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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Google Java Style Guide</title>
<link rel="stylesheet" href="javaguide.css">
<script src="include/styleguide.js"></script>
<link rel="shortcut icon" href="/styleguide/favicon.ico">
<script src="https://cdn.jsdelivr.net/gh/google/code-prettify@master/loader/run_prettify.js"></script>
</head>
<body onload="initStyleGuide();">
<div id="content">
<h1>Google Java Style Guide</h1>
<div id="tocDiv" class="vertical_toc"></div>
<div class="main_body">
<h2 id="s1-introduction">1 Introduction</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>
<h3 id="s1.1-terminology">1.1 Terminology notes</h3>
<p>In this document, unless otherwise clarified:</p>
<ol>
<li>The term <em>class</em> is used inclusively to mean an "ordinary" class, record class, enum
class, interface or annotation type (<code class="prettyprint lang-java">@interface</code>).</li>
<li>The term <em>member</em> (of a class) is used inclusively to mean a nested class, field,
method, <em>or constructor</em>; that is, all top-level contents of a class except initializers
and comments.
</li><li>The term <em>comment</em> always refers to <em>implementation</em> comments. We do not
use the phrase "documentation comments", and instead use the common term "Javadoc."</li>
</ol>
<p>Other "terminology notes" will appear occasionally throughout the document.</p>
<h3 id="s1.2-guide-notes">1.2 Guide notes</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>
<h2 id="s2-source-file-basics">2 Source file basics</h2>
<h3 id="s2.1-file-name">2.1 File name</h3>
<p>For a source file containing classes, the file name consists of the case-sensitive name of the
top-level class (of which there is <a href="#s3.4.1-one-top-level-class">exactly one</a>), plus the
<code>.java</code> extension.</p>
<h3 id="s2.2-file-encoding">2.2 File encoding: UTF-8</h3>
<p>Source files are encoded in <strong>UTF-8</strong>.</p>
<h3 id="s2.3-special-characters">2.3 Special characters</h3>
<h4 id="s2.3.1-whitespace-characters">2.3.1 Whitespace characters</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>
<h4 id="s2.3.2-special-escape-sequences">2.3.2 Special escape sequences</h4>
<p>For any character that has a
<a href="http://docs.oracle.com/javase/tutorial/java/data/characters.html">
special escape sequence</a>
(<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">\s</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="badcode">\012</code>) or Unicode
(e.g. <code class="badcode">\u000a</code>) escape.</p>
<h4 id="s2.3.3-non-ascii-characters">2.3.3 Non-ASCII characters</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. The choice depends only on
which makes the code <strong>easier to read and understand</strong>, although Unicode escapes
outside string literals and comments are strongly discouraged.</p>
<p class="tip"><strong>Tip:</strong> 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>
<tbody><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="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>
</tbody></table>
<p class="tip"><strong>Tip:</strong> 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 id="filestructure"></a>
<h2 id="s3-source-file-structure">3 Source file structure</h2>
<div>
<p>An ordinary 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>
<p>A <code>package-info.java</code> file is the same, but without the top-level class.</p>
<p>A <code>module-info.java</code> file does not contain a package statement and replaces the
single top-level class with a module declaration, but otherwise follows the same structure.</p>
<h3 id="s3.1-copyright-statement">3.1 License or copyright information, if present</h3>
<p>If license or copyright information belongs in a file, it belongs here.</p>
<h3 id="s3.2-package-statement">3.2 Package statement</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: 100</a>) does not apply to package statements.</p>
<a id="imports"></a>
<h3 id="s3.3-import-statements">3.3 Import statements</h3>
<h4 id="s3.3.1-wildcard-imports">3.3.1 No wildcard imports</h4>
<p><strong>Wildcard imports</strong>, static or otherwise, <strong>are not used</strong>.</p>
<h4 id="s3.3.2-import-line-wrapping">3.3.2 No line-wrapping</h4>
<p>Import statements are <strong>not line-wrapped</strong>. The column limit (Section 4.4,
<a href="#s4.4-column-limit">Column limit: 100</a>) does not apply to import
statements.</p>
<h4 id="s3.3.3-import-ordering-and-spacing">3.3.3 Ordering and spacing</h4>
<p>Imports are ordered as follows:</p>
<ol>
<li>All static imports in a single block.</li>
<li>All non-static imports in a single block.</li>
</ol>
<p>If there are both static and non-static imports, a single blank line separates the two
blocks. There are no other blank lines between import statements.</p>
<p>Within each block 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, since '.'
