| <?xml version="1.0" encoding="UTF-8"?> |
| <!-- |
| Licensed to the Apache Software Foundation (ASF) under one or more |
| contributor license agreements. See the NOTICE file distributed with |
| this work for additional information regarding copyright ownership. |
| The ASF licenses this file to You under the Apache License, Version 2.0 |
| (the "License"); you may not use this file except in compliance with |
| the License. You may obtain a copy of the License at |
| |
| http://www.apache.org/licenses/LICENSE-2.0 |
| |
| Unless required by applicable law or agreed to in writing, software |
| distributed under the License is distributed on an "AS IS" BASIS, |
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| See the License for the specific language governing permissions and |
| limitations under the License. |
| --> |
| <document xmlns="http://maven.apache.org/XDOC/2.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" |
| xsi:schemaLocation="http://maven.apache.org/XDOC/2.0 http://maven.apache.org/xsd/xdoc-2.0.xsd"> |
| |
| <properties> |
| <title>Java Style Guidelines</title> |
| </properties> |
| |
| <body> |
| <section name="Apache Log4j Code Style Guidelines"> |
| <a name="intro"/> |
| <subsection name="Introduction"> |
| <p>This document serves as the <strong>complete</strong> definition of the Log4j project's coding standards for |
| source code in the Java™ Programming Language. It originated from the Google coding standards but incorporates |
| modifications that reflect the desires of the Log4j community.</p> |
| <p>Like other programming style guides, the issues covered span not only aesthetic issues of |
| formatting, but other types of conventions or coding standards as well. However, this document |
| focuses primarily on the <strong>hard-and-fast rules</strong> that we follow universally, and |
| avoids giving <em>advice</em> that isn't clearly enforceable (whether by human or tool).</p> |
| <a name="terminology"/> |
| <h3>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, enum class, interface or |
| annotation type (<code>@interface</code>).</li> |
| <li>The term <em>comment</em> always refers to <em>implementation</em> comments. We do not |
| use the phrase "documentation comments", instead using the common term "Javadoc."</li> |
| </ol> |
| <p>Other "terminology notes" will appear occasionally throughout the document.</p> |
| <a name="guide-notes"/> |
| <h3>Guide notes</h3> |
| <p>Example code in this document is <strong>non-normative</strong>. That is, while the examples |
| are in Log4j 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> |
| </subsection> |
| <a name="source-file-basics"/> |
| <subsection name="Source File Basics"> |
| <a name="file-name"/> |
| <h3>File name</h3> |
| <p>The source file name consists of the case-sensitive name of the top-level class it contains, |
| plus the <code>.java</code> extension.</p> |
| <a name="file-encoding"/> |
| <h3>2.2 File encoding: UTF-8</h3> |
| <p>Source files are encoded in <strong>UTF-8</strong>.</p> |
| <a name="special-characters"/> |
| <h3>Special characters</h3> |
| <a name="whitespace-characters"/> |
| <h4>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> |
| <a name="special-escape-sequences"/> |
| <h4>Special escape sequences</h4> |
| <p>For any character that has a special escape sequence |
| (<code>\b</code>, |
| <code>\t</code>, |
| <code>\n</code>, |
| <code>\f</code>, |
| <code>\r</code>, |
| <code>\"</code>, |
| <code>\'</code> and |
| <code>\\</code>), that sequence is used rather than the corresponding octal |
| (e.g. <code>\012</code>) or Unicode (e.g. <code>\u000a</code>) escape.</p> |
| <a name="non-ascii-characters"/> |
| <h4>Non-ASCII characters</h4> |
| <p>For the remaining non-ASCII characters, either the actual Unicode character |
| (e.g. <code>∞</code>) or the equivalent Unicode escape (e.g. <code>\u221e</code>) is used, depending only on which |
| makes the code <strong>easier to read and understand</strong>.</p> |
| <p><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> |
| <tr><th>Example</th><th>Discussion</th></tr> |
| <tr><td><code>String unitAbbrev = "μs";</code></td><td>Best: perfectly clear even without a comment.</td></tr> |
| <tr><td><code>String unitAbbrev = "\u03bcs"; // "μs"</code></td><td>Allowed, but there's no reason to do this.</td></tr> |
| <tr><td><code>String unitAbbrev = "\u03bcs"; // Greek letter mu, "s"</code></td><td>Allowed, but awkward and prone to mistakes.</td></tr> |
