceMappingsTask<K, V, U> rights, nextRight;
        
        MapReduceMappingsTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceMappingsTask<K, V, U> nextRight, BiFunction<? super K, ? super V, ? extends U> transformer, BiFunction<? super U, ? super U, ? extends U> reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.reducer = reducer;
        }
        
        public final U getRawResult() {
            return result;
        }
        
        public final void compute() {
            final BiFunction<? super K, ? super V, ? extends U> transformer;
            final BiFunction<? super U, ? super U, ? extends U> reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceMappingsTask<K, V, U>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, reducer)).fork();
                }
                
                U r = null;
                for(Node<K, V> p; (p = advance()) != null; ) {
                    U u;
                    if((u = transformer.apply(p.key, p.val)) != null) {
                        r = (r == null) ? u : reducer.apply(r, u);
                    }
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceMappingsTask<K, V, U> t = (MapReduceMappingsTask<K, V, U>) c, s = t.rights;
                    while(s != null) {
                        U tr, sr;
                        if((sr = s.result) != null) {
                            t.result = (((tr = t.result) == null) ? sr : reducer.apply(tr, sr));
                        }
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    
    
    @SuppressWarnings("serial")
    static final class MapReduceKeysToIntTask<K, V>     extends BulkTask<K, V, Integer> {
        final ToIntFunction<? super K> transformer;
        final IntBinaryOperator reducer;
        final int basis;
        int result;
        MapReduceKeysToIntTask<K, V> rights, nextRight;
        
        MapReduceKeysToIntTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceKeysToIntTask<K, V> nextRight, ToIntFunction<? super K> transformer, int basis, IntBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Integer getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToIntFunction<? super K> transformer;
            final IntBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                int r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceKeysToIntTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsInt(r, transformer.applyAsInt(p.key));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceKeysToIntTask<K, V> t = (MapReduceKeysToIntTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsInt(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceValuesToIntTask<K, V>   extends BulkTask<K, V, Integer> {
        final ToIntFunction<? super V> transformer;
        final IntBinaryOperator reducer;
        final int basis;
        int result;
        MapReduceValuesToIntTask<K, V> rights, nextRight;
        
        MapReduceValuesToIntTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceValuesToIntTask<K, V> nextRight, ToIntFunction<? super V> transformer, int basis, IntBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Integer getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToIntFunction<? super V> transformer;
            final IntBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                int r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceValuesToIntTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsInt(r, transformer.applyAsInt(p.val));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceValuesToIntTask<K, V> t = (MapReduceValuesToIntTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsInt(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceEntriesToIntTask<K, V>  extends BulkTask<K, V, Integer> {
        final ToIntFunction<Map.Entry<K, V>> transformer;
        final IntBinaryOperator reducer;
        final int basis;
        int result;
        MapReduceEntriesToIntTask<K, V> rights, nextRight;
        
        MapReduceEntriesToIntTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceEntriesToIntTask<K, V> nextRight, ToIntFunction<Map.Entry<K, V>> transformer, int basis, IntBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Integer getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToIntFunction<Map.Entry<K, V>> transformer;
            final IntBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                int r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceEntriesToIntTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsInt(r, transformer.applyAsInt(p));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceEntriesToIntTask<K, V> t = (MapReduceEntriesToIntTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsInt(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceMappingsToIntTask<K, V> extends BulkTask<K, V, Integer> {
        final ToIntBiFunction<? super K, ? super V> transformer;
        final IntBinaryOperator reducer;
        final int basis;
        int result;
        MapReduceMappingsToIntTask<K, V> rights, nextRight;
        
        MapReduceMappingsToIntTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceMappingsToIntTask<K, V> nextRight, ToIntBiFunction<? super K, ? super V> transformer, int basis, IntBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Integer getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToIntBiFunction<? super K, ? super V> transformer;
            final IntBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                int r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceMappingsToIntTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsInt(r, transformer.applyAsInt(p.key, p.val));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceMappingsToIntTask<K, V> t = (MapReduceMappingsToIntTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsInt(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    
    
