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1410 lines
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1410 lines
58 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="utf-8">
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<title>Google Java Style Guide</title>
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<link rel="stylesheet" href="javaguide.css">
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<script src="include/styleguide.js"></script>
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<link rel="shortcut icon" href="/styleguide/favicon.ico">
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<script src="https://cdn.jsdelivr.net/gh/google/code-prettify@master/loader/run_prettify.js"></script>
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</head>
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<body onload="initStyleGuide();">
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<div id="content">
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<h1>Google Java Style Guide</h1>
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<div id="tocDiv" class="vertical_toc"></div>
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<div class="main_body">
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<h2 id="s1-introduction">1 Introduction</h2>
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<p>This document serves as the <strong>complete</strong> definition of Google's coding standards for
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source code in the Java™ Programming Language. A Java source file is described as being <em>in
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Google Style</em> if and only if it adheres to the rules herein.</p>
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<p>Like other programming style guides, the issues covered span not only aesthetic issues of
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formatting, but other types of conventions or coding standards as well. However, this document
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focuses primarily on the <strong>hard-and-fast rules</strong> that we follow universally, and
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avoids giving <em>advice</em> that isn't clearly enforceable (whether by human or tool).
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</p>
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<h3 id="s1.1-terminology">1.1 Terminology notes</h3>
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<p>In this document, unless otherwise clarified:</p>
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<ol>
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<li>The term <em>class</em> is used inclusively to mean a normal class, record class, enum
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class, interface or annotation type (<code class="prettyprint lang-java">@interface</code>).</li>
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<li>The term <em>member</em> (of a class) is used inclusively to mean a nested class, field,
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method, <em>or constructor</em>; that is, all top-level contents of a class except initializers.
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</li><li>The term <em>comment</em> always refers to <em>implementation</em> comments. We do not
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use the phrase "documentation comments", and instead use the common term "Javadoc."</li>
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</ol>
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<p>Other "terminology notes" will appear occasionally throughout the document.</p>
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<h3 id="s1.2-guide-notes">1.2 Guide notes</h3>
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<p>Example code in this document is <strong>non-normative</strong>. That is, while the examples
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are in Google Style, they may not illustrate the <em>only</em> stylish way to represent the
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code. Optional formatting choices made in examples should not be enforced as rules.</p>
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<h2 id="s2-source-file-basics">2 Source file basics</h2>
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<h3 id="s2.1-file-name">2.1 File name</h3>
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<p>For a source file containing classes, the file name consists of the case-sensitive name of the
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top-level class (of which there is <a href="#s3.4.1-one-top-level-class">exactly one</a>), plus the
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<code>.java</code> extension.</p>
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<h3 id="s2.2-file-encoding">2.2 File encoding: UTF-8</h3>
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<p>Source files are encoded in <strong>UTF-8</strong>.</p>
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<h3 id="s2.3-special-characters">2.3 Special characters</h3>
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<h4 id="s2.3.1-whitespace-characters">2.3.1 Whitespace characters</h4>
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<p>Aside from the line terminator sequence, the <strong>ASCII horizontal space
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character</strong> (<strong>0x20</strong>) is the only whitespace character that appears
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anywhere in a source file. This implies that:</p>
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<ol>
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<li>All other whitespace characters are escaped in <code>char</code> and string literals and in
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text blocks.</li>
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<li>Tab characters are <strong>not</strong> used for indentation.</li>
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</ol>
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<h4 id="s2.3.2-special-escape-sequences">2.3.2 Special escape sequences</h4>
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<p>For any character that has a
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<a href="http://docs.oracle.com/javase/tutorial/java/data/characters.html">
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special escape sequence</a>
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(<code class="prettyprint lang-java">\b</code>,
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<code class="prettyprint lang-java">\t</code>,
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<code class="prettyprint lang-java">\n</code>,
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<code class="prettyprint lang-java">\f</code>,
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<code class="prettyprint lang-java">\r</code>,