| <tr><td><code>String unitAbbrev = "\u03bcs";</code></td><td>Poor: the reader has no idea what this is.</td></tr> |
| <tr><td><code>return '\ufeff' + content; // byte order mark</code></td><td>Good: use escapes for non-printable characters, and comment if necessary.</td></tr> |
| </table> |
| <p><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> |
| </subsection> |
| <a name="filestructure"/> |
| <a name="source-file-structure"/> |
| <subsection name="Source file structure"> |
| <p>A source file consists of, <strong>in order</strong>:</p> |
| <ol> |
| <li>Apache license</li> |
| <li>Package statement</li> |
| <li>Import statements</li> |
| <li>Exactly one top-level class</li> |
| </ol> |
| <p><strong>Exactly one blank line</strong> separates each section that is present.</p> |
| <a name="license"/> |
| <h3>Apache License</h3> |
| <p>The Apache license belongs here. No other license should appear. Other licenses that apply should be referenced in |
| a NOTICE file</p> |
| <a name="package-statement"/> |
| <h3>Package statement</h3> |
| <p>The package statement is <strong>not line-wrapped</strong>. The column limit |
| (<a href="#column-limit">Column limit: 120</a>) does not apply to package statements.</p> |
| <a name="imports"/> |
| <a name="import-statements"/> |
| <h3>Import statements</h3> |
| <a name="wildcard-imports"/> |
| <h4>No wildcard imports in the main tree</h4> |
| <p><strong>Wildcard imports</strong>, static or otherwise, <strong>are not used</strong>.</p> |
| <h4>Static wildcard imports in the test tree</h4> |
| <p><strong>Wildcard static imports</strong> are encouraged for test imports like JUnit, EasyMock, and Hamcrest.</p> |
| <a name="import-line-wrapping"/> |
| <h4>No line-wrapping</h4> |
| <p>Import statements are <strong>not line-wrapped</strong>. The column limit |
| (<a href="#column-limit">Column limit: 120</a>) does not apply to import statements.</p> |
| <a name="import-ordering-and-spacing"/> |
| <h4>Ordering and spacing</h4> |
| <p>Import statements are divided into the following groups, in this order, with each group |
| separated by a single blank line:</p> |
| <ol> |
| <li>java</li> |
| <li>javax</li> |
| <li>org</li> |
| <li>com</li> |
| <li>All static imports in a single group</li> |
| </ol> |
| <p>Within a group there are no blank lines, and the imported names appear in ASCII sort |
| order. (<strong>Note:</strong> this is not the same as the import <em>statements</em> being in |
| ASCII sort order; the presence of semicolons warps the result.)</p> |
| <p>IDE settings for ordering imports automatically can be found in the source distributions under |
| <code>src/ide</code>. For example:</p> |
| <ul> |
| <li>Eclipse: <code>src/ide/eclipse/4.3.2/organize-imports.importorder</code></li> |
| <li>IntelliJ: <code>src/ide/Intellij/13/IntellijSettings.jar</code></li> |
| </ul> |
| <a name="class-declaration"/> |
| <h3>Class declaration</h3> |
| <a name="oneclassperfile"/> |
| <a name="one-top-level-class"/> |
| <h4>Exactly one top-level class declaration</h4> |
| <p>Each top-level class resides in a source file of its own.</p> |
| <a name="class-member-ordering"/> |
| <h4>Class member ordering</h4> |
| <p>Class members should be grouped in the following order>.</p> |
| <ol> |
| <li>static variables grouped in the order shown below. Within a group variables may appear in any order.</li> |
| <li> |
| <ol> |
| <li>public</li> |
| <li>protected</li> |
| <li>package</li> |
| <li>private</li> |
| </ol> |
| </li> |
| <li>instance variables grouped in the order shown below. Within a group variables may appear in any order</li> |
| <li> |
| <ol> |
| <li>public</li> |
| <li>protected</li> |
| <li>package</li> |
| <li>private</li> |
| </ol> |
| </li> |
| <li>constructors</li> |
| <li>methods may be specified in the following order but may appear in another order if it improves the |
| clarity of the program.</li> |
| <li> |
| <ol> |
| <li>public</li> |
| <li>protected</li> |
| <li>package</li> |
| <li>private</li> |
| </ol> |
| </li> |
| </ol> |
| <a name="overloads"/> |
| <a name="never-split"/> |
| <h5>Overloads: never split</h5> |
| <p>When a class has multiple constructors, or multiple methods with the same name, these appear |
| sequentially, with no intervening members.</p> |
| </subsection> |
| <a name="formatting"/> |
| <subsection name="Formatting"> |
| <p><strong>Terminology Note:</strong> <em>block-like construct</em> refers to |
