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<html lang="en">
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8" />
<link rel="stylesheet" type="text/css" href="javaguide.css"/>
<script src="http://google-code-prettify.googlecode.com/svn/loader/run_prettify.js"
type="text/javascript"></script>
<link href="http://www.google.com/favicon.ico"
type="image/x-icon" rel="shortcut icon" />
<title>Google Java Style</title>
</head>
<body>
<h1>Google Java Style</h1>
<div class="change">Last changed: December 19, 2013</div>
<table border="0">
<tr>
<td>
<dl>
<br>
<dt class="toc1">
<a href="#s1-introduction">1 Introduction</a>
</dt>
<dd>
<a href="#s1.1-terminology">1.1 Terminology notes</a>
</dd>
<dd>
<a href="#s1.2-guide-notes">1.2 Guide notes</a>
</dd>
<br>
<dt class="toc1">
<a href="#s2-source-file-basics">2 Source file basics</a>
</dt>
<dd>
<a href="#s2.1-file-name">2.1 File name</a>
</dd>
<dd>
<a href="#s2.2-file-encoding">2.2 File encoding: UTF-8</a>
</dd>
<dd>
<a href="#s2.3-special-characters">2.3 Special characters</a>
</dd>
<dd class="toc3">
<a href="#s2.3.1-whitespace-characters">2.3.1 Whitespace characters</a>
</dd>
<dd class="toc3">
<a href="#s2.3.2-special-escape-sequences">2.3.2 Special escape sequences</a>
</dd>
<dd class="toc3">
<a href="#s2.3.3-non-ascii-characters">2.3.3 Non-ASCII characters</a>
</dd>
<br>
<dt class="toc1">
<a href="#s3-source-file-structure">3 Source file structure</a>
</dt>
<dd>
<a href="#s3.1-copyright-statement">3.1 License or copyright information, if present</a>
</dd>
<dd>
<a href="#s3.2-package-statement">3.2 Package statement</a>
</dd>
<dd>
<a href="#s3.3-import-statements">3.3 Import statements</a>
</dd>
<dd class="toc3">
<a href="#s3.3.1-wildcard-imports">3.3.1 No wildcard imports</a>
</dd>
<dd class="toc3">
<a href="#s3.3.2-import-line-wrapping">3.3.2 No line-wrapping</a>
</dd>
<dd class="toc3">
<a href="#s3.3.3-import-ordering-and-spacing">3.3.3 Ordering and spacing</a>
</dd>
<dd>
<a href="#s3.4-class-declaration">3.4 Class declaration</a>
</dd>
<dd class="toc3">
<a href="#s3.4.1-one-top-level-class">3.4.1 Exactly one top-level class declaration</a>
</dd>
<dd class="toc3">
<a href="#s3.4.2-class-member-ordering">3.4.2 Class member ordering</a>
</dd>
</dl>
</td><td>
<dl>
<br>
<dt class="toc1">
<a href="#s4-formatting">4 Formatting</a>
</dt>
<dd>
<a href="#s4.1-braces">4.1 Braces</a>
</dd>
<dd class="toc3">
<a href="#s4.1.1-braces-always-used">4.1.1 Braces are used where optional</a>
</dd>
<dd class="toc3">
<a href="#s4.1.2-blocks-k-r-style">4.1.2 Nonempty blocks: K & R style</a>
</dd>
<dd class="toc3">
<a href="#s4.1.3-braces-empty-blocks">4.1.3 Empty blocks: may be concise</a>
</dd>
<dd>
<a href="#s4.2-block-indentation">4.2 Block indentation: +2 spaces</a>
</dd>
<dd>
<a href="#s4.3-one-statement-per-line">4.3 One statement per line</a>
</dd>
<dd>
<a href="#s4.4-column-limit">4.4 Column limit: 80 or 100</a>
</dd>
<dd>
<a href="#s4.5-line-wrapping">4.5 Line-wrapping</a>
</dd>
<dd class="toc3">
<a href="#s4.5.1-line-wrapping-where-to-break">4.5.1 Where to break</a>
</dd>
<dd class="toc3">
<a href="#s4.5.2-line-wrapping-indent">4.5.2 Indent continuation lines at least +4 spaces</a>
</dd>
<dd>
<a href="#s4.6-whitespace">4.6 Whitespace</a>
</dd>
<dd class="toc3">
<a href="#s4.6.1-vertical-whitespace">4.6.1 Vertical Whitespace</a>
</dd>
<dd class="toc3">
<a href="#s4.6.2-horizontal-whitespace">4.6.2 Horizontal whitespace</a>
</dd>
<dd class="toc3">
<a href="#s4.6.3-horizontal-alignment">4.6.3 Horizontal alignment: never required</a>
</dd>
<dd>
<a href="#s4.7-grouping-parentheses">4.7 Grouping parentheses: recommended</a>
</dd>
<dd>
<a href="#s4.8-specific-constructs">4.8 Specific constructs</a>
</dd>
<dd class="toc3">
<a href="#s4.8.1-enum-classes">4.8.1 Enum classes</a>
</dd>
<dd class="toc3">
<a href="#s4.8.2-variable-declarations">4.8.2 Variable declarations</a>
</dd>
<dd class="toc3">
<a href="#s4.8.3-arrays">4.8.3 Arrays</a>
</dd>
<dd class="toc3">
<a href="#s4.8.4-switch">4.8.4 Switch statements</a>
</dd>
<dd class="toc3">
<a href="#s4.8.5-annotations">4.8.5 Annotations</a>
</dd>
<dd class="toc3">
<a href="#s4.8.6-comments">4.8.6 Comments</a>
</dd>
<dd class="toc3">
<a href="#s4.8.7-modifiers">4.8.7 Modifiers</a>
</dd>
</dl>
</td><td>
<dl>
<br>
<dt class="toc1">
<a href="#s5-naming">5 Naming</a>
</dt>
<dd>
<a href="#s5.1-identifier-names">5.1 Rules common to all identifiers</a>
</dd>
<dd>
<a href="#s5.2-specific-identifier-names">5.2 Rules by identifier type</a>
</dd>
<dd class="toc3">
<a href="#s5.2.1-package-names">5.2.1 Package names</a>
</dd>
<dd class="toc3">
<a href="#s5.2.2-class-names">5.2.2 Class names</a>
</dd>
<dd class="toc3">
<a href="#s5.2.3-method-names">5.2.3 Method names</a>
</dd>
<dd class="toc3">
<a href="#s5.2.4-constant-names">5.2.4 Constant names</a>
</dd>
<dd class="toc3">
<a href="#s5.2.5-non-constant-field-names">5.2.5 Non-constant field names</a>
</dd>
<dd class="toc3">
<a href="#s5.2.6-parameter-names">5.2.6 Parameter names</a>
</dd>
<dd class="toc3">
<a href="#s5.2.7-local-variable-names">5.2.7 Local variable names</a>
</dd>
<dd class="toc3">
<a href="#s5.2.8-type-variable-names">5.2.8 Type variable names</a>
</dd>
<dd>
<a href="#s5.3-camel-case">5.3 Camel case: defined</a>
</dd>
<br>
<dt class="toc1">
<a href="#s6-programming-practices">6 Programming Practices</a>
</dt>
<dd>
<a href="#s6.1-override-annotation">6.1 @Override: always used</a>
</dd>
<dd>
<a href="#s6.2-caught-exceptions">6.2 Caught exceptions: not ignored</a>
</dd>
<dd>
<a href="#s6.3-static-members">6.3 Static members: qualified using class</a>
</dd>
<dd>
<a href="#s6.4-finalizers">6.4 Finalizers: not used</a>
</dd>
<br>
<dt class="toc1">
<a href="#s7-javadoc">7 Javadoc</a>
</dt>
<dd>
<a href="#s7.1-javadoc-formatting">7.1 Formatting</a>
</dd>
<dd class="toc3">
<a href="#s7.1.1-javadoc-multi-line">7.1.1 General form</a>
</dd>
<dd class="toc3">
<a href="#s7.1.2-javadoc-paragraphs">7.1.2 Paragraphs</a>
</dd>
<dd class="toc3">
<a href="#s7.1.3-javadoc-at-clauses">7.1.3 At-clauses</a>
</dd>
<dd>
<a href="#s7.2-summary-fragment">7.2 The summary fragment</a>
</dd>
<dd>
<a href="#s7.3-javadoc-where-required">7.3 Where Javadoc is used</a>
</dd>
<dd class="toc3">
<a href="#s7.3.1-javadoc-exception-self-explanatory">7.3.1 Exception: self-explanatory methods</a>
</dd>
<dd class="toc3">
<a href="#s7.3.2-javadoc-exception-overrides">7.3.2 Exception: overrides</a>
</dd>
<dd class="toc3">
<a href="#s7.3.3-javadoc-optional">7.3.3 Optional javadoc</a>
</dd>
</dl>
</td>
</tr>
</table>
<div><div id="with-header-links"><a name="s1-introduction"/>
<h2>1 Introduction <a href="#s1-introduction"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
<p>This document serves as the <strong>complete</strong> definition of Google's coding standards for
source code in the Java™ Programming Language. A Java source file is described as being <em>in