    @SuppressWarnings("serial")
    static final class MapReduceKeysToLongTask<K, V>     extends BulkTask<K, V, Long> {
        final ToLongFunction<? super K> transformer;
        final LongBinaryOperator reducer;
        final long basis;
        long result;
        MapReduceKeysToLongTask<K, V> rights, nextRight;
        
        MapReduceKeysToLongTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceKeysToLongTask<K, V> nextRight, ToLongFunction<? super K> transformer, long basis, LongBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Long getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToLongFunction<? super K> transformer;
            final LongBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                long r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceKeysToLongTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsLong(r, transformer.applyAsLong(p.key));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceKeysToLongTask<K, V> t = (MapReduceKeysToLongTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsLong(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceValuesToLongTask<K, V>   extends BulkTask<K, V, Long> {
        final ToLongFunction<? super V> transformer;
        final LongBinaryOperator reducer;
        final long basis;
        long result;
        MapReduceValuesToLongTask<K, V> rights, nextRight;
        
        MapReduceValuesToLongTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceValuesToLongTask<K, V> nextRight, ToLongFunction<? super V> transformer, long basis, LongBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Long getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToLongFunction<? super V> transformer;
            final LongBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                long r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceValuesToLongTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsLong(r, transformer.applyAsLong(p.val));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceValuesToLongTask<K, V> t = (MapReduceValuesToLongTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsLong(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceEntriesToLongTask<K, V>  extends BulkTask<K, V, Long> {
        final ToLongFunction<Map.Entry<K, V>> transformer;
        final LongBinaryOperator reducer;
        final long basis;
        long result;
        MapReduceEntriesToLongTask<K, V> rights, nextRight;
        
        MapReduceEntriesToLongTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceEntriesToLongTask<K, V> nextRight, ToLongFunction<Map.Entry<K, V>> transformer, long basis, LongBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Long getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToLongFunction<Map.Entry<K, V>> transformer;
            final LongBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                long r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceEntriesToLongTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsLong(r, transformer.applyAsLong(p));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceEntriesToLongTask<K, V> t = (MapReduceEntriesToLongTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsLong(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceMappingsToLongTask<K, V> extends BulkTask<K, V, Long> {
        final ToLongBiFunction<? super K, ? super V> transformer;
        final LongBinaryOperator reducer;
        final long basis;
        long result;
        MapReduceMappingsToLongTask<K, V> rights, nextRight;
        
        MapReduceMappingsToLongTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceMappingsToLongTask<K, V> nextRight, ToLongBiFunction<? super K, ? super V> transformer, long basis, LongBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Long getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToLongBiFunction<? super K, ? super V> transformer;
            final LongBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                long r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceMappingsToLongTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsLong(r, transformer.applyAsLong(p.key, p.val));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceMappingsToLongTask<K, V> t = (MapReduceMappingsToLongTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsLong(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    
    
    @SuppressWarnings("serial")
    static final class MapReduceKeysToDoubleTask<K, V>     extends BulkTask<K, V, Double> {
        final ToDoubleFunction<? super K> transformer;
        final DoubleBinaryOperator reducer;
        final double basis;
        double result;
        MapReduceKeysToDoubleTask<K, V> rights, nextRight;
        
        MapReduceKeysToDoubleTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceKeysToDoubleTask<K, V> nextRight, ToDoubleFunction<? super K> transformer, double basis, DoubleBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Double getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToDoubleFunction<? super K> transformer;
            final DoubleBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                double r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceKeysToDoubleTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsDouble(r, transformer.applyAsDouble(p.key));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceKeysToDoubleTask<K, V> t = (MapReduceKeysToDoubleTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsDouble(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceValuesToDoubleTask<K, V>   extends BulkTask<K, V, Double> {
        final ToDoubleFunction<? super V> transformer;
        final DoubleBinaryOperator reducer;
        final double basis;
        double result;
        MapReduceValuesToDoubleTask<K, V> rights, nextRight;
        
        MapReduceValuesToDoubleTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceValuesToDoubleTask<K, V> nextRight, ToDoubleFunction<? super V> transformer, double basis, DoubleBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Double getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToDoubleFunction<? super V> transformer;
            final DoubleBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                double r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceValuesToDoubleTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsDouble(r, transformer.applyAsDouble(p.val));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceValuesToDoubleTask<K, V> t = (MapReduceValuesToDoubleTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsDouble(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceEntriesToDoubleTask<K, V>  extends BulkTask<K, V, Double> {
        final ToDoubleFunction<Map.Entry<K, V>> transformer;
        final DoubleBinaryOperator reducer;
        final double basis;
        double result;
        MapReduceEntriesToDoubleTask<K, V> rights, nextRight;
        
        MapReduceEntriesToDoubleTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceEntriesToDoubleTask<K, V> nextRight, ToDoubleFunction<Map.Entry<K, V>> transformer, double basis, DoubleBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Double getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToDoubleFunction<Map.Entry<K, V>> transformer;
            final DoubleBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                double r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceEntriesToDoubleTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsDouble(r, transformer.applyAsDouble(p));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceEntriesToDoubleTask<K, V> t = (MapReduceEntriesToDoubleTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsDouble(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    @SuppressWarnings("serial")
    static final class MapReduceMappingsToDoubleTask<K, V> extends BulkTask<K, V, Double> {
        final ToDoubleBiFunction<? super K, ? super V> transformer;
        final DoubleBinaryOperator reducer;
        final double basis;
        double result;
        MapReduceMappingsToDoubleTask<K, V> rights, nextRight;
        
        MapReduceMappingsToDoubleTask(BulkTask<K, V, ?> p, int b, int i, int f, Node<K, V>[] t, MapReduceMappingsToDoubleTask<K, V> nextRight, ToDoubleBiFunction<? super K, ? super V> transformer, double basis, DoubleBinaryOperator reducer) {
            super(p, b, i, f, t);
            this.nextRight = nextRight;
            this.transformer = transformer;
            this.basis = basis;
            this.reducer = reducer;
        }
        
        public final Double getRawResult() {
            return result;
        }
        
        public final void compute() {
            final ToDoubleBiFunction<? super K, ? super V> transformer;
            final DoubleBinaryOperator reducer;
            if((transformer = this.transformer) != null && (reducer = this.reducer) != null) {
                double r = this.basis;
                for(int i = baseIndex, f, h; batch>0 && (h = ((f = baseLimit) + i) >>> 1)>i; ) {
                    addToPendingCount(1);
                    (rights = new MapReduceMappingsToDoubleTask<K, V>(this, batch >>>= 1, baseLimit = h, f, tab, rights, transformer, r, reducer)).fork();
                }
                
                for(Node<K, V> p; (p = advance()) != null; ) {
                    r = reducer.applyAsDouble(r, transformer.applyAsDouble(p.key, p.val));
                }
                
                result = r;
                
                for(CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
                    @SuppressWarnings("unchecked")
                    MapReduceMappingsToDoubleTask<K, V> t = (MapReduceMappingsToDoubleTask<K, V>) c, s = t.rights;
                    while(s != null) {
                        t.result = reducer.applyAsDouble(t.result, s.result);
                        s = t.rights = s.nextRight;
                    }
                }
            }
        }
    }
    
    
    
    /**
     * A padded cell for distributing counts.  Adapted from LongAdder
     * and Striped64.  See their internal docs for explanation.
     */
    @jdk.internal.vm.annotation.Contended
    static final class CounterCell {
        volatile long value;
        
        CounterCell(long x) {
            value = x;
        }
    }
    
    
    
    /**
     * Stripped-down version of helper class used in previous version,
     * declared for the sake of serialization compatibility.
     */
    static class Segment<K,V> extends ReentrantLock implements Serializable {
        private static final long serialVersionUID = 2249069246763182397L;
        final float loadFactor;
        Segment(float lf) { this.loadFactor = lf; }
    }
    
}