| the body of a class, method or constructor. Note that, by |
| <a href="array-initializers">array initializers</a>, any array initializer |
| <em>may</em> optionally be treated as if it were a block-like construct.</p> |
| <a name="braces"/> |
| <h3>Braces</h3> |
| <a name="braces-always-used"/> |
| <h4>Braces are used where optional</h4> |
| <p>Braces are used with |
| <code>if</code>, |
| <code>else</code>, |
| <code>for</code>, |
| <code>do</code> and |
| <code>while</code> statements, even when the |
| body is empty or contains only a single statement.</p> |
| <a name="blocks-k-r-style"/> |
| <h4>Nonempty blocks: K & R style</h4> |
| <p>Braces follow the Kernighan and Ritchie style |
| ("<a href="http://www.codinghorror.com/blog/2012/07/new-programming-jargon.html">Egyptian brackets</a>") |
| for <em>nonempty</em> blocks and block-like constructs:</p><ul><li>No line break before the opening brace.</li><li>Line break after the opening brace.</li><li>Line break before the closing brace.</li><li>Line break after the closing brace <em>if</em> that brace terminates a statement or the body |
| of a method, constructor or <em>named</em> class. For example, there is <em>no</em> line break |
| after the brace if it is followed by <code>else</code> or a |
| comma.</li></ul><p>Example:</p> |
| <pre> |
| return new MyClass() { |
| @Override public void method() { |
| if (condition()) { |
| try { |
| something(); |
| } catch (ProblemException e) { |
| recover(); |
| } |
| } |
| } |
| }; |
| </pre><p>A few exceptions for enum classes are given in Section 4.8.1, |
| <a href="enum-classes">Enum classes</a>.</p> |
| <a name="emptyblocks"/> |
| <a name="braces-empty-blocks"/> |
| <h4>Empty blocks: may be concise</h4> |
| <p>An empty block or block-like construct <em>may</em> be closed immediately after it is |
| opened, with no characters or line break in between |
| (<code>{}</code>), <strong>unless</strong> it is part of a |
| <em>multi-block statement</em> (one that directly contains multiple blocks: |
| <code>if/else-if/else</code> or |
| <code>try/catch/finally</code>).</p> |
| <p>Example:</p><pre> |
| void doNothing() {} |
| </pre><a name="block-indentation"/> |
| <h3>Block indentation: +4 spaces</h3> |
| <p>Each time a new block or block-like construct is opened, the indent increases by four |
| 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="#blocks-k-r-style">Nonempty blocks: K & R Style</a>.)</p> |
| <a name="one-statement-per-line"/> |
| <h3>One statement per line</h3> |
| <p>Each statement is followed by a line-break.</p> |
| <a name="columnlimit"/> |
| <a name="column-limit"/> |
| <h3>Column limit: 120</h3> |
| <p> |
| The column limit for Log4j is 120 characters. |
| |
| Except as noted below, any line that would exceed this limit must be line-wrapped, as explained in |
| <a href="#line-wrapping">Line-wrapping</a>. |
| </p><p><strong>Exceptions:</strong></p> |
| <ol> |
| <li>Lines where obeying the column limit is not possible (for example, a long URL in Javadoc, |
| or a long JSNI method reference).</li> |
| <li><code>package</code> and <code>import</code> statements (see <a href="#package-statement">Package statement</a> and |
| <a href="#import-statements">Import statements</a>).</li> |
| <li>Command lines in a comment that may be cut-and-pasted into a shell.</li> |
| </ol><a name="line-wrapping"/> |
| <h3>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, typically to avoid overflowing the column |
| limit, this activity is called |
| <em>line-wrapping</em>.</p> |
| <p>There is no comprehensive, deterministic formula showing <em>exactly</em> how to line-wrap in |
| every situation. Very often there are several valid ways to line-wrap the same piece of code.</p> |
| <p class="tip"><strong>Tip:</strong> Extracting a method or local variable may solve the problem |
| without the need to line-wrap.</p> |
| <a name="line-wrapping-where-to-break"/> |
| <h4>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: the dot separator |
| (<code>.</code>), the ampersand in type bounds |
| (<code><T extends Foo & Bar></code>), and the pipe in |
| catch blocks |
| (<code>catch (FooException | BarException e)</code>).</li> |
| </ul> |
| </li> |
| <li>When a line is broken at an <em>assignment</em> operator the break typically comes |
| <em>after</em> the symbol, but either way is acceptable. |
| <ul> |
| <li>This also applies to the "assignment-operator-like" colon in an enhanced |
