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    1 <?xml version="1.0" encoding="UTF-8"?>
    2 <!--
    3  Licensed to the Apache Software Foundation (ASF) under one or more
    4  contributor license agreements. See the NOTICE file distributed with
    5  this work for additional information regarding copyright ownership.
    6  The ASF licenses this file to You under the Apache License, Version 2.0
    7  (the "License"); you may not use this file except in compliance with
    8  the License. You may obtain a copy of the License at
   10          http://www.apache.org/licenses/LICENSE-2.0
   12  Unless required by applicable law or agreed to in writing, software
   13  distributed under the License is distributed on an "AS IS" BASIS,
   14  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   15  See the License for the specific language governing permissions and
   16  limitations under the License.
   17 -->
   18 <document xmlns="http://maven.apache.org/XDOC/2.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   19           xsi:schemaLocation="http://maven.apache.org/XDOC/2.0 http://maven.apache.org/xsd/xdoc-2.0.xsd">
   21   <properties>
   22     <title>Java Style Guidelines</title>
   23   </properties>
   25   <body>
   26     <section name="Apache Log4j Code Style Guidelines">
   27       <a name="intro"/>
   28       <subsection name="Introduction">
   29         <p>This document serves as the <strong>complete</strong> definition of the Log4j project's coding standards for
   30           source code in the Java™ Programming Language. It originated from the Google coding standards but incorporates
   31           modifications that reflect the desires of the Log4j community.</p>
   32         <p>Like other programming style guides, the issues covered span not only aesthetic issues of
   33         formatting, but other types of conventions or coding standards as well. However, this document
   34         focuses primarily on the <strong>hard-and-fast rules</strong> that we follow universally, and
   35         avoids giving <em>advice</em> that isn't clearly enforceable (whether by human or tool).</p>
   36       <a name="terminology"/>
   37       <h3>Terminology notes</h3>
   38         <p>In this document, unless otherwise clarified:</p>
   39         <ol>
   40           <li>The term <em>class</em> is used inclusively to mean an "ordinary" class, enum class, interface or
   41              annotation type (<code>@interface</code>).</li>
   42           <li>The term <em>comment</em> always refers to <em>implementation</em> comments. We do not
   43             use the phrase "documentation comments", instead using the common term "Javadoc."</li>
   44         </ol>
   45         <p>Other "terminology notes" will appear occasionally throughout the document.</p>
   46       <a name="guide-notes"/>
   47       <h3>Guide notes</h3>
   48       <p>Example code in this document is <strong>non-normative</strong>. That is, while the examples
   49         are in Log4j Style, they may not illustrate the <em>only</em> stylish way to represent the
   50         code. Optional formatting choices made in examples should not be enforced as rules.</p>
   51       </subsection>
   52       <a name="source-file-basics"/>
   53       <subsection name="Source File Basics">
   54       <a name="file-name"/>
   55       <h3>File name</h3>
   56       <p>The source file name consists of the case-sensitive name of the top-level class it contains,
   57         plus the <code>.java</code> extension.</p>
   58       <a name="file-encoding"/>
   59       <h3>2.2 File encoding: UTF-8</h3>
   60       <p>Source files are encoded in <strong>UTF-8</strong>.</p>
   61       <a name="special-characters"/>
   62       <h3>Special characters</h3>
   63       <a name="whitespace-characters"/>
   64       <h4>Whitespace characters</h4>
   65       <p>Aside from the line terminator sequence, the <strong>ASCII horizontal space
   66         character</strong> (<strong>0x20</strong>) is the only whitespace character that appears
   67         anywhere in a source file. This implies that:</p>
   68         <ol>
   69           <li>All other whitespace characters in string and character literals are escaped.</li>
   70           <li>Tab characters are <strong>not</strong> used for indentation.</li>
   71         </ol>
   72       <a name="special-escape-sequences"/>
   73       <h4>Special escape sequences</h4>
   74       <p>For any character that has a special escape sequence
   75         (<code>\b</code>,
   76         <code>\t</code>,
   77         <code>\n</code>,
   78         <code>\f</code>,
   79         <code>\r</code>,
   80         <code>\"</code>,
   81         <code>\'</code> and
   82         <code>\\</code>), that sequence is used rather than the corresponding octal
   83         (e.g. <code>\012</code>) or Unicode (e.g. <code>\u000a</code>) escape.</p>
   84       <a name="non-ascii-characters"/>
   85       <h4>Non-ASCII characters</h4>