sorts before ';'.)</p>
<h4 id="s3.3.4-import-class-not-static">3.3.4 No static import for classes</h4>
<p>Static import is not used for static nested classes. They are imported with
normal imports.</p>
<h3 id="s3.4-class-declaration">3.4 Class declaration</h3>
<a id="oneclassperfile"></a>
<h4 id="s3.4.1-one-top-level-class">3.4.1 Exactly one top-level class declaration</h4>
<p>Each top-level class resides in a source file of its own.</p>
<a id="s3.4.2-class-member-ordering"></a>
<h4 id="s3.4.2-ordering-class-contents">3.4.2 Ordering of class contents</h4>
<p>The order you choose for the members and initializers of your class can have a great effect on
learnability. However, there's no single correct recipe for how to do it; different classes may
order their contents in different ways.</p>
<p>What is important is that each class uses <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 id="overloads"></a>
<h5 id="s3.4.2.1-overloads-never-split">3.4.2.1 Overloads: never split</h5>
<p>Methods of a class that share the same name appear in a single contiguous group with no other
members in between. The same applies to multiple constructors (which always have the same name).
This rule applies even when modifiers such as <code class="prettyprint lang-java">static</code> or
<code class="prettyprint lang-java">private</code> differ between the methods.</p>
<h3 id="s3.5-module-declaration">3.5 Module declaration</h3>
<h4 id="s3.5.1-ordering-module-directives">3.5.1 Ordering and spacing of module directives</h4>
<p>Module directives are ordered as follows:</p>
<ol>
<li>All <code>requires</code> directives in a single block.</li>
<li>All <code>exports</code> directives in a single block.</li>
<li>All <code>opens</code> directives in a single block.</li>
<li>All <code>uses</code> directives in a single block.</li>
<li>All <code>provides</code> directives in a single block.</li>
</ol>
<p>A single blank line separates each block that is present.</p>
<h2 id="s4-formatting">4 Formatting</h2>
<p class="terminology"><strong>Terminology Note:</strong> <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 id="braces"></a>
<h3 id="s4.1-braces">4.1 Braces</h3>
<h4 id="s4.1.1-braces-always-used">4.1.1 Use of optional braces</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>
<p>Other optional braces, such as those in a lambda expression, remain optional.</p>
<h4 id="s4.1.2-blocks-k-r-style">4.1.2 Nonempty blocks: K & R style</h4>
<p>Braces follow the Kernighan and Ritchie style
("<a href="https://blog.codinghorror.com/new-programming-jargon/#3">Egyptian brackets</a>")
for <em>nonempty</em> blocks and block-like constructs:</p>
<ul>
<li>No line break before the opening brace, except as detailed below.</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>only if</em> that brace terminates a statement or
terminates 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>Exception: In places where these rules allow a single statement ending with a semicolon
(<code class="prettyprint lang-java">;</code>), a block of statements can appear, and the opening
brace of this block is preceded by a line break. Blocks like these are typically introduced to
limit the scope of local variables.</p>
<p>Examples:</p>
<pre class="prettyprint lang-java">return () -> {
while (condition()) {
method();
}
};
return new MyClass() {
@Override public void method() {
if (condition()) {
try {
something();
} catch (ProblemException e) {
recover();
}
} else if (otherCondition()) {
somethingElse();
} else {
lastThing();
}
{
int x = foo();
frob(x);
}
}
};
</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 id="emptyblocks"></a>
<h4 id="s4.1.3-braces-empty-blocks">4.1.3 Empty blocks: may be concise</h4>
<p>An empty block or block-like construct may be in K & R style (as described in
<a href="#s4.1.2-blocks-k-r-style">Section 4.1.2</a>). Alternatively, it may 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</code> or
<code class="prettyprint lang-java">try/catch/finally</code>).</p>
<p>Examples:</p>
<pre class="prettyprint lang-java"> // This is acceptable
void doNothing() {}
// This is equally acceptable
void doNothingElse() {
}
</pre>
<pre class="prettyprint lang-java badcode"> // This is not acceptable: No concise empty blocks in a multi-block statement
try {
doSomething();
} catch (Exception e) {}
</pre>
<h3 id="s4.2-block-indentation">4.2 Block indentation: +2 spaces</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>
<h3 id="s4.3-one-statement-per-line">4.3 One statement per line</h3>
<p>Each statement is followed by a line break.</p>
<a id="columnlimit"></a>
<h3 id="s4.4-column-limit">4.4 Column limit: 100</h3>
<p>Java code has a column limit of 100 characters. A "character" means any Unicode code point.