Google Style</em> if and only if it adheres to the rules herein.</p><p>Like other programming style guides, the issues covered span not only aesthetic issues of
formatting, but other types of conventions or coding standards as well. However, this document
focuses primarily on the <strong>hard-and-fast rules</strong> that we follow universally, and
avoids giving <em>advice</em> that isn't clearly enforceable (whether by human or tool).
</p><a name="s1.1-terminology"/>
<h3>1.1 Terminology notes <a href="#s1.1-terminology"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>In this document, unless otherwise clarified:</p><ol><li>The term <em>class</em> is used inclusively to mean an "ordinary" class, enum class,
interface or annotation type (<code class="prettyprint lang-java">@interface</code>).</li><li>The term <em>comment</em> always refers to <em>implementation</em> comments. We do not
use the phrase "documentation comments", instead using the common term "Javadoc."</li></ol><p>Other "terminology notes" will appear occasionally throughout the document.</p><a name="s1.2-guide-notes"/>
<h3>1.2 Guide notes <a href="#s1.2-guide-notes"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>Example code in this document is <strong>non-normative</strong>. That is, while the examples
are in Google Style, they may not illustrate the <em>only</em> stylish way to represent the
code. Optional formatting choices made in examples should not be enforced as rules.</p><a name="s2-source-file-basics"/>
<h2>2 Source file basics <a href="#s2-source-file-basics"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
<a name="s2.1-file-name"/>
<h3>2.1 File name <a href="#s2.1-file-name"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>The source file name consists of the case-sensitive name of the top-level class it contains,
plus the <code>.java</code> extension (aside from <code>package-info.java</code> files).</p><a name="s2.2-file-encoding"/>
<h3>2.2 File encoding: UTF-8 <a href="#s2.2-file-encoding"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>Source files are encoded in <strong>UTF-8</strong>.</p><a name="s2.3-special-characters"/>
<h3>2.3 Special characters <a href="#s2.3-special-characters"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<a name="s2.3.1-whitespace-characters"/>
<h4>2.3.1 Whitespace characters <a href="#s2.3.1-whitespace-characters"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Aside from the line terminator sequence, the <strong>ASCII horizontal space
character</strong> (<strong>0x20</strong>) is the only whitespace character that appears
anywhere in a source file. This implies that:</p><ol><li>All other whitespace characters in string and character literals are escaped.</li><li>Tab characters are <strong>not</strong> used for indentation.</li></ol><a name="s2.3.2-special-escape-sequences"/>
<h4>2.3.2 Special escape sequences <a href="#s2.3.2-special-escape-sequences"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>For any character that has a special escape sequence
(<code class="prettyprint lang-java">\b</code>,
<code class="prettyprint lang-java">\t</code>,
<code class="prettyprint lang-java">\n</code>,
<code class="prettyprint lang-java">\f</code>,
<code class="prettyprint lang-java">\r</code>,
<code class="prettyprint lang-java">\"</code>,
<code class="prettyprint lang-java">\'</code> and
<code class="prettyprint lang-java">\\</code>), that sequence
is used rather than the corresponding octal
(e.g. <code class="prettyprint lang-java badcode">\012</code>) or Unicode
(e.g. <code class="prettyprint lang-java badcode">\u000a</code>) escape.</p><a name="s2.3.3-non-ascii-characters"/>
<h4>2.3.3 Non-ASCII characters <a href="#s2.3.3-non-ascii-characters"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>For the remaining non-ASCII characters, either the actual Unicode character
(e.g. <code class="prettyprint lang-java">∞</code>) or the equivalent Unicode escape
(e.g. <code class="prettyprint lang-java">\u221e</code>) is used, depending only on which
makes the code <strong>easier to read and understand</strong>.</p><p class="tip"><b>Tip:</b> in the Unicode escape case, and occasionally even when actual Unicode characters
are used, an explanatory comment can be very helpful.</p><p>Examples:</p><table><tr><th>Example</th><th>Discussion</th></tr><tr><td><code class="prettyprint lang-java">String unitAbbrev = "μs";</code></td><td>Best: perfectly clear even without a comment.</td></tr><tr><td><code class="prettyprint lang-java">String unitAbbrev = "\u03bcs"; // "μs"</code></td><td>Allowed, but there's no reason to do this.</td></tr><tr><td><code class="prettyprint lang-java">String unitAbbrev = "\u03bcs";
// Greek letter mu, "s"</code></td><td>Allowed, but awkward and prone to mistakes.</td></tr><tr><td><code class="prettyprint lang-java badcode">String unitAbbrev = "\u03bcs";</code></td><td>Poor: the reader has no idea what this is.</td></tr><tr><td><code class="prettyprint lang-java">return '\ufeff' + content;
// byte order mark</code></td><td>Good: use escapes for non-printable characters, and comment if necessary.</td></tr></table><p class="tip"><b>Tip:</b> Never make your code less readable simply out of fear that some programs might
not handle non-ASCII characters properly. If that should happen, those programs
are <strong>broken</strong> and they must be <strong>fixed</strong>.</p><a name="filestructure"/><a name="s3-source-file-structure"/>
<h2>3 Source file structure <a href="#s3-source-file-structure"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
<div><p>A source file consists of, <strong>in order</strong>:</p><ol><li>License or copyright information, if present</li><li>Package statement</li><li>Import statements</li><li>Exactly one top-level class</li></ol></div><p><strong>Exactly one blank line</strong> separates each section that is present.</p><a name="s3.1-copyright-statement"/>
<h3>3.1 License or copyright information, if present <a href="#s3.1-copyright-statement"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>If license or copyright information belongs in a file, it belongs here.</p><a name="s3.2-package-statement"/>
<h3>3.2 Package statement <a href="#s3.2-package-statement"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>The package statement is <strong>not line-wrapped</strong>. The column limit (Section 4.4,
<a href="#s4.4-column-limit">Column limit: 80 or 100</a>) does not apply to package statements.</p><a name="imports"/><a name="s3.3-import-statements"/>
<h3>3.3 Import statements <a href="#s3.3-import-statements"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<a name="s3.3.1-wildcard-imports"/>
<h4>3.3.1 No wildcard imports <a href="#s3.3.1-wildcard-imports"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p><strong>Wildcard imports</strong>, static or otherwise, <strong>are not used</strong>.</p><a name="s3.3.2-import-line-wrapping"/>
