java.util contains the collections framework, legacy collection classes, event model, date and time facilities, internationalization, and miscellaneous utility classes (a string tokenizer, a random-number generator, and a bit array).
Java Collection simply means a single unit of objects. Java Collection framework provides many interfaces (Set, List, Queue, Deque etc.) and classes (ArrayList, Vector, LinkedList, PriorityQueue, HashSet, LinkedHashSet, TreeSet etc).
| Method | Description |
|---|---|
| public boolean add(Object element) | This method is used to insert an element in a collection. |
| public boolean addAll(Collection c) | This method is used to insert all elements of Collection c in the invoking collection. |
| public boolean remove(Object element) | This method is used to delete an element from a collection. |
| public boolean removeAll(Collection c) | This method is used to delete all the elements of Collection c from the invoking collection. |
| public boolean retainAll(Collection c) | This method is used to delete all the elements of invoking collection except the elements which are present in specified Collection c. |
| public int size() | This method returns the total number of elements in the collection. |
| public void clear() | This method removes all the elements from the collection. |
| public boolean contains(Object element) | This method is used to search an element in invoking collection. |
| public boolean containsAll(Collection c) | This method is used to check if all the elements of Collection c are present in invoking collection or not. |
| public Iterator iterator() | This method returns an iterator. |
| public boolean isEmpty() | This method checks if collection is empty. |
| public boolean equals(Object element) | This method is used to compare objects of collections. |
NOTE!- The above methods work for Set and List Interfaces.
| Method | Description |
|---|---|
| public boolean hasNext() | It returns true if iterator has more elements. |
| public Object next() | It returns the element and moves the cursor pointer to the next element. |
| public void remove() | It removes the last elements returned by the iterator. It is rarely used. |
//This example demonstrates the use of all methods listed above
import java.util.*;
public class Main
{
public static void main(String args[])
{
ArrayList <Integer> num= new ArrayList<Integer>();
ArrayList <Integer> even_num= new ArrayList<Integer>();
ArrayList <Integer> odd_num= new ArrayList<Integer>();
//inserting elements using add() method
for(int i=1;i<=10;i++)
num.add(i);
System.out.println("Original list of numbers is: ");
//Print numbers using iterator
Iterator it = num.iterator();
while(it.hasNext())
System.out.println(it.next());
for(int i=2;i<=10;i=i+2)
even_num.add(i);
System.out.println("List of even numbers is: ");
Iterator even_it = even_num.iterator();
while(even_it.hasNext())
System.out.println(even_it.next());
//inserting all elements of num in odd_num
odd_num.addAll(num);
//removing elements of even_num from odd_num
odd_num.removeAll(even_num);
System.out.println("List of odd numbers is: ");
Iterator odd_it = odd_num.iterator();
while(odd_it.hasNext())
System.out.println(odd_it.next());
num.clear();
int n = num.size();
int e = even_num.size();
System.out.println("Size of num list is "+n+" and that of even_num list is "+e);
//check if even_num list contains 10
boolean check = even_num.contains(10);
System.out.println("even_num contains 10? "+check);
//check if even_num contains all elements of odd_num
check = even_num.containsAll(odd_num);
System.out.println("even_num contains odd_num? "+check);
boolean empty = num.isEmpty();
System.out.println("num is empty? "+empty);
//even if any one element of odd_num is not present in even_num, the method will return false
//check if temp is equl to odd_num
ArrayList <Integer> temp= new ArrayList<Integer>();
temp.addAll(odd_num);
check= temp.equals(odd_num);
System.out.println("temp list equal to odd_num? "+check);
}
}Original list of numbers is:
1
2
3
4
5
6
7
8
9
10
List of even numbers is:
2
4
6
8
10
List of odd numbers is:
1
3
5
7
9
Size of num list is 0 and that of even_num list is 5
even_num contains 10? true
even_num contains odd_num? false
num is empty? true
temp list equal to odd_num? true
| Method | Description |
|---|---|
| Object put(Object key, Object value) | It is used to insert an entry in this map. |
| void putAll(Map map) | It is used to insert the specified map in this map. |
| Object remove(Object key) | It is used to delete an entry for the specified key. |
| Object get(Object key) | It is used to return the value for the specified key. |
| boolean containsKey(Object key) | It is used to search the specified key from this map. |
| Set keySet() | It is used to return the Set view containing all the keys. |
| Set entrySet() | It is used to return the Set view containing all the keys and values. |
Entry is the sub interface of Map. So we will be accessed it by Map.Entry name. It provides methods to get key and value.
| Method | Description |
|---|---|
| Object getKey() | It is used to obtain key. |
| Object getValue() | It is used to obtain value. |
Maps can also be traversed using Iterator.
//This example demonstrates use of all Map Interface methods
import java.util.*;
class Main
{
public static void main(String args[])
{
Map<Integer,String> map=new HashMap<Integer,String>();
Map<Integer,String> copy=new HashMap<Integer,String>();
map.put(100,"ABC");
map.put(101,"XYZ");
map.put(102,"PQR");
//traversing map
System.out.println("Traversing map:");
for(Map.Entry m:map.entrySet())
System.out.println(m.getKey()+" "+m.getValue());
System.out.println("Value of key 100 in map is " +map.get(100));
//inserting all elements of map into copy
copy.putAll(map);
System.out.println("Traversing copy: ");
for(Map.Entry c:copy.entrySet())
System.out.println(c.getKey()+" "+c.getValue());
map.remove(102);
System.out.println("Removed key 102 from map.");
boolean check = map.containsKey(102);
System.out.println("map contains key 102? "+check);
}
} Traversing map:
100 ABC
101 XYZ
102 PQR
Value of key 100 in map is ABC
Traversing copy:
100 ABC
101 XYZ
102 PQR
Removed key 102 from map.
map contains key 102? false