   86         <p>For the remaining non-ASCII characters, either the actual Unicode character
   87           (e.g. <code>∞</code>) or the equivalent Unicode escape (e.g. <code>\u221e</code>) is used, depending only on which
   88           makes the code <strong>easier to read and understand</strong>.</p>
   89         <p><strong>Tip:</strong> In the Unicode escape case, and occasionally even when actual
   90           Unicode characters are used, an explanatory comment can be very helpful.</p>
   91         <p>Examples:</p>
   92           <table>
   93             <tr><th>Example</th><th>Discussion</th></tr>
   94             <tr><td><code>String unitAbbrev = "μs";</code></td><td>Best: perfectly clear even without a comment.</td></tr>
   95             <tr><td><code>String unitAbbrev = "\u03bcs"; // "μs"</code></td><td>Allowed, but there's no reason to do this.</td></tr>
   96             <tr><td><code>String unitAbbrev = "\u03bcs"; // Greek letter mu, "s"</code></td><td>Allowed, but awkward and prone to mistakes.</td></tr>
   97             <tr><td><code>String unitAbbrev = "\u03bcs";</code></td><td>Poor: the reader has no idea what this is.</td></tr>
   98             <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>
   99           </table>
  100         <p><strong>Tip:</strong> Never make your code less readable simply out of fear that
  101           some programs might not handle non-ASCII characters properly. If that should happen, those
  102           programs are <strong>broken</strong> and they must be <strong>fixed</strong>.</p>
  103       </subsection>
  104       <a name="filestructure"/>
  105       <a name="source-file-structure"/>
  106       <subsection name="Source file structure">
  107         <p>A source file consists of, <strong>in order</strong>:</p>
  108           <ol>
  109             <li>Apache license</li>
  110             <li>Package statement</li>
  111             <li>Import statements</li>
  112             <li>Exactly one top-level class</li>
  113           </ol>
  114         <p><strong>Exactly one blank line</strong> separates each section that is present.</p>
  115         <a name="license"/>
  116       <h3>Apache License</h3>
  117         <p>The Apache license belongs here. No other license should appear. Other licenses that apply should be referenced in
  118           a NOTICE file</p>
  119       <a name="package-statement"/>
  120       <h3>Package statement</h3>
  121       <p>The package statement is <strong>not line-wrapped</strong>. The column limit
  122         (<a href="#column-limit">Column limit: 120</a>) does not apply to package statements.</p>
  123       <a name="imports"/>
  124       <a name="import-statements"/>
  125       <h3>Import statements</h3>
  126       <a name="wildcard-imports"/>
  127       <h4>No wildcard imports in the main tree</h4>
  128         <p><strong>Wildcard imports</strong>, static or otherwise, <strong>are not used</strong>.</p>
  129       <h4>Static wildcard imports in the test tree</h4>
  130         <p><strong>Wildcard static imports</strong> are encouraged for test imports like JUnit, EasyMock, and Hamcrest.</p>
  131       <a name="import-line-wrapping"/>
  132       <h4>No line-wrapping</h4>
  133       <p>Import statements are <strong>not line-wrapped</strong>. The column limit
  134         (<a href="#column-limit">Column limit: 120</a>) does not apply to import statements.</p>
  135       <a name="import-ordering-and-spacing"/>
  136       <h4>Ordering and spacing</h4>
  137       <p>Import statements are divided into the following groups, in this order, with each group
  138         separated by a single blank line:</p>
  139         <ol>
  140           <li>java</li>
  141           <li>javax</li>
  142           <li>org</li>
  143           <li>com</li>
  144           <li>All static imports in a single group</li>
  145         </ol>
  146       <p>Within a group there are no blank lines, and the imported names appear in ASCII sort
  147       order. (<strong>Note:</strong> this is not the same as the import <em>statements</em> being in
  148       ASCII sort order; the presence of semicolons warps the result.)</p>
  149       <p>IDE settings for ordering imports automatically can be found in the source distributions under
  150       <code>src/ide</code>. For example:</p>
  151       <ul>
  152         <li>Eclipse: <code>src/ide/eclipse/4.3.2/organize-imports.importorder</code></li>
  153         <li>IntelliJ: <code>src/ide/Intellij/13/IntellijSettings.jar</code></li>
  154       </ul>
  155       <a name="class-declaration"/>
  156       <h3>Class declaration</h3>
  157       <a name="oneclassperfile"/>
  158       <a name="one-top-level-class"/>
  159       <h4>Exactly one top-level class declaration</h4>
  160         <p>Each top-level class resides in a source file of its own.</p>
  161       <a name="class-member-ordering"/>
  162       <h4>Class member ordering</h4>
  163       <p>Class members should be grouped in the following order>.</p>