<h4>3.3.2 No line-wrapping <a href="#s3.3.2-import-line-wrapping"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Import statements are <strong>not line-wrapped</strong>. The column limit (Section 4.4,
<a href="#s4.4-column-limit">Column limit: 80 or 100</a>) does not apply to import
statements.</p><a name="s3.3.3-import-ordering-and-spacing"/>
<h4>3.3.3 Ordering and spacing <a href="#s3.3.3-import-ordering-and-spacing"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Import statements are divided into the following groups, in this order, with each group
separated by a single blank line:</p><ol><li>All static imports in a single group</li><li><code>com.google</code> imports
(only if this source file is in the <code>com.google</code> package
space)</li><li>Third-party imports, one group per top-level package, in ASCII sort order
<ul><li>for example: <code>android</code>, <code>com</code>, <code>junit</code>, <code>org</code>,
<code>sun</code></li></ul></li><li><code>java</code> imports</li><li><code>javax</code> imports</li></ol><p>Within a group there are no blank lines, and the imported names appear in ASCII sort
order. (<strong>Note:</strong> this is not the same as the import <em>statements</em> being in
ASCII sort order; the presence of semicolons warps the result.)</p><a name="s3.4-class-declaration"/>
<h3>3.4 Class declaration <a href="#s3.4-class-declaration"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<a name="oneclassperfile"/><a name="s3.4.1-one-top-level-class"/>
<h4>3.4.1 Exactly one top-level class declaration <a href="#s3.4.1-one-top-level-class"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Each top-level class resides in a source file of its own.</p><p class="exception"><b>Exception:</b> of course, no such class appears in <code>package-info.java</code>
files.</p><a name="s3.4.2-class-member-ordering"/>
<h4>3.4.2 Class member ordering <a href="#s3.4.2-class-member-ordering"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>The ordering of the members of a class can have a great effect on learnability, but there is
no single correct recipe for how to do it. Different classes may order their members
differently.</p><p>What is important is that each class order its members in <strong><em>some</em> logical
order</strong>, which its maintainer could explain if asked. For example, new methods are not
just habitually added to the end of the class, as that would yield "chronological by date
added" ordering, which is not a logical ordering.</p><a name="overloads"/><a name="s3.4.2.1-overloads-never-split"/>
<h5>3.4.2.1 Overloads: never split <a href="#s3.4.2.1-overloads-never-split"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
<p>When a class has multiple constructors, or multiple methods with the same name, these appear
sequentially, with no intervening members.</p><a name="s4-formatting"/>
<h2>4 Formatting <a href="#s4-formatting"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
<p class="terminology"><b>Terminology Note:</b> <em>block-like construct</em> refers to the body of a class, method or
constructor. Note that, by Section 4.8.3.1 on
<a href="#s4.8.3.1-array-initializers">array initializers</a>, any array initializer
<em>may</em> optionally be treated as if it were a block-like construct.</p><a name="braces"/><a name="s4.1-braces"/>
<h3>4.1 Braces <a href="#s4.1-braces"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<a name="s4.1.1-braces-always-used"/>
<h4>4.1.1 Braces are used where optional <a href="#s4.1.1-braces-always-used"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Braces are used with
<code class="prettyprint lang-java">if</code>,
<code class="prettyprint lang-java">else</code>,
<code class="prettyprint lang-java">for</code>,
<code class="prettyprint lang-java">do</code> and
<code class="prettyprint lang-java">while</code> statements, even when the
body is empty or contains only a single statement.</p><a name="s4.1.2-blocks-k-r-style"/>
<h4>4.1.2 Nonempty blocks: K & R style <a href="#s4.1.2-blocks-k-r-style"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Braces follow the Kernighan and Ritchie style
("<a href="http://www.codinghorror.com/blog/2012/07/new-programming-jargon.html">Egyptian brackets</a>")
for <em>nonempty</em> blocks and block-like constructs:</p><ul><li>No line break before the opening brace.</li><li>Line break after the opening brace.</li><li>Line break before the closing brace.</li><li>Line break after the closing brace <em>if</em> that brace terminates a statement or the body
of a method, constructor or <em>named</em> class. For example, there is <em>no</em> line break
after the brace if it is followed by <code class="prettyprint lang-java">else</code> or a
comma.</li></ul><p>Example:</p><pre class="prettyprint lang-java">
return new MyClass() {
@Override public void method() {
if (condition()) {
try {
something();
} catch (ProblemException e) {
recover();
}
}
}
};
</pre><p>A few exceptions for enum classes are given in Section 4.8.1,
<a href="#s4.8.1-enum-classes">Enum classes</a>.</p><a name="emptyblocks"/><a name="s4.1.3-braces-empty-blocks"/>
<h4>4.1.3 Empty blocks: may be concise <a href="#s4.1.3-braces-empty-blocks"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>An empty block or block-like construct <em>may</em> be closed immediately after it is
opened, with no characters or line break in between
(<code class="prettyprint lang-java">{}</code>), <strong>unless</strong> it is part of a
<em>multi-block statement</em> (one that directly contains multiple blocks:
<code class="prettyprint lang-java">if/else-if/else</code> or
<code class="prettyprint lang-java">try/catch/finally</code>).</p><p>Example:</p><pre class="prettyprint lang-java">
void doNothing() {}
</pre><a name="s4.2-block-indentation"/>
<h3>4.2 Block indentation: +2 spaces <a href="#s4.2-block-indentation"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>Each time a new block or block-like construct is opened, the indent increases by two
spaces. When the block ends, the indent returns to the previous indent level. The indent level
applies to both code and comments throughout the block. (See the example in Section 4.1.2,
<a href="#s4.1.2-blocks-k-r-style">Nonempty blocks: K & R Style</a>.)</p><a name="s4.3-one-statement-per-line"/>
<h3>4.3 One statement per line <a href="#s4.3-one-statement-per-line"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>Each statement is followed by a line-break.</p><a name="columnlimit"/><a name="s4.4-column-limit"/>
<h3>4.4 Column limit: 80 or 100 <a href="#s4.4-column-limit"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>
Projects are free to choose a column limit of either 80 or 100 characters.