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 class="tip">Each Unicode code point counts as one character, even if its display width is
greater or less. For example, if using
<a href="https://en.wikipedia.org/wiki/Halfwidth_and_fullwidth_forms">fullwidth characters</a>,
you may choose to wrap the line earlier than where this rule strictly requires.</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>Contents of <a href="s4.8.9-text-blocks">text blocks</a>.</li>
<li>Command lines in a comment that may be copied-and-pasted into a shell.</li>
<li>Very long identifiers, on the rare occasions they are called for, are allowed to exceed the
column limit. In that case, the valid wrapping for the surrounding code is as produced by
<a href="https://github.com/google/google-java-format">google-java-format</a>.
</li>
</ol>
<h3 id="s4.5-line-wrapping">4.5 Line-wrapping</h3>
<p class="terminology"><strong>Terminology Note:</strong> When code that might otherwise legally
occupy a single line is divided into multiple lines, 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="note"><strong>Note:</strong> While the typical reason for line-wrapping is to avoid
overflowing the column limit, even code that would in fact fit within the column limit <em>may</em>
be line-wrapped at the author's discretion.</p>
<p class="tip"><strong>Tip:</strong> Extracting a method or local variable may solve the problem
without the need to line-wrap.</p>
<h4 id="s4.5.1-line-wrapping-where-to-break">4.5.1 Where to break</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:
<ul>
<li>the dot separator (<code class="prettyprint lang-java">.</code>)</li>
<li>the two colons of a method reference
(<code class="prettyprint lang-java">::</code>)</li>
<li>an ampersand in a type bound
(<code class="prettyprint lang-java"><T extends Foo & Bar></code>)</li>
<li>a pipe in a catch block
(<code class="prettyprint lang-java">catch (FooException | BarException e)</code>).</li>
</ul>
</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, constructor, or record-class 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>
<li>A line is never broken adjacent to the arrow in a lambda or a switch rule, except that a
break may come immediately after the arrow if the text following it consists of a single unbraced
expression. Examples:
<pre class="prettyprint lang-java">MyLambda<String, Long, Object> lambda =
(String label, Long value, Object obj) -> {
...
};
Predicate<String> predicate = str ->
longExpressionInvolving(str);
switch (x) {
case ColorPoint(Color color, Point(int x, int y)) ->
handleColorPoint(color, x, y);
...
}
</pre>
</li>
</ol>
<p class="note"><strong>Note:</strong> The primary goal for line wrapping is to have clear
code, <em>not necessarily</em> code that fits in the smallest number of lines.</p>
<a id="indentation"></a>
<h4 id="s4.5.2-line-wrapping-indent">4.5.2 Indent continuation lines at least +4 spaces</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>
<h3 id="s4.6-whitespace">4.6 Whitespace</h3>
<h4 id="s4.6.1-vertical-whitespace">4.6.1 Vertical Whitespace</h4>
<p>A single blank line always appears:</p>
<ol>
<li><em>Between</em> consecutive members or initializers of a class: fields, constructors,
methods, nested classes, static initializers, and instance initializers.
<ul>
<li><span class="exception"><strong>Exception:</strong> 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>
<li><span class="exception"><strong>Exception:</strong> Blank lines between enum constants are
covered in <a href="#s4.8.1-enum-classes">Section 4.8.1</a>.</span></li>
</ul>
</li>
<li>As required by other sections of this document (such as Section 3,
<a href="#s3-source-file-structure">Source file structure</a>, and Section 3.3,
<a href="#s3.3-import-statements">Import statements</a>).</li>
</ol>
<p>A single blank line may also appear anywhere it improves readability, for example between
statements to organize the code into logical subsections. A blank line before the first member or
initializer, or after the last member or initializer of the class, is neither encouraged nor
discouraged.