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<code class="prettyprint lang-java">\s</code>,
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<code class="prettyprint lang-java">\"</code>,
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<code class="prettyprint lang-java">\'</code> and
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<code class="prettyprint lang-java">\\</code>), that sequence
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is used rather than the corresponding octal
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(e.g. <code class="badcode">\012</code>) or Unicode
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(e.g. <code class="badcode">\u000a</code>) escape.</p>
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<h4 id="s2.3.3-non-ascii-characters">2.3.3 Non-ASCII characters</h4>
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<p>For the remaining non-ASCII characters, either the actual Unicode character
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(e.g. <code class="prettyprint lang-java">∞</code>) or the equivalent Unicode escape
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(e.g. <code class="prettyprint lang-java">\u221e</code>) is used. The choice depends only on
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which makes the code <strong>easier to read and understand</strong>, although Unicode escapes
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outside string literals and comments are strongly discouraged.</p>
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<p class="tip"><strong>Tip:</strong> In the Unicode escape case, and occasionally even when actual
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Unicode characters are used, an explanatory comment can be very helpful.</p>
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<p>Examples:</p>
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<table>
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<tbody><tr>
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<th>Example</th>
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<th>Discussion</th>
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</tr>
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<tr>
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<td><code class="prettyprint lang-java">String unitAbbrev = "μs";</code></td>
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<td>Best: perfectly clear even without a comment.</td>
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</tr>
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<tr>
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<td><code class="prettyprint lang-java">String unitAbbrev = "\u03bcs"; // "μs"</code></td>
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<td>Allowed, but there's no reason to do this.</td>
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</tr>
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<tr>
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<td><code class="prettyprint lang-java">String unitAbbrev = "\u03bcs";
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// Greek letter mu, "s"</code></td>
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<td>Allowed, but awkward and prone to mistakes.</td>
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</tr>
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<tr>
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<td><code class="badcode">String unitAbbrev = "\u03bcs";</code></td>
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<td>Poor: the reader has no idea what this is.</td>
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</tr>
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<tr>
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<td><code class="prettyprint lang-java">return '\ufeff' + content;
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// byte order mark</code></td>
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<td>Good: use escapes for non-printable characters, and comment if necessary.</td>
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</tr>
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</tbody></table>
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<p class="tip"><strong>Tip:</strong> Never make your code less readable simply out of fear that
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some programs might not handle non-ASCII characters properly. If that should happen, those
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programs are <strong>broken</strong> and they must be <strong>fixed</strong>.</p>
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<a id="filestructure"></a>
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<h2 id="s3-source-file-structure">3 Source file structure</h2>
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<div>
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<p>An ordinary source file consists of these sections, <strong>in order</strong>:</p>
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<ol>
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<li>License or copyright information, if present</li>
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<li>Package declaration</li>
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<li>Imports</li>
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<li>Exactly one top-level class declaration</li>
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</ol>
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</div>
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<p><strong>Exactly one blank line</strong> separates each section that is present.</p>
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<p>A <code>package-info.java</code> file is the same, but without the class declaration.</p>
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<p>A <code>module-info.java</code> file does not contain a package declaration and replaces the
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class declaration with a module declaration, but otherwise follows the same structure.</p>
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<h3 id="s3.1-copyright-statement">3.1 License or copyright information, if present</h3>
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<p>If license or copyright information belongs in a file, it belongs here.</p>
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<a id="s3.2-package-statement"></a>
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<h3 id="s3.2-package-declaration">3.2 Package declaration</h3>
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<p>Every source file must have a package declaration. <a href="https://openjdk.org/jeps/512">
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Compact source files</a> are not used. (This rule obviously does not apply to
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<code>module-info.java</code> files, which have a different syntax that does not include a
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package declaration.)