Except as noted below, any line that would exceed this limit must be line-wrapped, as explained in
Section 4.5, <a href="#s4.5-line-wrapping">Line-wrapping</a>.
</p><p><strong>Exceptions:</strong></p><ol><li>Lines where obeying the column limit is not possible (for example, a long URL in Javadoc,
or a long JSNI method reference).</li><li><code class="prettyprint lang-java">package</code> and
<code class="prettyprint lang-java">import</code> statements (see Sections
3.2 <a href="#s3.2-package-statement">Package statement</a> and
3.3 <a href="#s3.3-import-statements">Import statements</a>).</li><li>Command lines in a comment that may be cut-and-pasted into a shell.</li></ol><a name="s4.5-line-wrapping"/>
<h3>4.5 Line-wrapping <a href="#s4.5-line-wrapping"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p class="terminology"><b>Terminology Note:</b> When code that might otherwise legally occupy a single line is divided into
multiple lines, typically to avoid overflowing the column limit, this activity is called
<em>line-wrapping</em>.</p><p>There is no comprehensive, deterministic formula showing <em>exactly</em> how to line-wrap in
every situation. Very often there are several valid ways to line-wrap the same piece of code.</p><p class="tip"><b>Tip:</b> extracting a method or local variable may solve the problem without the need to
line-wrap.</p><a name="s4.5.1-line-wrapping-where-to-break"/>
<h4>4.5.1 Where to break <a href="#s4.5.1-line-wrapping-where-to-break"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>The prime directive of line-wrapping is: prefer to break at a
<strong>higher syntactic level</strong>. Also:</p><ol><li>When a line is broken at a <em>non-assignment</em> operator the break comes <em>before</em>
the symbol. (Note that this is not the same practice used in Google style for other languages,
such as C++ and JavaScript.)
<ul><li>This also applies to the following "operator-like" symbols: the dot separator
(<code class="prettyprint lang-java">.</code>), the ampersand in type bounds
(<code class="prettyprint lang-java"><T extends Foo & Bar></code>), and the pipe in
catch blocks
(<code class="prettyprint lang-java">catch (FooException | BarException e)</code>).</li></ul></li><li>When a line is broken at an <em>assignment</em> operator the break typically comes
<em>after</em> the symbol, but either way is acceptable.
<ul><li>This also applies to the "assignment-operator-like" colon in an enhanced
<code class="prettyprint lang-java">for</code> ("foreach") statement.</li></ul></li><li>A method or constructor name stays attached to the open parenthesis
(<code class="prettyprint lang-java">(</code>) that follows it.</li><li>A comma (<code class="prettyprint lang-java">,</code>) stays attached to the token that
precedes it.</li></ol><a name="indentation"/><a name="s4.5.2-line-wrapping-indent"/>
<h4>4.5.2 Indent continuation lines at least +4 spaces <a href="#s4.5.2-line-wrapping-indent"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>When line-wrapping, each line after the first (each <em>continuation line</em>) is indented
at least +4 from the original line.</p><p>When there are multiple continuation lines, indentation may be varied beyond +4 as
desired. In general, two continuation lines use the same indentation level if and only if they
begin with syntactically parallel elements.</p><p>Section 4.6.3 on <a href="#s4.6.3-horizontal-alignment">Horizontal alignment</a> addresses
the discouraged practice of using a variable number of spaces to align certain tokens with
previous lines.</p><a name="s4.6-whitespace"/>
<h3>4.6 Whitespace <a href="#s4.6-whitespace"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<a name="s4.6.1-vertical-whitespace"/>
<h4>4.6.1 Vertical Whitespace <a href="#s4.6.1-vertical-whitespace"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>A single blank line appears:</p><ol><li><em>Between</em> consecutive members (or initializers) of a class: fields, constructors,
methods, nested classes, static initializers, instance initializers.