</p><p><em>Multiple</em> consecutive blank lines are permitted, but never required (or encouraged).</p>
<h4 id="s4.6.2-horizontal-whitespace">4.6.2 Horizontal whitespace</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 9 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>
<li>the arrow in a lambda expression:
<code class="prettyprint lang-java">(String str) -> str.length()</code><br>
or switch rule:
<code class="prettyprint lang-java">case "FOO" -> bar();</code></li>
</ul>
but not
<ul>
<li>the two colons (<code class="prettyprint lang-java">::</code>) of a method reference, which
is written like <code class="prettyprint lang-java">Object::toString</code></li>
<li>the dot separator (<code class="prettyprint lang-java">.</code>), which is written like
<code class="prettyprint lang-java">object.toString()</code></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>Between any content and a double slash (<code class="prettyprint lang-java">//</code>) which
begins a comment. Multiple spaces are allowed.</li>
<li>Between a double slash (<code class="prettyprint lang-java">//</code>) which begins a comment
and the comment's text. Multiple spaces are allowed.</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>
<li>Between a type annotation and <code class="prettyprint lang-java">[]</code> or
<code class="prettyprint lang-java">...</code>.</li>
</ol>
<p>This rule is never interpreted as requiring or forbidding additional space at the start or
end of a line; it addresses only <em>interior</em> space.</p>
<h4 id="s4.6.3-horizontal-alignment">4.6.3 Horizontal alignment: never required</h4>
<p class="terminology"><strong>Terminology Note:</strong> <em>Horizontal alignment</em> 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"><strong>Tip:</strong> Alignment can aid readability, but attempts to preserve
alignment for its own sake create future problems. For example, consider a change that touches only
one line. If that change disrupts the previous alignment, it's important **not** to introduce
additional changes on nearby lines simply to realign them. Introducing formatting changes on
otherwise unaffected lines corrupts version history, slows down reviewers, and exacerbates merge
conflicts. These practical concerns
take priority over alignment.</p>
<a id="parentheses"></a>
<h3 id="s4.7-grouping-parentheses">4.7 Grouping parentheses: recommended</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>
<h3 id="s4.8-specific-constructs">4.8 Specific constructs</h3>
<h4 id="s4.8.1-enum-classes">4.8.1 Enum classes</h4>
<p>After each comma that follows an enum constant, a line break is optional. Additional blank
lines (usually just one) are also allowed. This is one possibility:
</p><pre class="prettyprint lang-java">private enum Answer {
YES {
@Override public String toString() {
return "yes";
}
},
NO,
MAYBE
}
</pre>
<p>An enum class with no methods and no documentation on its constants may optionally be formatted
as if it were an array initializer (see Section 4.8.3.1 on
<a href="#s4.8.3.1-array-initializers">array initializers</a>).</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 id="localvariables"></a>
<h4 id="s4.8.2-variable-declarations">4.8.2 Variable declarations</h4>
<h5 id="s4.8.2.1-variables-per-declaration">4.8.2.1 One variable per declaration</h5>
<p>Every variable declaration (field or local) declares only one variable: declarations such as
<code class="badcode">int a, b;</code> are not used.</p>
<p><strong>Exception:</strong> Multiple variable declarations are acceptable in the header of a
<code class="prettyprint lang-java">for</code> loop.</p>
<h5 id="s4.8.2.2-variables-limited-scope">4.8.2.2 Declared when needed</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>
<h4 id="s4.8.3-arrays">4.8.3 Arrays</h4>
<h5 id="s4.8.3.1-array-initializers">4.8.3.1 Array initializers: can be "block-like"</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>
<h5 id="s4.8.3.2-array-declarations">4.8.3.2 No C-style array declarations</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="badcode">String args[]</code>.</p>
<h4 id="s4.8.4-switch">4.8.4 Switch statements and expressions</h4>
<p>For historical reasons, the Java language has two distinct syntaxes for <code class="prettyprint lang-java">switch</code>, which we can call <em>old-style</em> and
<em>new-style</em>. New-style switches use an arrow
(<code class="prettyprint lang-java">-></code>) after the switch labels, while old-style switches
use a colon (<code class="prettyprint lang-java">:</code>).