| <code>for</code> ("foreach") statement.</li> |
| </ul> |
| </li> |
| <li>A method or constructor name stays attached to the open parenthesis |
| (<code>(</code>) that follows it.</li> |
| <li>A comma (<code>,</code>) stays attached to the token that |
| precedes it.</li> |
| </ol> |
| <a name="indentation"/> |
| <a name="line-wrapping-indent"/> |
| <h4>Indent continuation lines at least +8 spaces</h4> |
| <p>When line-wrapping, each line after the first (each <em>continuation line</em>) is indented |
| at least +8 from the original line.</p> |
| <p>When there are multiple continuation lines, indentation may be varied beyond +8 as |
| desired. In general, two continuation lines use the same indentation level if and only if they |
| begin with syntactically parallel elements.</p> |
| <p>The section on <a href="#horizontal-alignment">Horizontal alignment</a> addresses |
| the discouraged practice of using a variable number of spaces to align certain tokens with |
| previous lines.</p> |
| <a name="whitespace"/> |
| <h3>Whitespace</h3> |
| <a name="vertical-whitespace"/> |
| <h4>Vertical Whitespace</h4> |
| <p>A single blank line appears:</p> |
| <ol> |
| <li><em>Between</em> consecutive members (or initializers) of a class: fields, constructors, |
| methods, nested classes, static initializers, instance initializers. |
| <ul> |
| <li><span class="exception"><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> |
| </ul> |
| </li> |
| <li>Within method bodies, as needed to create <em>logical groupings</em> of statements.</li><li><em>Optionally</em> before the first member or after the last member of the class (neither |
| encouraged nor discouraged).</li> |
| <li>As required by other sections of this document (such as |
| <a href="#import-statements">Import statements</a>).</li> |
| </ol> |
| <p><em>Multiple</em> consecutive blank lines are permitted, but never required (or encouraged).</p> |
| <a name="horizontal-whitespace"/> |
| <h4>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>if</code>, |
| <code>for</code> or |
| <code>catch</code>, from an open parenthesis |
| (<code>(</code>) |
| that follows it on that line</li> |
| <li>Separating any reserved word, such as |
| <code>else</code> or |
| <code>catch</code>, from a closing curly brace |
| (<code>}</code>) that precedes it on that line</li> |
| <li>Before any open curly brace |
| (<code>{</code>), with two exceptions: |
| <ul> |
| <li><code>String[][] x = {{"foo"}};</code> (no space is required |
| between <code>{{</code>, by item 8 below)</li> |
| </ul> |
| </li> |
| <li>On both sides of any binary or ternary operator. This also applies to the following |
| "operator-like" symbols: |
| <ul> |
| <li>the ampersand in a conjunctive type bound: |
| <code><T extends Foo & Bar></code></li> |
| <li>the pipe for a catch block that handles multiple exceptions: |
| <code>catch (FooException | BarException e)</code></li> |
| <li>the colon (<code>:</code>) in an enhanced |
| <code>for</code> ("foreach") statement</li> |
| </ul> |
| </li> |
| <li>After <code>,:;</code> or the closing parenthesis |
| (<code>)</code>) of a cast</li> |
| <li>On both sides of the double slash (<code>//</code>) that |
| begins an end-of-line comment. Here, multiple spaces are allowed, but not required.</li> |
| <li>Between the type and variable of a declaration: |
| <code>List<String> list</code></li> |
| <li><em>Optional</em> just inside both braces of an array initializer |
| <ul> |
| <li><code>new int[] {5, 6}</code> and |
| <code>new int[] { 5, 6 }</code> are both valid</li> |
| </ul> |
| </li> |
| </ol> |
| <p class="note"><strong>Note:</strong> This rule never requires or forbids additional space at the |
| start or end of a line, only <em>interior</em> space.</p> |
| <a name="horizontal-alignment"/> |
| <h4>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> |
| 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 it creates problems for |
| future maintenance. Consider a future change that needs to touch just one line. This change may |
| leave the formerly-pleasing formatting mangled, and that is <strong>allowed</strong>. More often |
| it prompts the coder (perhaps you) to adjust whitespace on nearby lines as well, possibly |
| triggering a cascading series of reformattings. That one-line change now has a "blast radius." |
| This can at worst result in pointless busywork, but at best it still corrupts version history |