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</p><p>The package declaration is <strong>not line-wrapped</strong>. The column limit (Section 4.4,
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<a href="#s4.4-column-limit">Column limit: 100</a>) does not apply to package declarations.</p>
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<a id="imports"></a>
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<h3 id="s3.3-import-statements">3.3 Imports</h3>
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<h4 id="s3.3.1-wildcard-imports">3.3.1 No wildcard imports</h4>
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<p><strong>Wildcard ("on-demand") imports</strong>, static or otherwise, <strong>are not
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used</strong>.</p>
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<h4 id="s3.3.1.1-module-imports">3.3.1.1 No module imports</h4>
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<p><a href="https://docs.oracle.com/en/java/javase/25/language/module-import-declarations.html">
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Module imports</a> <strong>are not used</strong>.</p>
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<p>Example:</p>
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<pre class="prettyprint lang-java">import module java.base;
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</pre>
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<h4 id="s3.3.2-import-line-wrapping">3.3.2 No line-wrapping</h4>
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<p>Imports are <strong>not line-wrapped</strong>. The column limit (Section 4.4,
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<a href="#s4.4-column-limit">Column limit: 100</a>) does not apply to imports.</p>
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<h4 id="s3.3.3-import-ordering-and-spacing">3.3.3 Ordering and spacing</h4>
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<p>Imports are ordered as follows:</p>
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<ol>
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<li>All static imports in a single group.</li>
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<li>All non-static imports in a single group.</li>
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</ol>
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<p>If there are both static and non-static imports, a single blank line separates the two
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groups. There are no other blank lines between imports.</p>
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<p>Within each group the imported names appear in ASCII sort order. (<strong>Note:</strong>
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this is not the same as the import <em>lines</em> being in ASCII sort order, since '.'
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sorts before ';'.)</p>
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<h4 id="s3.3.4-import-class-not-static">3.3.4 No static import for classes</h4>
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<p>Static import is not used for static nested classes. They are imported with
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normal imports.</p>
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<h3 id="s3.4-class-declaration">3.4 Class declaration</h3>
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<a id="oneclassperfile"></a>
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<h4 id="s3.4.1-one-top-level-class">3.4.1 Exactly one top-level class declaration</h4>
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<p>Each top-level class resides in a source file of its own.</p>
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<a id="s3.4.2-class-member-ordering"></a>
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<h4 id="s3.4.2-ordering-class-contents">3.4.2 Ordering of class contents</h4>
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<p>The order you choose for the members and initializers of your class can have a great effect on
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learnability. However, there's no single correct recipe for how to do it; different classes may
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order their contents in different ways.</p>
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<p>What is important is that each class uses <strong><em>some</em> logical order</strong>, which its
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maintainer could explain if asked. For example, new methods are not just habitually added to the end
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of the class, as that would yield "chronological by date added" ordering, which is not a logical
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ordering.</p>
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<a id="overloads"></a>
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<h5 id="s3.4.2.1-overloads-never-split">3.4.2.1 Overloads: never split</h5>
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<p>Methods of a class that share the same name appear in a single contiguous group with no other
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members in between. The same applies to multiple constructors. This rule applies even when
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modifiers such as <code class="prettyprint lang-java">static</code> or
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<code class="prettyprint lang-java">private</code> differ between the methods or constructors.</p>
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<h3 id="s3.5-module-declaration">3.5 Module declaration</h3>
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<h4 id="s3.5.1-ordering-module-directives">3.5.1 Ordering and spacing of module directives</h4>
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<p>Module directives are ordered as follows:</p>
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<ol>
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<li>All <code>requires</code> directives in a single block.</li>
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<li>All <code>exports</code> directives in a single block.</li>
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<li>All <code>opens</code> directives in a single block.</li>
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<li>All <code>uses</code> directives in a single block.</li>
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<li>All <code>provides</code> directives in a single block.</li>
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</ol>
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<p>A single blank line separates each block that is present.</p>
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<h2 id="s4-formatting">4 Formatting</h2>
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<p class="terminology"><strong>Terminology Note:</strong> <em>block-like construct</em> refers to
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the body of a class, method, constructor, or switch. Note that, by Section 4.8.3.1 on
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<a href="#s4.8.3.1-array-initializers">array initializers</a>, any array initializer
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<em>may</em> optionally be treated as if it were a block-like construct.</p>
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<a id="braces"></a>
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<h3 id="s4.1-braces">4.1 Braces</h3>
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<h4 id="s4.1.1-braces-always-used">4.1.1 Use of optional braces</h4>
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<p>Braces are used with
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<code class="prettyprint lang-java">if</code>,
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<code class="prettyprint lang-java">else</code>,
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<code class="prettyprint lang-java">for</code>,
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<code class="prettyprint lang-java">do</code> and
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<code class="prettyprint lang-java">while</code> statements, even when the
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body is empty or contains only a single statement.</p>
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<p>Other optional braces, such as those in a lambda expression, remain optional.</p>
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<h4 id="s4.1.2-blocks-k-r-style">4.1.2 Nonempty blocks: K & R style</h4>
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<p>Braces follow the Kernighan and Ritchie style for <em>nonempty</em> blocks and block-like
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constructs:</p>
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<ul>
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<li>No line break before the opening brace, except as detailed below.</li>
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<li>Line break after the opening brace.</li>
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<li>Line break before the closing brace.</li>
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<li>Line break after the closing brace, <em>only if</em> that brace terminates a statement or
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terminates the body of a method, constructor, or <em>named</em> class.