</p><p class="terminology"><strong>Terminology Note:</strong> Inside the braces of a
<em>switch block</em> are either one or more <em>switch rules</em> (new-style);
or one or more <em>statement groups</em> (old-style). A <em>switch
rule</em> consists of a <em>switch label</em> (<code class="prettyprint lang-java">case ...</code>
or <code class="prettyprint lang-java">default</code>) followed by <code class="prettyprint lang-java">-></code> and an expression, block, or <code class="prettyprint
lang-java">throw</code>. A statement group consists of one or more switch labels each followed by
a colon, then one or more statements, or, for the <em>last</em> statement group, <em>zero</em> or
more statements. (These definitions match the Java Language Specification,
<a href="https://docs.oracle.com/javase/specs/jls/se21/html/jls-14.html#jls-14.11">§14.11</a>.)</p>
<h5 id="s4.8.4.1-switch-indentation">4.8.4.1 Indentation</h5>
<p>As with any other block, the contents of a switch block are indented +2. Each switch label
starts with this +2 indentation.</p>
<p>In a new-style switch, a switch rule can be written on a single line if it otherwise follows
Google style. (It must not exceed the column limit, and if it contains a non-empty block then
there must be a line break after <code class="prettyprint lang-java">{</code>.) The line-wrapping
rules of <a href="#s4.5-line-wrapping">Section 4.5</a> apply, including the +4 indent for
continuation lines. For a switch rule with a non-empty block after the arrow, the same rules apply
as for blocks elsewhere: lines between <code class="prettyprint lang-java">{</code> and
<code class="prettyprint lang-java">}</code> are indented a further +2 relative to the line with the
switch label.
</p><pre class="prettyprint lang-java">switch (number) {
case 0, 1 -> handleZeroOrOne();
case 2 ->
handleTwoWithAnExtremelyLongMethodCallThatWouldNotFitOnTheSameLine();
default -> {
logger.atInfo().log("Surprising number %s", number);
handleSurprisingNumber(number);
}
}
</pre>
<p>In an old-style switch, the colon of each switch label is followed by a line break. The
statements within a statement group start with a further +2 indentation.</p>
<a id="fallthrough"></a>
<h5 id="s4.8.4.2-switch-fall-through">4.8.4.2 Fall-through: commented</h5>
<p>Within an old-style 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>
<p>Notice that no comment is needed after <code class="prettyprint lang-java">case 1:</code>, only
at the end of the statement group.</p>
<p>There is no fall-through in new-style switches.</p>
<h5 id="s4.8.4.3-switch-default">4.8.4.3 Exhaustiveness and presence of the <code>default</code> label</h5>
<p>The Java language requires switch expressions and many kinds of switch statements to be
<em>exhaustive</em>. That effectively means that every possible value that could be switched on will
be matched by one of the switch labels. A switch is exhaustive if it has a <code class="prettyprint lang-java">default</code> label, but also for example if the value being switched
on is an enum and every value of the enum is matched by a switch label. Google Style requires
<em>every</em> switch to be exhaustive, even those where the language itself does not require it.
This may require adding a <code class="prettyprint lang-java">default</code> label, even if it
contains no code.</p>
<h5 id="s4.8.4.4-switch-expressions">4.8.4.4 Switch expressions</h5>
<p>Switch expressions must be new-style switches:</p>
<pre class="prettyprint lang-java"> return switch (list.size()) {
case 0 -> "";
case 1 -> list.getFirst();
default -> String.join(", ", list);
};
</pre>
<a id="annotations"></a>
<h4 id="s4.8.5-annotations">4.8.5 Annotations</h4>
<h5 id="s4.8.5.1-type-use-annotation-style">4.8.5.1 Type-use annotations</h5>
<p>Type-use annotations appear immediately before the annotated type. An annotation is a type-use
annotation if it is meta-annotated with
<code class="prettyprint lang-java">@Target(ElementType.TYPE_USE)</code>. Example:</p>
<pre class="prettyprint lang-java">final @Nullable String name;
public @Nullable Person getPersonByName(String name);
</pre>
<h5 id="s4.8.5.2-class-annotation-style">4.8.5.2 Class, package, and module annotations</h5>
<p>Annotations applying to a class, package, or module declaration 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. Examples:</p>
<pre class="prettyprint lang-java">/** This is a class. */
@Deprecated
@CheckReturnValue
public final class Frozzler { ... }
</pre>
<pre class="prettyprint lang-java">/** This is a package. */
@Deprecated
@CheckReturnValue
package com.example.frozzler;
</pre>
<pre class="prettyprint lang-java">/** This is a module. */
@Deprecated
@SuppressWarnings("CheckReturnValue")
module com.example.frozzler { ... }
</pre>
<h5 id="s4.8.5.3-method-annotation-style">4.8.5.3 Method and constructor annotations</h5>
<p>The rules for annotations on method and constructor declarations are the same as the
<a href="#s4.8.5.2-class-annotation-style">previous section</a>. Example: </p>
<pre class="prettyprint lang-java">@Deprecated
@Override
public String getNameIfPresent() { ... }
</pre>
<p class="exception"><strong>Exception:</strong> 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>
<h5 id="s4.8.5.4-field-annotation-style">4.8.5.4 Field annotations</h5>
<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>
<h5 id="s4.8.5.5-local-parameter-annotation-style">4.8.5.5 Parameter and local variable annotations</h5>
<p>There are no specific rules for formatting annotations on parameters or local variables (except,
of course, when the annotation is a type-use annotation).</p>
<a id="comments"></a>
<h4 id="s4.8.6-comments">4.8.6 Comments</h4>
<p>This section addresses <em>implementation comments</em>. Javadoc is addressed separately in
Section 7, <a href="#s7-javadoc">Javadoc</a>.</p>
<p>Any line break may be preceded by arbitrary whitespace followed by an implementation comment.