<ul><li><span class="exception"><b>Exception:</b> a blank line between two consecutive fields (having no other code
between them) is optional. Such blank lines are used as needed to create <em>logical
groupings</em> of fields.</span></li></ul></li><li>Within method bodies, as needed to create <em>logical groupings</em> of statements.</li><li><em>Optionally</em> before the first member or after the last member of the class (neither
encouraged nor discouraged).</li><li>As required by other sections of this document (such as Section 3.3,
<a href="#s3.3-import-statements">Import statements</a>).</li></ol><p><em>Multiple</em> consecutive blank lines are permitted, but never required (or encouraged).</p><a name="s4.6.2-horizontal-whitespace"/>
<h4>4.6.2 Horizontal whitespace <a href="#s4.6.2-horizontal-whitespace"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Beyond where required by the language or other style rules, and apart from literals, comments and
Javadoc, a single ASCII space also appears in the following places <strong>only</strong>.</p><ol><li>Separating any reserved word, such as
<code class="prettyprint lang-java">if</code>,
<code class="prettyprint lang-java">for</code> or
<code class="prettyprint lang-java">catch</code>, from an open parenthesis
(<code class="prettyprint lang-java">(</code>)
that follows it on that line</li><li>Separating any reserved word, such as
<code class="prettyprint lang-java">else</code> or
<code class="prettyprint lang-java">catch</code>, from a closing curly brace
(<code class="prettyprint lang-java">}</code>) that precedes it on that line</li><li>Before any open curly brace
(<code class="prettyprint lang-java">{</code>), with two exceptions:
<ul><li><code class="prettyprint lang-java">@SomeAnnotation({a, b})</code> (no space is used)</li><li><code class="prettyprint lang-java">String[][] x = {{"foo"}};</code> (no space is required
between <code class="prettyprint lang-java">{{</code>, by item 8 below)</li></ul></li><li>On both sides of any binary or ternary operator. This also applies to the following
"operator-like" symbols:
<ul><li>the ampersand in a conjunctive type bound:
<code class="prettyprint lang-java"><T extends Foo & Bar></code></li><li>the pipe for a catch block that handles multiple exceptions:
<code class="prettyprint lang-java">catch (FooException | BarException e)</code></li><li>the colon (<code class="prettyprint lang-java">:</code>) in an enhanced
<code class="prettyprint lang-java">for</code> ("foreach") statement</li></ul></li><li>After <code class="prettyprint lang-java">,:;</code> or the closing parenthesis
(<code class="prettyprint lang-java">)</code>) of a cast</li><li>On both sides of the double slash (<code class="prettyprint lang-java">//</code>) that
begins an end-of-line comment. Here, multiple spaces are allowed, but not required.</li><li>Between the type and variable of a declaration:
<code class="prettyprint lang-java">List<String> list</code></li><li><em>Optional</em> just inside both braces of an array initializer
<ul><li><code class="prettyprint lang-java">new int[] {5, 6}</code> and
<code class="prettyprint lang-java">new int[] { 5, 6 }</code> are both valid</li></ul></li></ol><p class="note"><b>Note:</b> this rule never requires or forbids additional space at the start or end of a
line, only <em>interior</em> space.</p><a name="s4.6.3-horizontal-alignment"/>
<h4>4.6.3 Horizontal alignment: never required <a href="#s4.6.3-horizontal-alignment"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p class="terminology"><b>Terminology Note:</b> Horizontal alignment is the practice of adding a variable number of
additional spaces in your code with the goal of making certain tokens appear directly below certain
other tokens on previous lines.</p><p>This practice is permitted, but is <strong>never required</strong> by Google Style. It is not
even required to <em>maintain</em> horizontal alignment in places where it was already used.</p><p>Here is an example without alignment, then using alignment:</p><pre class="prettyprint lang-java">
private int x; // this is fine
private Color color; // this too
private int x; // permitted, but future edits
private Color color; // may leave it unaligned
</pre><p class="tip"><b>Tip:</b> Alignment can aid readability, but it creates problems for future maintenance.
Consider a future change that needs to touch just one line. This change may leave the
formerly-pleasing formatting mangled, and that is <strong>allowed</strong>. More often it prompts
the coder (perhaps you) to adjust whitespace on nearby lines as well, possibly triggering a
cascading series of reformattings. That one-line change now has a "blast radius." This can at worst
result in pointless busywork, but at best it still corrupts version history information, slows down
reviewers and exacerbates merge conflicts.</p><a name="parentheses"/><a name="s4.7-grouping-parentheses"/>
<h3>4.7 Grouping parentheses: recommended <a href="#s4.7-grouping-parentheses"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>Optional grouping parentheses are omitted only when author and reviewer agree that there is no
reasonable chance the code will be misinterpreted without them, nor would they have made the code
easier to read. It is <em>not</em> reasonable to assume that every reader has the entire Java
operator precedence table memorized.</p><a name="s4.8-specific-constructs"/>
<h3>4.8 Specific constructs <a href="#s4.8-specific-constructs"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<a name="s4.8.1-enum-classes"/>
<h4>4.8.1 Enum classes <a href="#s4.8.1-enum-classes"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>After each comma that follows an enum constant, a line-break is optional.</p><p>An enum class with no methods and no documentation on its constants may optionally be formatted
as if it were an array initializer:</p><pre class="prettyprint lang-java">
private enum Suit { CLUBS, HEARTS, SPADES, DIAMONDS }
</pre><p>Since enum classes <em>are classes</em>, all other rules for formatting classes apply.</p><a name="localvariables"/><a name="s4.8.2-variable-declarations"/>
<h4>4.8.2 Variable declarations <a href="#s4.8.2-variable-declarations"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<a name="s4.8.2.1-variables-per-declaration"/>
<h5>4.8.2.1 One variable per declaration <a href="#s4.8.2.1-variables-per-declaration"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
<p>Combined declarations such as <code class="prettyprint lang-java badcode">int a, b;</code> are
<strong>not</strong> used.</p><a name="s4.8.2.2-variables-limited-scope"/>
<h5>4.8.2.2 Declared when needed, initialized as soon as
possible <a href="#s4.8.2.2-variables-limited-scope"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
<p>Local variables are <strong>not</strong> habitually declared at the start of their containing
block or block-like construct. Instead, local variables are declared close to the point they are
first used (within reason), to minimize their scope. Local variable declarations typically have
initializers, or are initialized immediately after declaration.</p><a name="s4.8.3-arrays"/>
<h4>4.8.3 Arrays <a href="#s4.8.3-arrays"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<a name="s4.8.3.1-array-initializers"/>
<h5>4.8.3.1 Array initializers: can be "block-like" <a href="#s4.8.3.1-array-initializers"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
<p>Any array initializer may <em>optionally</em> be formatted as if it were a "block-like