| information, slows down reviewers and exacerbates merge conflicts.</p> |
| <a name="parentheses"/> |
| <a name="grouping-parentheses"/> |
| <h3>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> |
| <a name="specific-constructs"/> |
| <h3>Specific constructs</h3> |
| <a name="enum-classes"/> |
| <h4>Enum classes</h4> |
| <p>After each comma that follows an enum constant, a line-break is optional.</p><p>An enum class with no methods |
| and no documentation on its constants may optionally be formatted |
| as if it were an array initializer (see |
| <a href="array-initializers">array initializers</a>).</p><pre> |
| private enum Suit { CLUBS, HEARTS, SPADES, DIAMONDS } |
| </pre> |
| <p>Since enum classes <em>are classes</em>, all other rules for formatting classes apply.</p> |
| <a name="localvariables"/> |
| <a name="variable-declarations"/> |
| <h4>Variable declarations</h4> |
| <a name="variables-per-declaration"/> |
| <h5>One variable per declaration</h5> |
| <p>Every variable declaration (field or local) declares only one variable: declarations such as |
| <code>int a, b;</code> are not used.</p> |
| <a name="variables-limited-scope"/> |
| <h5>Declared when needed, initialized as soon as possible</h5> |
| <p>Local variables are <strong>not</strong> habitually declared at the start of their containing |
| block or block-like construct. Instead, local variables are declared close to the point they are |
| first used (within reason), to minimize their scope. Local variable declarations typically have |
| initializers, or are initialized immediately after declaration.</p><a name="s4.8.3-arrays"/> |
| <h4>Arrays</h4> |
| <a name="array-initializers"/> |
| <h5>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> |
| new int[] { new int[] { |
| 0, 1, 2, 3 0, |
| } 1, |
| 2, |
| new int[] { 3, |
| 0, 1, } |
| 2, 3 |
| } new int[] |
| {0, 1, 2, 3} |
| </pre><a name="array-declarations"/> |
| <h5>No C-style array declarations</h5> |
| <p>The square brackets form a part of the <em>type</em>, not the variable: |
| <code>String[] args</code>, not |
| <code>String args[]</code>.</p> |
| <a name="switch"/> |
| <h4>Switch statements</h4> |
| <p class="terminology"><strong>Terminology Note:</strong> Inside the braces of a |
| <em>switch block</em> are one or more <em>statement groups</em>. Each statement group consists of |
| one or more <em>switch labels</em> (either <code>case FOO:</code> or |
| <code>default:</code>), followed by one or more statements.</p> |
| <a name="switch-indentation"/> |
| <h5>Indentation</h5> |
| <p>As with any other block, the contents of a switch block are indented +2.</p> |
| <p>After a switch label, a newline appears, and the indentation level is increased +2, exactly as |
| if a block were being opened. The following switch label returns to the previous indentation |
| level, as if a block had been closed.</p> |
| <a name="fallthrough"/> |
| <a name="switch-fall-through"/> |
| <h5>Fall-through: commented</h5> |
| <p>Within a switch block, each statement group either terminates abruptly (with a |
| <code>break</code>, |
| <code>continue</code>, |
| <code>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>// fall through</code>). This special comment is not required in |
| the last statement group of the switch block. Example:</p><pre> |
| switch (input) { |
| case 1: |
| case 2: |
| prepareOneOrTwo(); |
| // fall through |
| case 3: |
| handleOneTwoOrThree(); |
| break; |
| default: |
| handleLargeNumber(input); |
| } |
| </pre><a name="switch-default"/> |
| <h5>The default case is present</h5> |
| <p>Each switch statement includes a <code>default</code> statement |
| group, even if it contains no code.</p> |
| <a name="annotations"/> |
| <h4>Annotations</h4> |
| <p>Annotations applying to a class, method or constructor appear immediately after the |
| documentation block, and each annotation is listed on a line of its own (that is, one annotation |
| per line). These line breaks do not constitute line-wrapping (Section |
| 4.5, <a href="#line-wrapping">Line-wrapping</a>), so the indentation level is not |
| increased. Example:</p><pre> |
| @Override |
| @Nullable |
| 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> |
| @Override public int hashCode() { ... } |
| </pre><p>Annotations applying to a field also appear immediately after the documentation block, but in |
| this case, <em>multiple</em> annotations (possibly parameterized) may be listed on the same line; |