  164       <ol>
  165         <li>static variables grouped in the order shown below. Within a group variables may appear in any order.</li>
  166         <li>
  167           <ol>
  168             <li>public</li>
  169             <li>protected</li>
  170             <li>package</li>
  171             <li>private</li>
  172           </ol>
  173         </li>
  174         <li>instance variables grouped in the order shown below. Within a group variables may appear in any order</li>
  175         <li>
  176           <ol>
  177             <li>public</li>
  178             <li>protected</li>
  179             <li>package</li>
  180             <li>private</li>
  181           </ol>
  182         </li>
  183         <li>constructors</li>
  184         <li>methods may be specified in the following order but may appear in another order if it improves the
  185           clarity of the program.</li>
  186         <li>
  187           <ol>
  188             <li>public</li>
  189             <li>protected</li>
  190             <li>package</li>
  191             <li>private</li>
  192           </ol>
  193         </li>
  194       </ol>
  195       <a name="overloads"/>
  196       <a name="never-split"/>
  197       <h5>Overloads: never split</h5>
  198       <p>When a class has multiple constructors, or multiple methods with the same name, these appear
  199         sequentially, with no intervening members.</p>
  200       </subsection>
  201       <a name="formatting"/>
  202       <subsection name="Formatting">
  203       <p><strong>Terminology Note:</strong> <em>block-like construct</em> refers to
  204         the body of a class, method or constructor. Note that, by
  205         <a href="array-initializers">array initializers</a>, any array initializer
  206         <em>may</em> optionally be treated as if it were a block-like construct.</p>
  207       <a name="braces"/>
  208       <h3>Braces</h3>
  209       <a name="braces-always-used"/>
  210       <h4>Braces are used where optional</h4>
  211       <p>Braces are used with
  212         <code>if</code>,
  213         <code>else</code>,
  214         <code>for</code>,
  215         <code>do</code> and
  216         <code>while</code> statements, even when the
  217         body is empty or contains only a single statement.</p>
  218       <a name="blocks-k-r-style"/>
  219       <h4>Nonempty blocks: K &amp; R style</h4>
  220       <p>Braces follow the Kernighan and Ritchie style
  221         ("<a href="http://www.codinghorror.com/blog/2012/07/new-programming-jargon.html">Egyptian brackets</a>")
  222         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
  223       of a method, constructor or <em>named</em> class. For example, there is <em>no</em> line break
  224       after the brace if it is followed by <code>else</code> or a
  225       comma.</li></ul><p>Example:</p>
  226 <pre>
  227       return new MyClass() {
  228           @Override public void method() {
  229               if (condition()) {
  230                   try {
  231                       something();
  232                   } catch (ProblemException e) {
  233                       recover();
  234                   }
  235               }
  236           }
  237       };
  238     </pre><p>A few exceptions for enum classes are given in Section 4.8.1,
  239       <a href="enum-classes">Enum classes</a>.</p>
  240       <a name="emptyblocks"/>
  241       <a name="braces-empty-blocks"/>
  242       <h4>Empty blocks: may be concise</h4>
  243       <p>An empty block or block-like construct <em>may</em> be closed immediately after it is
  244         opened, with no characters or line break in between
  245         (<code>{}</code>), <strong>unless</strong> it is part of a
  246         <em>multi-block statement</em> (one that directly contains multiple blocks:
  247         <code>if/else-if/else</code> or
  248         <code>try/catch/finally</code>).</p>
  249         <p>Example:</p><pre>
  250       void doNothing() {}
  251     </pre><a name="block-indentation"/>
  252       <h3>Block indentation: +4 spaces</h3>
  253       <p>Each time a new block or block-like construct is opened, the indent increases by four
  254         spaces. When the block ends, the indent returns to the previous indent level. The indent level
  255         applies to both code and comments throughout the block. (See the example in Section 4.1.2,
  256         <a href="#blocks-k-r-style">Nonempty blocks: K &amp; R Style</a>.)</p>
  257         <a name="one-statement-per-line"/>
  258       <h3>One statement per line</h3>
  259       <p>Each statement is followed by a line-break.</p>
  260         <a name="columnlimit"/>
  261         <a name="column-limit"/>
  262       <h3>Column limit: 120</h3>
  263       <p>
  264         The column limit for Log4j is 120 characters.
  266         Except as noted below, any line that would exceed this limit must be line-wrapped, as explained in
  267         <a href="#line-wrapping">Line-wrapping</a>.