construct." For example, the following are all valid (<strong>not</strong> an exhaustive
list):</p><pre class="prettyprint lang-java">
new int[] { new int[] {
0, 1, 2, 3 0,
} 1,
2,
new int[] { 3,
0, 1, }
2, 3
} new int[]
{0, 1, 2, 3}
</pre><a name="s4.8.3.2-array-declarations"/>
<h5>4.8.3.2 No C-style array declarations <a href="#s4.8.3.2-array-declarations"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
<p>The square brackets form a part of the <em>type</em>, not the variable:
<code class="prettyprint lang-java">String[] args</code>, not
<code class="prettyprint lang-java badcode">String args[]</code>.</p><a name="s4.8.4-switch"/>
<h4>4.8.4 Switch statements <a href="#s4.8.4-switch"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p class="terminology"><b>Terminology Note:</b> Inside the braces of a <em>switch block</em> are one or more
<em>statement groups</em>. Each statement group consists of one or more <em>switch labels</em>
(either <code class="prettyprint lang-java">case FOO:</code> or
<code class="prettyprint lang-java">default:</code>), followed by one or more statements.</p><a name="s4.8.4.1-switch-indentation"/>
<h5>4.8.4.1 Indentation <a href="#s4.8.4.1-switch-indentation"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
<p>As with any other block, the contents of a switch block are indented +2.</p><p>After a switch label, a newline appears, and the indentation level is increased +2, exactly as
if a block were being opened. The following switch label returns to the previous indentation
level, as if a block had been closed.</p><a name="fallthrough"/><a name="s4.8.4.2-switch-fall-through"/>
<h5>4.8.4.2 Fall-through: commented <a href="#s4.8.4.2-switch-fall-through"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
<p>Within a switch block, each statement group either terminates abruptly (with a
<code class="prettyprint lang-java">break</code>,
<code class="prettyprint lang-java">continue</code>,
<code class="prettyprint lang-java">return</code> or thrown exception), or is marked with a comment
to indicate that execution will or <em>might</em> continue into the next statement group. Any
comment that communicates the idea of fall-through is sufficient (typically
<code class="prettyprint lang-java">// fall through</code>). This special comment is not required in
the last statement group of the switch block. Example:</p><pre class="prettyprint lang-java">
switch (input) {
case 1:
case 2:
prepareOneOrTwo();
// fall through
case 3:
handleOneTwoOrThree();
break;
default:
handleLargeNumber(input);
}
</pre><a name="s4.8.4.3-switch-default"/>
<h5>4.8.4.3 The default case is present <a href="#s4.8.4.3-switch-default"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
<p>Each switch statement includes a <code class="prettyprint lang-java">default</code> statement
group, even if it contains no code.</p><a name="annotations"/><a name="s4.8.5-annotations"/>
<h4>4.8.5 Annotations <a href="#s4.8.5-annotations"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Annotations applying to a class, method or constructor appear immediately after the
documentation block, and each annotation is listed on a line of its own (that is, one annotation
per line). These line breaks do not constitute line-wrapping (Section
4.5, <a href="#s4.5-line-wrapping">Line-wrapping</a>), so the indentation level is not
increased. Example:</p><pre class="prettyprint lang-java">
@Override
@Nullable
public String getNameIfPresent() { ... }
</pre><p class="exception"><b>Exception:</b> a <em>single</em> parameterless annotation <em>may</em> instead appear
together with the first line of the signature, for example:</p><pre class="prettyprint lang-java">
@Override public int hashCode() { ... }
</pre><p>Annotations applying to a field also appear immediately after the documentation block, but in
this case, <em>multiple</em> annotations (possibly parameterized) may be listed on the same line;
for example:</p><pre class="prettyprint lang-java">
@Partial @Mock DataLoader loader;
</pre><p>There are no specific rules for formatting parameter and local variable annotations.</p><a name="comments"/><a name="s4.8.6-comments"/>
<h4>4.8.6 Comments <a href="#s4.8.6-comments"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<a name="s4.8.6.1-block-comment-style"/>
<h5>4.8.6.1 Block comment style <a href="#s4.8.6.1-block-comment-style"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
<p>Block comments are indented at the same level as the surrounding code. They may be in
<code class="prettyprint lang-java">/* ... */</code> style or
<code class="prettyprint lang-java">// ...</code> style. For multi-line
<code class="prettyprint lang-java">/* ... */</code> comments, subsequent lines must start with
<code>*</code> aligned with the <code>*</code> on the previous line.</p><pre class="prettyprint lang-java">
/*
* This is // And so /* Or you can
* okay. // is this. * even do this. */
*/
</pre><p>Comments are not enclosed in boxes drawn with asterisks or other characters.</p><p class="tip"><b>Tip:</b> When writing multi-line comments, use the
<code class="prettyprint lang-java">/* ... */</code> style if you want automatic code formatters to
re-wrap the lines when necessary (paragraph-style). Most formatters don't re-wrap lines in
<code class="prettyprint lang-java">// ...</code> style comment blocks.</p><a name="modifiers"/><a name="s4.8.7-modifiers"/>
<h4>4.8.7 Modifiers <a href="#s4.8.7-modifiers"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Class and member modifiers, when present, appear in the order
recommended by the Java Language Specification:
</p><pre>
public protected private abstract static final transient volatile synchronized native strictfp
</pre><a name="naming"/><a name="s5-naming"/>
<h2>5 Naming <a href="#s5-naming"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
<a name="s5.1-identifier-names"/>
<h3>5.1 Rules common to all identifiers <a href="#s5.1-identifier-names"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>Identifiers use only ASCII letters and digits, and in two cases noted below, underscores. Thus
each valid identifier name is matched by the regular expression <code>\w+</code> .</p><p> In Google Style special prefixes or
suffixes, like those seen in the examples <code class="prettyprint lang-java badcode">name_</code>,
<code class="prettyprint lang-java badcode">mName</code>,
<code class="prettyprint lang-java badcode">s_name</code> and
<code class="prettyprint lang-java badcode">kName</code>, are <strong>not</strong> used.</p><a name="s5.2-specific-identifier-names"/>
<h3>5.2 Rules by identifier type <a href="#s5.2-specific-identifier-names"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<a name="s5.2.1-package-names"/>
<h4>5.2.1 Package names <a href="#s5.2.1-package-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Package names are all lowercase, with consecutive words simply concatenated together (no
underscores).</p><a name="s5.2.2-class-names"/>
<h4>5.2.2 Class names <a href="#s5.2.2-class-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Class names are written in <a href="#s5.3-camel-case">UpperCamelCase</a>.</p><p>Class names are typically nouns or noun phrases. Interface names may sometimes be adjectives or
adjective phrases instead. There are no specific rules or even well-established conventions for
naming annotation types.</p><p><em>Test</em> classes are named starting with the name of the class they are testing, and ending
with <code class="prettyprint lang-java">Test</code>. For example,
<code class="prettyprint lang-java">HashTest</code> or