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For example, there is <em>no</em> line break after the brace if it is followed by
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<code class="prettyprint lang-java">else</code> or a comma.</li>
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</ul>
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<p>Exception: In places where these rules allow a single statement ending with a semicolon
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(<code class="prettyprint lang-java">;</code>), a block of statements can appear, and the opening
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brace of this block is preceded by a line break. Blocks like these are typically introduced to
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limit the scope of local variables.</p>
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<p>Examples:</p>
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<pre class="prettyprint lang-java">return () -> {
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while (condition()) {
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method();
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}
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};
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return new MyClass() {
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||
@Override public void method() {
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if (condition()) {
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try {
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something();
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} catch (ProblemException e) {
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recover();
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}
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} else if (otherCondition()) {
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somethingElse();
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} else {
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lastThing();
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}
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{
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int x = foo();
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frob(x);
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}
|
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}
|
||
};
|
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</pre>
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|
||
<p>A few exceptions for enum classes are given in Section 4.8.1,
|
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<a href="#s4.8.1-enum-classes">Enum classes</a>.</p>
|
||
|
||
<a id="emptyblocks"></a>
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<h4 id="s4.1.3-braces-empty-blocks">4.1.3 Empty blocks: may be concise</h4>
|
||
|
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<p>An empty block or block-like construct may be in K & R style (as described in
|
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<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() {}
|
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|
||
// This is equally acceptable
|
||
void doNothingElse() {
|
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}
|
||
</pre>
|
||
<pre class="prettyprint lang-java badcode"> // This is not acceptable: No concise empty blocks in a multi-block statement
|
||
try {
|
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doSomething();
|
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} catch (Exception e) {}
|
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</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> declarations and
|
||
imports (see Sections 3.2 <a href="#s3.2-package-statement">Package declarations</a> and
|
||
3.3 <a href="#s3.3-import-statements">Imports</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
|
||
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 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 (blank lines)</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">Imports</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 within literals,
|
||
comments and Javadoc, a single ASCII space also appears in the following places
|
||
<strong>only</strong>.</p>
|
||
|
||
<ol>
|
||
<li>Separating any keyword, 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 keyword, 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 10 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 that separates multiple type bounds:
|
||
<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 identifier 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 attempting to preserve
|
||
alignment for its own sake creates 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 the 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 are indented a further +2.</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") // TODO(b/123): Fix existing CRV violations.
|
||
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> If the method or constructor only has a
|
||
<em>single</em>, <em>parameterless</em> annotation, it <em>may</em> 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>
|
||
|
||
|
||
|
||
<h3 id="s5.2-specific-identifier-names">5.2 Rules by identifier type</h3>
|
||
|
||
<h4 id="s5.2.1-package-names">5.2.1 Package and module names</h4>
|
||
|
||
<p>Package and module names use only lowercase letters and digits (no underscores). Consecutive
|
||
words are simply concatenated together. For example, <code>com.example.deepspace</code>, not
|
||
<code class="badcode">com.example.deepSpace</code> or
|
||
<code class="badcode">com.example.deep_space</code>.</p>
|
||
|
||
<h4 id="s5.2.2-class-names">5.2.2 Class names</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. For example,
|
||
<code class="prettyprint lang-java">Character</code> or
|
||
<code class="prettyprint lang-java">ImmutableList</code>. Interface names may also be nouns or
|
||
noun phrases (for example, <code class="prettyprint lang-java">List</code>), but may sometimes be
|
||
adjectives or adjective phrases instead (for example,
|
||
<code class="prettyprint lang-java">Readable</code>).</p>
|
||
|
||
<p>There are no specific rules or even well-established conventions for naming annotation types.</p>
|
||
|
||
<p>A <em>test</em> class has a name that ends with <code class="prettyprint lang-java">Test</code>,
|
||
for example, <code class="prettyprint lang-java">HashIntegrationTest</code>.