| for example:</p><pre> |
| @Partial @Mock DataLoader loader; |
| </pre><p>There are no specific rules for formatting parameter and local variable annotations.</p> |
| <a name="comments"/> |
| <h4>Comments</h4> |
| <a name="block-comment-style"/> |
| <h5>Block comment style</h5> |
| <p>Block comments are indented at the same level as the surrounding code. They may be in |
| <code>/* ... */</code> style or |
| <code>// ...</code> style. For multi-line |
| <code>/* ... */</code> comments, subsequent lines must start with |
| <code>*</code> aligned with the <code>*</code> on the previous line.</p><pre> |
| /* |
| * 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><strong>Tip:</strong> When writing multi-line comments, use the |
| <code>/* ... */</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>// ...</code> style comment blocks.</p> |
| <a name="modifiers"/> |
| <h4>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 static final transient volatile synchronized native strictfp |
| </pre> |
| <a name="numeric-literals"/> |
| <h4>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>3000000000l</code>.</p> |
| </subsection> |
| <a name="naming"/> |
| <subsection name="Naming"> |
| <a name="identifier-names"/> |
| <h3>Rules common to all identifiers</h3> |
| <p>Identifiers use only ASCII letters and digits, and in two cases noted below, underscores. Thus |
| each valid identifier name is matched by the regular expression <code>\w+</code> .</p> |
| <p> In Google Style special prefixes or |
| suffixes, like those seen in the examples <code>name_</code>, |
| <code>mName</code>, <code>s_name</code> and |
| <code>kName</code>, are <strong>not</strong> used.</p> |
| <a name="specific-identifier-names"/> |
| <h3>Rules by identifier type</h3> |
| <a name="package-names"/> |
| <h4>Package names</h4> |
| <p>Package names are all lowercase, with consecutive words simply concatenated together (no |
| underscores). For example, <code>com.example.deepspace</code>, not |
| <code>com.example.deepSpace</code> or |
| <code>com.example.deep_space</code>.</p> |
| <a name="class-names"/> |
| <h4>Class names</h4> |
| <p>Class names are written in <a href="#camel-case">UpperCamelCase</a>.</p> |
| <p>Class names are typically nouns or noun phrases. For example, |
| <code>Character</code> or |
| <code>ImmutableList</code>. Interface names may also be nouns or |
| noun phrases (for example, <code>List</code>), but may sometimes be |
| adjectives or adjective phrases instead (for example, |
| <code>Readable</code>).</p><p>There are no specific rules or even well-established conventions for naming annotation types.</p><p><em>Test</em> classes are named starting with the name of the class they are testing, and ending |
| with <code>Test</code>. For example, |
| <code>HashTest</code> or |
| <code>HashIntegrationTest</code>.</p> |
| <a name="method-names"/> |
| <h4>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>sendMessage</code> or |
| <code>stop</code>.</p><p>Underscores may appear in JUnit <em>test</em> method names to separate logical components of the |
| name. One typical pattern is <code>test<i><MethodUnderTest></i>_<i><state></i></code>, |
| for example <code>testPop_emptyStack</code>. There is no One Correct |
| Way to name test methods.</p> |
| <a name="constants"/> |
| <a name="constant-names"/> |
| <h4>Constant names</h4> |
| <p>Constant names use <code>CONSTANT_CASE</code>: all uppercase |
| letters, with words separated by underscores. But what <em>is</em> a constant, exactly?</p> |
| <p>Every constant is a static final field, but not all static final fields are constants. Before |
| choosing constant case, consider whether the field really <em>feels like</em> a constant. For |
| example, if any of that instance's observable state can change, it is almost certainly not a |
| constant. Merely <em>intending</em> to never mutate the object is generally not |
| enough. Examples:</p><pre> |
| // Constants |
| static final int NUMBER = 5; |
| static final ImmutableList<String> NAMES = ImmutableList.of("Ed", "Ann"); |
| static final Joiner COMMA_JOINER = Joiner.on(','); // because Joiner is immutable |
| static final SomeMutableType[] EMPTY_ARRAY = {}; |
| enum SomeEnum { ENUM_CONSTANT } |
| |
| // Not constants |
| static String nonFinal = "non-final"; |
| final String nonStatic = "non-static"; |
| static final Set<String> mutableCollection = new HashSet<String>(); |