  268       </p><p><strong>Exceptions:</strong></p>
  269         <ol>
  270           <li>Lines where obeying the column limit is not possible (for example, a long URL in Javadoc,
  271       or a long JSNI method reference).</li>
  272           <li><code>package</code> and <code>import</code> statements (see <a href="#package-statement">Package statement</a> and
  273       <a href="#import-statements">Import statements</a>).</li>
  274           <li>Command lines in a comment that may be cut-and-pasted into a shell.</li>
  275         </ol><a name="line-wrapping"/>
  276       <h3>Line-wrapping</h3>
  277       <p class="terminology"><strong>Terminology Note:</strong> When code that might otherwise legally
  278         occupy a single line is divided into multiple lines, typically to avoid overflowing the column
  279         limit, this activity is called
  280         <em>line-wrapping</em>.</p>
  281         <p>There is no comprehensive, deterministic formula showing <em>exactly</em> how to line-wrap in
  282       every situation. Very often there are several valid ways to line-wrap the same piece of code.</p>
  283         <p class="tip"><strong>Tip:</strong> Extracting a method or local variable may solve the problem
  284       without the need to line-wrap.</p>
  285       <a name="line-wrapping-where-to-break"/>
  286       <h4>Where to break</h4>
  287       <p>The prime directive of line-wrapping is: prefer to break at a
  288         <strong>higher syntactic level</strong>. Also:</p>
  289         <ol>
  290           <li>When a line is broken at a <em>non-assignment</em> operator the break comes <em>before</em>
  291       the symbol. (Note that this is not the same practice used in Google style for other languages,
  292       such as C++ and JavaScript.)
  293             <ul>
  294               <li>This also applies to the following "operator-like" symbols: the dot separator
  295         (<code>.</code>), the ampersand in type bounds
  296         (<code>&lt;T extends Foo &amp; Bar&gt;</code>), and the pipe in
  297         catch blocks
  298         (<code>catch (FooException | BarException e)</code>).</li>
  299             </ul>
  300           </li>
  301           <li>When a line is broken at an <em>assignment</em> operator the break typically comes
  302       <em>after</em> the symbol, but either way is acceptable.
  303             <ul>
  304               <li>This also applies to the "assignment-operator-like" colon in an enhanced
  305         <code>for</code> ("foreach") statement.</li>
  306             </ul>
  307           </li>
  308           <li>A method or constructor name stays attached to the open parenthesis
  309       (<code>(</code>) that follows it.</li>
  310           <li>A comma (<code>,</code>) stays attached to the token that
  311       precedes it.</li>
  312         </ol>
  313         <a name="indentation"/>
  314         <a name="line-wrapping-indent"/>
  315       <h4>Indent continuation lines at least +8 spaces</h4>
  316         <p>When line-wrapping, each line after the first (each <em>continuation line</em>) is indented
  317           at least +8 from the original line.</p>
  318         <p>When there are multiple continuation lines, indentation may be varied beyond +8 as
  319       desired. In general, two continuation lines use the same indentation level if and only if they
  320       begin with syntactically parallel elements.</p>
  321         <p>The section on <a href="#horizontal-alignment">Horizontal alignment</a> addresses
  322       the discouraged practice of using a variable number of spaces to align certain tokens with
  323       previous lines.</p>
  324         <a name="whitespace"/>
  325       <h3>Whitespace</h3>
  326       <a name="vertical-whitespace"/>
  327       <h4>Vertical Whitespace</h4>
  328       <p>A single blank line appears:</p>
  329         <ol>
  330           <li><em>Between</em> consecutive members (or initializers) of a class: fields, constructors,
  331       methods, nested classes, static initializers, instance initializers.