<code class="prettyprint lang-java">HashIntegrationTest</code>.</p><a name="s5.2.3-method-names"/>
<h4>5.2.3 Method names <a href="#s5.2.3-method-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Method names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p><p>Method names are typically verbs or verb phrases.</p><p>Underscores may appear in JUnit <em>test</em> method names to separate logical components of the
name. One typical pattern is <code>test<i><MethodUnderTest></i>_<i><state></i></code>,
for example <code class="prettyprint lang-java">testPop_emptyStack</code>. There is no One Correct
Way to name test methods.</p><a name="constants"/><a name="s5.2.4-constant-names"/>
<h4>5.2.4 Constant names <a href="#s5.2.4-constant-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Constant names use <code class="prettyprint lang-java">CONSTANT_CASE</code>: all uppercase
letters, with words separated by underscores. But what <em>is</em> a constant, exactly?</p><p>Every constant is a static final field, but not all static final fields are constants. Before
choosing constant case, consider whether the field really <em>feels like</em> a constant. For
example, if any of that instance's observable state can change, it is almost certainly not a
constant. Merely <em>intending</em> to never mutate the object is generally not
enough. Examples:</p><pre class="prettyprint lang-java">
// Constants
static final int NUMBER = 5;
static final ImmutableList<String> NAMES = ImmutableList.of("Ed", "Ann");
static final Joiner COMMA_JOINER = Joiner.on(','); // because Joiner is immutable
static final SomeMutableType[] EMPTY_ARRAY = {};
enum SomeEnum { ENUM_CONSTANT }
// Not constants
static String nonFinal = "non-final";
final String nonStatic = "non-static";
static final Set<String> mutableCollection = new HashSet<String>();
static final ImmutableSet<SomeMutableType> mutableElements = ImmutableSet.of(mutable);
static final Logger logger = Logger.getLogger(MyClass.getName());
static final String[] nonEmptyArray = {"these", "can", "change"};
</pre><p>These names are typically nouns or noun phrases.</p><a name="s5.2.5-non-constant-field-names"/>
<h4>5.2.5 Non-constant field names <a href="#s5.2.5-non-constant-field-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Non-constant field names (static or otherwise) are written
in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p><p>These names are typically nouns or noun phrases.</p><a name="s5.2.6-parameter-names"/>
<h4>5.2.6 Parameter names <a href="#s5.2.6-parameter-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Parameter names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p><p>One-character parameter names should be avoided.</p><a name="s5.2.7-local-variable-names"/>
<h4>5.2.7 Local variable names <a href="#s5.2.7-local-variable-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Local variable names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>, and can be
abbreviated more liberally than other types of names.</p><p>However, one-character names should be avoided, except for temporary and looping variables.</p><p>Even when final and immutable, local variables are not considered to be constants, and should not
be styled as constants.</p><a name="s5.2.8-type-variable-names"/>
<h4>5.2.8 Type variable names <a href="#s5.2.8-type-variable-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Each type variable is named in one of two styles:</p><ul><li>A single capital letter, optionally followed by a single numeral (such as
<code class="prettyprint lang-java">E</code>, <code class="prettyprint lang-java">T</code>,
<code class="prettyprint lang-java">X</code>, <code class="prettyprint lang-java">T2</code>)
</li><li>A name in the form used for classes (see Section 5.2.2,
<a href="#s5.2.2-class-names">Class names</a>), followed by the capital letter
<code class="prettyprint lang-java">T</code> (examples:
<code class="prettyprint lang-java">RequestT</code>,
<code class="prettyprint lang-java">FooBarT</code>).</li></ul><a name="acronyms"/><a name="camelcase"/><a name="s5.3-camel-case"/>
<h3>5.3 Camel case: defined <a href="#s5.3-camel-case"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>Sometimes there is more than one reasonable way to convert an English phrase into camel case,
such as when acronyms or unusual constructs like "IPv6" or "iOS" are present. To improve
predictability, Google Style specifies the following (nearly) deterministic scheme.</p><p>Beginning with the prose form of the name:</p><ol><li>Convert the phrase to plain ASCII and remove any apostrophes. For example, "Müller's
algorithm" might become "Muellers algorithm".</li><li>Divide this result into words, splitting on spaces and any remaining punctuation (typically
hyphens).
<ul><li><em>Recommended:</em> if any word already has a conventional camel-case appearance in common
usage, split this into its constituent parts (e.g., "AdWords" becomes "ad words"). Note
that a word such as "iOS" is not really in camel case <em>per se</em>; it defies <em>any</em>
convention, so this recommendation does not apply.</li></ul></li><li>Now lowercase <em>everything</em> (including acronyms), then uppercase only the first
character of:
<ul><li>... each word, to yield <em>upper camel case</em>, or</li><li>... each word except the first, to yield <em>lower camel case</em></li></ul></li><li>Finally, join all the words into a single identifier.</li></ol><p>Note that the casing of the original words is almost entirely disregarded. Examples:</p><table><tr><th>Prose form</th><th>Correct</th><th>Incorrect</th></tr><tr><td>"XML HTTP request"</td><td><code class="prettyprint lang-java">XmlHttpRequest</code></td><td><code class="prettyprint lang-java badcode">XMLHTTPRequest</code></td></tr><tr><td>"new customer ID"</td><td><code class="prettyprint lang-java">newCustomerId</code></td><td><code class="prettyprint lang-java badcode">newCustomerID</code></td></tr><tr><td>"inner stopwatch"</td><td><code class="prettyprint lang-java">innerStopwatch</code></td><td><code class="prettyprint lang-java badcode">innerStopWatch</code></td></tr><tr><td>"supports IPv6 on iOS?"</td><td><code class="prettyprint lang-java">supportsIpv6OnIos</code></td><td><code class="prettyprint lang-java badcode">supportsIPv6OnIOS</code></td></tr><tr><td>"YouTube importer"</td><td><code class="prettyprint lang-java">YouTubeImporter</code><br/><code class="prettyprint lang-java">YoutubeImporter</code>*</td><td/></tr></table><p>*Acceptable, but not recommended.</p><p class="note"><b>Note:</b> Some words are ambiguously hyphenated in the English language: for example
"nonempty" and "non-empty" are both correct, so the method names
<code class="prettyprint lang-java">checkNonempty</code> and
<code class="prettyprint lang-java">checkNonEmpty</code> are likewise both correct.</p><a name="s6-programming-practices"/>
<h2>6 Programming Practices <a href="#s6-programming-practices"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
<a name="s6.1-override-annotation"/>
<h3>6.1 @Override: always used <a href="#s6.1-override-annotation"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>The <code class="prettyprint lang-java">@Override</code> annotation is used in any context in
which it is legal.</p><a name="caughtexceptions"/><a name="s6.2-caught-exceptions"/>
<h3>6.2 Caught exceptions: not ignored <a href="#s6.2-caught-exceptions"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>Except as noted below, it is very rarely correct to do nothing in response to a caught