| static final ImmutableSet<SomeMutableType> mutableElements = ImmutableSet.of(mutable); |
| static final Logger logger = Logger.getLogger(MyClass.getName()); |
| static final String[] nonEmptyArray = {"these", "can", "change"}; |
| </pre> |
| <p>These names are typically nouns or noun phrases.</p> |
| <a name="non-constant-field-names"/> |
| <h4>Non-constant field names</h4> |
| <p>Non-constant field names (static or otherwise) are written |
| in <a href="#camel-case">lowerCamelCase</a>.</p> |
| <p>These names are typically nouns or noun phrases. For example, |
| <code>computedValues</code> or |
| <code>index</code>.</p> |
| <a name="parameter-names"/> |
| <h4>Parameter names</h4> |
| <p>Parameter names are written in <a href="#camel-case">lowerCamelCase</a>.</p> |
| <p>One-character parameter names should be avoided.</p> |
| <a name="local-variable-names"/> |
| <h4>Local variable names</h4> |
| <p>Local variable names are written in <a href="#camel-case">lowerCamelCase</a>, and can be |
| abbreviated more liberally than other types of names.</p><p>However, one-character names should be avoided, except for temporary and looping variables.</p><p>Even when final and immutable, local variables are not considered to be constants, and should not |
| be styled as constants.</p> |
| <a name="type-variable-names"/> |
| <h4>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>E</code>, <code>T</code>, |
| <code>X</code>, <code>T2</code>) |
| </li><li>A name in the form used for classes (see |
| <a href="#class-names">Class names</a>), followed by the capital letter |
| <code>T</code> (examples: |
| <code>RequestT</code>, |
| <code>FooBarT</code>).</li></ul><a name="acronyms"/> |
| <a name="camelcase"/> |
| <a name="camel-case"/> |
| <h3>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.</li> |
| </ol> |
| <p>Note that the casing of the original words is almost entirely disregarded. Examples:</p> |
| <table> |
| <tr><th>Prose form</th><th>Correct</th><th>Incorrect</th></tr> |
| <tr><td>"XML HTTP request"</td><td><code>XmlHttpRequest</code></td><td><code>XMLHTTPRequest</code></td></tr> |
| <tr><td>"new customer ID"</td><td><code>newCustomerId</code></td><td><code>newCustomerID</code></td></tr> |
| <tr><td>"inner stopwatch"</td><td><code>innerStopwatch</code></td><td><code>innerStopWatch</code></td></tr> |
| <tr><td>"supports IPv6 on iOS?"</td><td><code>supportsIpv6OnIos</code></td><td><code>supportsIPv6OnIOS</code></td></tr> |
| <tr><td>"YouTube importer"</td><td><code>YouTubeImporter</code><br/><code>YoutubeImporter</code>*</td><td/></tr> |
| </table> |
| <p>*Acceptable, but not recommended.</p> |
| <p><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>checkNonempty</code> and |
| <code>checkNonEmpty</code> are likewise both correct.</p> |
| </subsection> |
| <subsection name="Programming Practices"> |
| <a name="programming-practices"/> |
| <a name="override-annotation"/> |
| <h3>@Override: always used</h3> |
| <p>A method is marked with the <code>@Override</code> annotation |
| whenever it is legal. This includes a class method overriding a superclass method, a class method |
| implementing an interface method, and an interface method respecifying a superinterface |
| method.</p> |
| <p class="exception"><strong>Exception:</strong><code>@Override</code> may be omitted when the parent method is |
| <code>@Deprecated</code>.</p> |
| <a name="caughtexceptions"/> |
| <a name="caught-exceptions"/> |
| <h3>Caught exceptions: not ignored</h3> |
| <p>Except as noted below, it is very rarely correct to do nothing in response to a caught |
| exception. (Typical responses are to log it, or if it is considered "impossible", rethrow it as an |
| <code>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> |
| try { |
| int i = Integer.parseInt(response); |
| return handleNumericResponse(i); |
| } catch (NumberFormatException ok) { |
| // it's not numeric; that's fine, just continue |
| } |
| return handleTextResponse(response); |
| </pre><p><strong>Exception:</strong> In tests, a caught exception may be ignored |
| without comment <em>if</em> it is named <code>expected</code>. The |
| following is a very common idiom for ensuring that the method under test <em>does</em> throw an |
| exception of the expected type, so a comment is unnecessary here.</p><pre> |
| try { |
| emptyStack.pop(); |
| fail(); |
| } catch (NoSuchElementException expected) { |
| } |
| </pre><a name="static-members"/> |