|
||
If it covers a single class, its name is the name of that class
|
||
plus <code class="prettyprint lang-java">Test</code>, for example <code class="prettyprint
|
||
lang-java">HashImplTest</code>.</p>
|
||
|
||
<h4 id="s5.2.3-method-names">5.2.3 Method names</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. For example,
|
||
<code class="prettyprint lang-java">sendMessage</code> or
|
||
<code class="prettyprint lang-java">stop</code>.</p>
|
||
|
||
<p>Underscores may appear in JUnit <em>test</em> method names to separate logical components of the
|
||
name, with <em>each</em> component written in <a href="#s5.3-camel-case">lowerCamelCase</a>, for
|
||
example <code class="prettyprint lang-java">transferMoney_deductsFromSource</code>. There is no One
|
||
Correct Way to name test methods.</p>
|
||
|
||
<a id="constants"></a>
|
||
<h4 id="s5.2.4-constant-names">5.2.4 Constant names</h4>
|
||
|
||
<p>Constant names use <code class="prettyprint lang-java">UPPER_SNAKE_CASE</code>: all uppercase
|
||
letters, with each word separated from the next by a single underscore. But what <em>is</em> a
|
||
constant, exactly?</p>
|
||
|
||
<p>Constants are static final fields whose contents are deeply immutable and whose methods have no
|
||
detectable side effects. Examples include primitives, strings, immutable value classes, and anything
|
||
set to <code>null</code>. If any of the instance's observable state can change, it is not a
|
||
constant. Merely <em>intending</em> to never mutate the object is 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 Map<String, Integer> AGES = ImmutableMap.of("Ed", 35, "Ann", 32);
|
||
static final Joiner COMMA_JOINER = Joiner.on(','); // because Joiner is immutable
|
||
static final SomeMutableType[] EMPTY_ARRAY = {};
|
||
|
||
// 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 ImmutableMap<String, SomeMutableType> mutableValues =
|
||
ImmutableMap.of("Ed", mutableInstance, "Ann", mutableInstance2);
|
||
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>
|
||
|
||
<h4 id="s5.2.5-non-constant-field-names">5.2.5 Non-constant field names</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. For example,
|
||
<code class="prettyprint lang-java">computedValues</code> or
|
||
<code class="prettyprint lang-java">index</code>.</p>
|
||
|
||
<h4 id="s5.2.6-parameter-names">5.2.6 Parameter names</h4>
|
||
|
||
<p>Parameter names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p>
|
||
|
||
<p>One-character parameter names in public methods should be avoided.</p>
|
||
|
||
<h4 id="s5.2.7-local-variable-names">5.2.7 Local variable names</h4>
|
||
|
||
<p>Local variable names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p>
|
||
|
||
<p>Even when final and immutable, local variables are not considered to be constants, and should not
|
||
be styled as constants.</p>
|
||
|
||
<h4 id="s5.2.8-type-variable-names">5.2.8 Type variable names</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>
|
||
|
||
<h4 id="s5.2.9-unnamed-variables">5.2.9 Unnamed variables</h4>
|
||
|
||
<p>The <code class="prettyprint lang-java">_</code> syntax for unnamed variables and parameters is
|
||
allowed wherever it is applicable. For example:</p>
|
||
|
||
<pre class="prettyprint lang-java">Predicate<String> alwaysTrue = _ -> true;
|
||
</pre>
|
||
|
||
<a id="acronyms"></a>
|
||
<a id="camelcase"></a>
|
||
<h3 id="s5.3-camel-case">5.3 Camel case: defined</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. Note that the casing of the original
|
||
words is almost entirely disregarded.</li>
|
||
</ol>
|
||
|
||
<p>In very rare circumstances (for example, multipart version numbers), you may need to use
|
||
underscores to separate adjacent numbers, since numbers do not have upper and lower case variants.