exception. (Typical responses are to log it, or if it is considered "impossible", rethrow it as an
<code class="prettyprint lang-java">AssertionError</code>.)</p><p>When it truly is appropriate to take no action whatsoever in a catch block, the reason this is
justified is explained in a comment.</p><pre class="prettyprint lang-java">
try {
int i = Integer.parseInt(response);
return handleNumericResponse(i);
} catch (NumberFormatException ok) {
// it's not numeric; that's fine, just continue
}
return handleTextResponse(response);
</pre><p class="exception"><b>Exception:</b> in tests, a caught exception may be ignored without comment <em>if</em> it is
named <code class="prettyprint lang-java">expected</code>. The following is a very common idiom
for ensuring that the method under test <em>does</em> throw an exception of the expected type, so
a comment is unnecessary here.</p><pre class="prettyprint lang-java">
try {
emptyStack.pop();
fail();
} catch (NoSuchElementException expected) {
}
</pre><a name="s6.3-static-members"/>
<h3>6.3 Static members: qualified using class <a href="#s6.3-static-members"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>When a reference to a static class member must be qualified, it is qualified with that class's
name, not with a reference or expression of that class's type.</p><pre class="prettyprint lang-java">
Foo aFoo = ...;
Foo.aStaticMethod(); // good
<span class="badcode">aFoo.aStaticMethod();</span> // bad
<span class="badcode">somethingThatYieldsAFoo().aStaticMethod();</span> // very bad
</pre><a name="finalizers"/><a name="s6.4-finalizers"/>
<h3>6.4 Finalizers: not used <a href="#s6.4-finalizers"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>It is <strong>extremely rare</strong> to override <code class="prettyprint lang-java">Object.finalize</code>.</p><p class="tip"><b>Tip:</b> Don't do it. If you absolutely must, first read and understand
<a href="http://books.google.com/books?isbn=8131726592"><em>Effective Java</em></a>
Item 7, "Avoid Finalizers," very carefully, and <em>then</em> don't do it.</p><a name="javadoc"/><a name="s7-javadoc"/>
<h2>7 Javadoc <a href="#s7-javadoc"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
<a name="s7.1-javadoc-formatting"/>
<h3>7.1 Formatting <a href="#s7.1-javadoc-formatting"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<a name="s7.1.1-javadoc-multi-line"/>
<h4>7.1.1 General form <a href="#s7.1.1-javadoc-multi-line"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>The <em>basic</em> formatting of Javadoc blocks is as seen in this general example:</p><pre class="prettyprint lang-java">
/**
* Multiple lines of Javadoc text are written here,
* wrapped normally...
*/
public int method(String p1) { ... }
</pre><p>... or in this single-line example:</p><pre class="prettyprint lang-java">
/** An especially short bit of Javadoc. */
</pre><p>The general form is always acceptable. The single-line form may be substituted when there are no
at-clauses present, and the entirety of the Javadoc block (including comment markers) can fit on a
single line.</p><a name="s7.1.2-javadoc-paragraphs"/>
<h4>7.1.2 Paragraphs <a href="#s7.1.2-javadoc-paragraphs"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>One blank line—that is, a line containing only the aligned leading asterisk
(<code>*</code>)—appears between paragraphs, and before the group of "at-clauses" if
present. Each paragraph but the first has <code><p></code> immediately before the first word,
with no space after.</p><a name="s7.1.3-javadoc-at-clauses"/>
<h4>7.1.3 At-clauses <a href="#s7.1.3-javadoc-at-clauses"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Any of the standard "at-clauses" that are used appear in the order <code>@param</code>,
<code>@return</code>, <code>@throws</code>, <code>@deprecated</code>, and these four types never
appear with an empty description. When an at-clause doesn't fit on a single line, continuation lines
are indented four (or more) spaces from the position of the <code>@</code>.
</p><a name="s7.2-summary-fragment"/>
<h3>7.2 The summary fragment <a href="#s7.2-summary-fragment"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>The Javadoc for each class and member begins with a brief <strong>summary fragment</strong>. This
fragment is very important: it is the only part of the text that appears in certain contexts such as
class and method indexes.</p><p>This is a fragment—a noun phrase or verb phrase, not a complete sentence. It does
<strong>not</strong> begin with <code class="badcode">A {@code Foo} is a...</code>, or
<code class="badcode">This method returns...</code>, nor does it form a complete imperative sentence
like <code class="badcode">Save the record.</code>. However, the fragment is capitalized and
punctuated as if it were a complete sentence.</p><p class="tip"><b>Tip:</b> A common mistake is to write simple Javadoc in the form
<code class="prettyprint lang-java badcode">/** @return the customer ID */</code>. This is
incorrect, and should be changed to
<code class="prettyprint lang-java">/** Returns the customer ID. */</code>.</p><a name="s7.3-javadoc-where-required"/>
<h3>7.3 Where Javadoc is used <a href="#s7.3-javadoc-where-required"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
<p>At the <em>minimum</em>, Javadoc is present for every
<code class="prettyprint lang-java">public</code> class, and every
<code class="prettyprint lang-java">public</code> or
<code class="prettyprint lang-java">protected</code> member of such a class, with a few exceptions
noted below.</p><a name="s7.3.1-javadoc-exception-self-explanatory"/>
<h4>7.3.1 Exception: self-explanatory methods <a href="#s7.3.1-javadoc-exception-self-explanatory"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Javadoc is optional for "simple, obvious" methods like
<code class="prettyprint lang-java">getFoo</code>, in cases where there <em>really and truly</em> is
nothing else worthwhile to say but "Returns the foo".</p><p>The test methods of a unit test class are perhaps the most common example of this exemption.
These methods can <em>usually</em> be named descriptively enough that no additional documentation is
needed.</p><p class="tip"><b>Tip:</b> <strong>Important:</strong> it is not appropriate to cite this exception to justify
omitting relevant information that a typical reader might need to know. For example, for a method
named <code class="prettyprint lang-java">getCanonicalName</code>, don't omit its documentation
(with the rationale that it would say only <code class="prettyprint lang-java badcode">/** Returns
the canonical name. */</code>) if a typical reader may have no idea what the term "canonical name"
means!</p><a name="s7.3.2-javadoc-exception-overrides"/>
<h4>7.3.2 Exception: overrides <a href="#s7.3.2-javadoc-exception-overrides"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Javadoc is not always present on a method that overrides a supertype method.
</p><a name="s7.3.3-javadoc-optional"/>
<h4>7.3.3 Optional javadoc <a href="#s7.3.3-javadoc-optional"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
<p>Classes and members that are not visible outside their package still have Javadoc <em>as
needed</em>. Whenever an implementation comment would be used to define the overall purpose or
behavior of a class, method or field, that comment is written as Javadoc instead. (It's more
uniform, and more tool-friendly.)</p></div></div> <hr/>
<div class="change">Last changed: December 19, 2013</div>
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