Such a comment renders the line non-blank.</p>
<h5 id="s4.8.6.1-block-comment-style">4.8.6.1 Block comment style</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"><strong>Tip:</strong> 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 id="todo"></a>
<h5 id="s4.8.6.2-todo-comments">4.8.6.2 TODO comments</h5>
<div>
<p>Use <code>TODO</code> comments for code that is temporary, a short-term solution, or good-enough
but not perfect.</p>
<p>A <code>TODO</code> comment begins with the word <code>TODO</code> in all caps, a following
colon, and a link to a resource that contains the context, ideally a bug reference. A bug
reference is preferable because bugs are tracked and have follow-up comments. Follow this piece of
context with an explanatory string introduced with a hyphen <code>-</code>.</p>
<p>The purpose is to have a consistent <code>TODO</code> format that can be searched to find out how
to get more details.</p>
<pre class="good">// TODO: crbug.com/12345678 - Remove this after the 2047q4 compatibility window expires.
</pre>
<p>Avoid adding TODOs that refer to an individual or team as the context:</p>
<pre class="bad">// TODO: @yourusername - File an issue and use a '*' for repetition.
</pre>
<p>If your <code>TODO</code> is of the form "At a future date do something" make sure that you
either include a very specific date ("Fix by November 2005") or a very specific event ("Remove
this code when all clients can handle XML responses.").</p>
</div>
<a id="modifiers"></a>
<h4 id="s4.8.7-modifiers">4.8.7 Modifiers</h4>
<p>Class and member modifiers, when present, appear in the order
recommended by the Java Language Specification:
</p>
<pre>public protected private abstract default static final sealed non-sealed
transient volatile synchronized native strictfp
</pre>
<p>Modifiers on <code>requires</code> module directives, when present, appear in the following
order:</p>
<pre>transitive static</pre>
<h4 id="s4.8.8-numeric-literals">4.8.8 Numeric Literals</h4>
<p><code>long</code>-valued integer literals use an uppercase <code>L</code> suffix, never
lowercase (to avoid confusion with the digit <code>1</code>). For example, <code>3000000000L</code>
rather than <code class="badcode">3000000000l</code>.</p>
<h4 id="s4.8.9-text-blocks">4.8.9 Text Blocks</h4>
<p>The opening <code>"""</code> of a text block is always on a new line. That line may
either follow the same indentation rules as other constructs, or it may have no indentation at all
(so it starts at the left margin). The closing <code>"""</code> is on a new line
with the same indentation as the opening <code>"""</code>, and may be followed on the
same line by further code. Each line of text in the text block is indented at least as much as the
opening and closing <code>"""</code>. (If a line is indented further, then the string
literal defined by the text block will have space at the start of that line.)
</p><p>The contents of a text block may exceed the <a href="#columnlimit">column limit</a>.
<a id="naming"></a>
</p><h2 id="s5-naming">5 Naming</h2>
<h3 id="s5.1-identifier-names">5.1 Rules common to all identifiers</h3>
<p>Identifiers use only ASCII letters and digits, and, in a small number of 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 are <strong>not</strong> used. For example, these
names are not Google Style: <code class="badcode">name_</code>, <code class="badcode">mName</code>,
<code class="badcode">s_name</code> and <code class="badcode">kName</code>.</p>