|
||
</p>
|
||
|
||
<p>Examples:</p>
|
||
|
||
<table>
|
||
<tbody><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="badcode">XMLHTTPRequest</code></td>
|
||
</tr>
|
||
<tr>
|
||
<td>"new customer ID"</td>
|
||
<td><code class="prettyprint lang-java">newCustomerId</code></td>
|
||
<td><code class="badcode">newCustomerID</code></td>
|
||
</tr>
|
||
<tr>
|
||
<td>"inner stopwatch"</td>
|
||
<td><code class="prettyprint lang-java">innerStopwatch</code></td>
|
||
<td><code class="badcode">innerStopWatch</code></td>
|
||
</tr>
|
||
<tr>
|
||
<td>"supports IPv6 on iOS?"</td>
|
||
<td><code class="prettyprint lang-java">supportsIpv6OnIos</code></td>
|
||
<td><code class="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></td>
|
||
</tr>
|
||
<tr>
|
||
<td>"Turn on 2SV"</td>
|
||
<td><code class="prettyprint lang-java">turnOn2sv</code></td>
|
||
<td><code class="badcode">turnOn2Sv</code></td>
|
||
</tr>
|
||
<tr>
|
||
<td>"Guava 33.4.6"</td>
|
||
<td><code class="prettyprint lang-java">guava33_4_6</code></td>
|
||
<td><code class="badcode">guava3346</code></td>
|
||
</tr>
|
||
</tbody></table>
|
||
|
||
<p>*Acceptable, but not recommended.</p>
|
||
|
||
<p class="note"><strong>Note:</strong> 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>
|
||
|
||
|
||
<h2 id="s6-programming-practices">6 Programming Practices</h2>
|
||
|
||
<h3 id="s6.1-override-annotation">6.1 <code>@Override</code>: always used</h3>
|
||
|
||
<p>A method is marked with the <code class="prettyprint lang-java">@Override</code> annotation
|
||
whenever it is legal. This includes a class method overriding a superclass method, a class method
|
||
implementing an interface method, an interface method respecifying a superinterface method, and an
|
||
explicitly declared accessor method for a record component.</p>
|
||
|
||
<p class="exception"><strong>Exception:</strong>
|
||
<code class="prettyprint lang-java">@Override</code> may be omitted when the parent method is
|
||
<code class="prettyprint lang-java">@Deprecated</code>.</p>
|
||
|
||
<a id="caughtexceptions"></a>
|
||
<h3 id="s6.2-caught-exceptions">6.2 Caught exceptions: not ignored</h3>
|
||
|
||
<p>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 _) {
|
||
// it's not numeric; that's fine, just continue
|
||
}
|
||
return handleTextResponse(response);
|
||
</pre>
|
||
|
||
<h3 id="s6.3-static-members">6.3 Static members: qualified using class</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 id="finalizers"></a>
|
||
<h3 id="s6.4-finalizers">6.4 Finalizers: not used</h3>
|
||
|
||
<p>Do not override <code class="prettyprint lang-java">Object.finalize</code>. Finalization support
|
||
is <a href="https://openjdk.org/jeps/421"><em>scheduled for removal</em></a>.</p>
|
||
|
||
|
||
<a id="javadoc"></a>
|
||
<h2 id="s7-javadoc">7 Javadoc</h2>
|
||
|
||
|
||
|
||
<h3 id="s7.1-javadoc-formatting">7.1 Formatting</h3>
|
||
|
||
<h4 id="s7.1.1-javadoc-multi-line">7.1.1 General form</h4>
|
||
|
||
<p>Both Markdown Javadoc and Traditional Javadoc are acceptable. Markdown Javadoc looks like this
|
||
example:</p>
|
||
|
||
<pre class="prettyprint lang-java">/// Javadoc text is written here,
|
||
/// wrapped normally...