  332             <ul>
  333               <li><span class="exception"><strong>Exception:</strong> A blank line between two consecutive
  334         fields (having no other code between them) is optional. Such blank lines are used as needed to
  335         create <em>logical groupings</em> of fields.</span></li>
  336             </ul>
  337           </li>
  338           <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
  339       encouraged nor discouraged).</li>
  340           <li>As required by other sections of this document (such as
  341       <a href="#import-statements">Import statements</a>).</li>
  342         </ol>
  343         <p><em>Multiple</em> consecutive blank lines are permitted, but never required (or encouraged).</p>
  344         <a name="horizontal-whitespace"/>
  345       <h4>Horizontal whitespace</h4>
  346       <p>Beyond where required by the language or other style rules, and apart from literals, comments and
  347         Javadoc, a single ASCII space also appears in the following places <strong>only</strong>.</p>
  348         <ol>
  349           <li>Separating any reserved word, such as
  350       <code>if</code>,
  351       <code>for</code> or
  352       <code>catch</code>, from an open parenthesis
  353       (<code>(</code>)
  354       that follows it on that line</li>
  355           <li>Separating any reserved word, such as
  356       <code>else</code> or
  357       <code>catch</code>, from a closing curly brace
  358       (<code>}</code>) that precedes it on that line</li>
  359           <li>Before any open curly brace
  360       (<code>{</code>), with two exceptions:
  361             <ul>
  362               <li><code>String[][] x = {{"foo"}};</code> (no space is required
  363         between <code>{{</code>, by item 8 below)</li>
  364             </ul>
  365           </li>
  366           <li>On both sides of any binary or ternary operator. This also applies to the following
  367       "operator-like" symbols:
  368             <ul>
  369               <li>the ampersand in a conjunctive type bound:
  370         <code>&lt;T extends Foo &amp; Bar&gt;</code></li>
  371               <li>the pipe for a catch block that handles multiple exceptions:
  372         <code>catch (FooException | BarException e)</code></li>
  373               <li>the colon (<code>:</code>) in an enhanced
  374         <code>for</code> ("foreach") statement</li>
  375             </ul>
  376           </li>
  377           <li>After <code>,:;</code> or the closing parenthesis
  378       (<code>)</code>) of a cast</li>
  379           <li>On both sides of the double slash (<code>//</code>) that
  380       begins an end-of-line comment. Here, multiple spaces are allowed, but not required.</li>
  381           <li>Between the type and variable of a declaration:
  382       <code>List&lt;String&gt; list</code></li>
  383           <li><em>Optional</em> just inside both braces of an array initializer
  384             <ul>
  385               <li><code>new int[] {5, 6}</code> and
  386         <code>new int[] { 5, 6 }</code> are both valid</li>
  387             </ul>
  388           </li>
  389         </ol>
  390         <p class="note"><strong>Note:</strong> This rule never requires or forbids additional space at the
  391       start or end of a line, only <em>interior</em> space.</p>
  392         <a name="horizontal-alignment"/>
  393       <h4>Horizontal alignment: never required</h4>
  394         <p class="terminology"><strong>Terminology Note:</strong> <em>Horizontal alignment</em> is the
  395         practice of adding a variable number of additional spaces in your code with the goal of making
  396         certain tokens appear directly below certain other tokens on previous lines.</p>
  397         <p>This practice is permitted, but is <strong>never required</strong> by Google Style. It is not
  398       even required to <em>maintain</em> horizontal alignment in places where it was already used.</p>
  399         <p>Here is an example without alignment, then using alignment:</p>
  400         <pre>
  401       private int x; // this is fine
  402       private Color color; // this too
  404       private int   x;      // permitted, but future edits
  405       private Color color;  // may leave it unaligned
  406     </pre>
  407         <p class="tip"><strong>Tip:</strong> Alignment can aid readability, but it creates problems for
  408       future maintenance.  Consider a future change that needs to touch just one line. This change may
  409       leave the formerly-pleasing formatting mangled, and that is <strong>allowed</strong>. More often
  410       it prompts the coder (perhaps you) to adjust whitespace on nearby lines as well, possibly
  411       triggering a cascading series of reformattings. That one-line change now has a "blast radius."