| <h3>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> |
| Foo aFoo = ...; |
| Foo.aStaticMethod(); // good |
| <span>aFoo.aStaticMethod();</span> // bad |
| <span>somethingThatYieldsAFoo().aStaticMethod();</span> // very bad |
| </pre> |
| <a name="finalizers"/> |
| <h3>Finalizers: not used</h3> |
| <p>It is <strong>extremely rare</strong> to override <code>Object.finalize</code>.</p> |
| <p><strong>Tip:</strong> Don't do it. If you absolutely must, first read and understand |
| <a href="http://books.google.com/books?isbn=8131726592"><em>Effective Java</em></a> |
| Item 7, "Avoid Finalizers," very carefully, and <em>then</em> don't do it.</p> |
| </subsection> |
| <a name="javadoc"/> |
| <subsection name="Javadoc"> |
| <a name="javadoc-formatting"/> |
| <h3>Formatting</h3> |
| <a name="javadoc-multi-line"/> |
| <h4>General form</h4> |
| <p>The <em>basic</em> formatting of Javadoc blocks is as seen in this example:</p><pre> |
| /** |
| * 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> |
| /** An especially short bit of Javadoc. */ |
| </pre><p>The basic form is always acceptable. The single-line form may be substituted when there are no |
| at-clauses present, and the entirety of the Javadoc block (including comment markers) can fit on a |
| single line.</p> |
| <a name="javadoc-paragraphs"/> |
| <h4>Paragraphs</h4> |
| <p>One blank line—that is, a line containing only the aligned leading asterisk |
| (<code>*</code>)—appears between paragraphs, and before the group of "at-clauses" if |
| present. Each paragraph but the first has <code><p></code> immediately before the first word, |
| with no space after.</p> |
| <a name="javadoc-at-clauses"/> |
| <h4>At-clauses</h4> |
| <p>Any of the standard "at-clauses" that are used appear in the order <code>@param</code>, |
| <code>@return</code>, <code>@throws</code>, <code>@deprecated</code>, and these four types never |
| appear with an empty description. When an at-clause doesn't fit on a single line, continuation lines |
| are indented four (or more) spaces from the position of the <code>@</code>. |
| </p> |
| <a name="summary-fragment"/> |
| <h3>The summary fragment</h3> |
| <p>The Javadoc for each class and member begins with a brief <strong>summary fragment</strong>. This |
| fragment is very important: it is the only part of the text that appears in certain contexts such as |
| class and method indexes.</p><p>This is a fragment—a noun phrase or verb phrase, not a complete sentence. It does |
| <strong>not</strong> begin with <code>A {@code Foo} is a...</code>, or |
| <code>This method returns...</code>, nor does it form a complete imperative sentence |
| like <code>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>/** @return the customer ID */</code>. This is incorrect, and should be |
| changed to <code>/** Returns the customer ID. */</code>.</p> |
| <a name="javadoc-optional"/> |
| <a name="javadoc-where-required"/> |
| <h3>Where Javadoc is used</h3> |
| <p>At the <em>minimum</em>, Javadoc is present for every |
| <code>public</code> class, and every |
| <code>public</code> or |
| <code>protected</code> member of such a class, with a few exceptions |
| noted below.</p><p>Other classes and members still have Javadoc <em>as needed</em>. Whenever an implementation |
| comment would be used to define the overall purpose or behavior of a class, method or field, that |
| comment is written as Javadoc instead. (It's more uniform, and more tool-friendly.)</p> |
| <a name="javadoc-exception-self-explanatory"/> |
| <h4>Exception: self-explanatory methods</h4> |
| <p>Javadoc is optional for "simple, obvious" methods like |
| <code>getFoo</code>, in cases where there <em>really and truly</em> is |
| nothing else worthwhile to say but "Returns the foo".</p> |
| <p class="note"><strong>Important:</strong> it is not appropriate to cite this exception to justify |
| omitting relevant information that a typical reader might need to know. For example, for a method |
| named <code>getCanonicalName</code>, don't omit its documentation |
| (with the rationale that it would say only |
| <code>/** Returns the canonical name. */</code>) if a typical reader may have no idea |
| what the term "canonical name" means!</p> |
| <a name="javadoc-exception-overrides"/> |
| <h4>Exception: overrides</h4> |
| <p>Javadoc is not always present on a method that overrides a supertype method. |
| </p> |
| </subsection> |
| </section> |
| </body> |
| </document> |