|
||
public int method(String p1) { ... }
|
||
</pre>
|
||
|
||
<p>The <em>basic</em> formatting of Traditional Javadoc blocks is as seen in this 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 basic form is always acceptable. The single-line form may be substituted when the entirety
|
||
of the Javadoc block (including comment markers) can fit on a single line. Note that this only
|
||
applies when there are no block tags such as <code>@param</code>.</p>
|
||
|
||
<h4 id="s7.1.2-javadoc-paragraphs">7.1.2 Paragraphs</h4>
|
||
|
||
<p>With Traditional Javadoc, one blank line—that is, a line containing only the aligned
|
||
leading asterisk (<code>*</code>)—appears between paragraphs, and before the group of
|
||
block tags if present. Each paragraph except the first has <code><p></code> immediately
|
||
before the first word, with no space after it. HTML tags for other block-level elements, such
|
||
as <code><ul></code> or <code><table></code>, are <em>not</em> preceded with
|
||
<code><p></code>.</p>
|
||
|
||
<p>With Markdown Javadoc, paragraphs are separated as usual for Markdown, by a line that is blank
|
||
apart from the leading <code>///</code>. No <code><p></code> is needed.</p>
|
||
|
||
<a id="s7.1.3-javadoc-at-clauses"></a>
|
||
|
||
<h4 id="s7.1.3-javadoc-block-tags">7.1.3 Block tags</h4>
|
||
|
||
<p>Any of the standard "block tags" 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 a block tag doesn't fit on a single line, continuation lines
|
||
are indented four (or more) spaces from the position of the <code>@</code> for Traditional Javadoc,
|
||
and exactly two spaces for Markdown Javadoc. (A four-space indentation in Markdown is interpreted as
|
||
an indented code block in some contexts.)
|
||
</p>
|
||
|
||
<h3 id="s7.2-summary-fragment">7.2 The summary fragment</h3>
|
||
|
||
<p>Each Javadoc block 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 `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"><strong>Tip:</strong> A common mistake is to write simple Javadoc in the form
|
||
<code class="badcode prettyprint lang-java">/// @return the customer ID</code>. This is
|
||
incorrect, and should be changed to
|
||
<code class="prettyprint lang-java">/// Returns the customer ID.</code> or
|
||
<code class="prettyprint lang-java">/// {@return the customer ID}</code>.</p>
|
||
|
||
<a id="s7.3.3-javadoc-optional"></a>
|
||
<h3 id="s7.3-javadoc-where-required">7.3 Where Javadoc is used</h3>
|
||
|
||
<p>At the <em>minimum</em>, Javadoc is present for every <em>visible</em> class, member, or record
|
||
component, with a few exceptions noted below. A top-level class is visible if it is <code class="prettyprint lang-java">public</code>; a member is visible if it is <code class="prettyprint
|
||
lang-java">public</code> or <code class="prettyprint lang-java">protected</code> and its containing
|
||
class is visible; and a record component is visible if its containing record is visible.
|
||
|
||
</p><p>Additional Javadoc content may also be present, as explained in Section 7.3.4,
|
||
<a href="#s7.3.4-javadoc-non-required">Non-required Javadoc</a>.</p>
|
||
|
||
<h4 id="s7.3.1-javadoc-exception-self-explanatory">7.3.1 Exception: self-explanatory members</h4>
|
||
|
||
<p>Javadoc is optional for "simple, obvious" members and record components, such as a
|
||
<code class="prettyprint lang-java">getFoo()</code> method, <em>if</em> there <em>really and
|
||
truly</em> is nothing else worthwhile to say but "the foo".</p>
|
||
|
||
<p class="note important"><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 record
|
||
component named <code class="prettyprint lang-java">canonicalName</code>, don't omit its
|
||
documentation (with the rationale that it would say only
|
||
<code class="badcode">@param canonicalName the canonical name</code>) if a typical reader may have
|
||
no idea what the term "canonical name" means!</p>
|
||
|
||
<h4 id="s7.3.2-javadoc-exception-overrides">7.3.2 Exception: overrides</h4>
|
||
|
||
<p>Javadoc is not always present on a method that overrides a supertype method.
|
||
|
||
</p>
|
||
|
||
|
||
|
||
<h4 id="s7.3.4-javadoc-non-required">7.3.4 Non-required Javadoc</h4>
|
||
|
||
<p>Other classes, members, and record components have Javadoc <em>as needed or desired</em>.
|
||
|
||
</p><p>Whenever an implementation comment would be used to define the overall purpose or behavior of a
|
||
class or member, that comment is written as Javadoc instead (using <code>///</code> or
|
||
<code>/**</code>).</p>
|
||
|
||
<p>Non-required Javadoc is not strictly required to follow the formatting rules of Sections
|
||
7.1.1, 7.1.2, 7.1.3, and 7.2, though it is of course recommended.</p>
|
||
|
||
|
||
|
||
</div>
|
||
</div>
|
||
</body>
|
||
</html>
|