  412       This can at worst result in pointless busywork, but at best it still corrupts version history
  413       information, slows down reviewers and exacerbates merge conflicts.</p>
  414         <a name="parentheses"/>
  415         <a name="grouping-parentheses"/>
  416       <h3>Grouping parentheses: recommended</h3>
  417       <p>Optional grouping parentheses are omitted only when author and reviewer agree that there is no
  418         reasonable chance the code will be misinterpreted without them, nor would they have made the code
  419         easier to read. It is <em>not</em> reasonable to assume that every reader has the entire Java
  420         operator precedence table memorized.</p>
  421         <a name="specific-constructs"/>
  422       <h3>Specific constructs</h3>
  423       <a name="enum-classes"/>
  424       <h4>Enum classes</h4>
  425       <p>After each comma that follows an enum constant, a line-break is optional.</p><p>An enum class with no methods
  426         and no documentation on its constants may optionally be formatted
  427       as if it were an array initializer (see
  428       <a href="array-initializers">array initializers</a>).</p><pre>
  429       private enum Suit { CLUBS, HEARTS, SPADES, DIAMONDS }
  430     </pre>
  431         <p>Since enum classes <em>are classes</em>, all other rules for formatting classes apply.</p>
  432         <a name="localvariables"/>
  433         <a name="variable-declarations"/>
  434       <h4>Variable declarations</h4>
  435       <a name="variables-per-declaration"/>
  436       <h5>One variable per declaration</h5>
  437       <p>Every variable declaration (field or local) declares only one variable: declarations such as
  438         <code>int a, b;</code> are not used.</p>
  439         <a name="variables-limited-scope"/>
  440       <h5>Declared when needed, initialized as soon as possible</h5>
  441       <p>Local variables are <strong>not</strong> habitually declared at the start of their containing
  442         block or block-like construct. Instead, local variables are declared close to the point they are
  443         first used (within reason), to minimize their scope. Local variable declarations typically have
  444         initializers, or are initialized immediately after declaration.</p><a name="s4.8.3-arrays"/>
  445       <h4>Arrays</h4>
  446       <a name="array-initializers"/>
  447       <h5>Array initializers: can be "block-like"</h5>
  448       <p>Any array initializer may <em>optionally</em> be formatted as if it were a "block-like
  449         construct." For example, the following are all valid (<strong>not</strong> an exhaustive
  450         list):</p><pre>
  451       new int[] {           new int[] {
  452         0, 1, 2, 3            0,
  453       }                       1,
  454                               2,
  455       new int[] {             3,
  456         0, 1,               }
  457         2, 3
  458       }                     new int[]
  459                                 {0, 1, 2, 3}
  460     </pre><a name="array-declarations"/>
  461       <h5>No C-style array declarations</h5>
  462       <p>The square brackets form a part of the <em>type</em>, not the variable:
  463         <code>String[] args</code>, not
  464         <code>String args[]</code>.</p>
  465         <a name="switch"/>
  466       <h4>Switch statements</h4>
  467       <p class="terminology"><strong>Terminology Note:</strong> Inside the braces of a
  468         <em>switch block</em> are one or more <em>statement groups</em>. Each statement group consists of
  469         one or more <em>switch labels</em> (either <code>case FOO:</code> or
  470         <code>default:</code>), followed by one or more statements.</p>
  471         <a name="switch-indentation"/>
  472       <h5>Indentation</h5>
  473       <p>As with any other block, the contents of a switch block are indented +2.</p>
  474         <p>After a switch label, a newline appears, and the indentation level is increased +2, exactly as
  475       if a block were being opened. The following switch label returns to the previous indentation
  476       level, as if a block had been closed.</p>
  477         <a name="fallthrough"/>
  478         <a name="switch-fall-through"/>
  479       <h5>Fall-through: commented</h5>
  480       <p>Within a switch block, each statement group either terminates abruptly (with a
  481         <code>break</code>,
  482         <code>continue</code>,
  483         <code>return</code> or thrown exception), or is marked with a comment
  484         to indicate that execution will or <em>might</em> continue into the next statement group. Any
  485         comment that communicates the idea of fall-through is sufficient (typically
  486         <code>// fall through</code>). This special comment is not required in
  487         the last statement group of the switch block. Example:</p><pre>
  488       switch (input) {
  489         case 1:
  490         case 2:
  491           prepareOneOrTwo();
  492           // fall through
  493         case 3:
  494           handleOneTwoOrThree();
  495           break;
  496       default:
  497           handleLargeNumber(input);
  498       }
  499     </pre><a name="switch-default"/>
  500       <h5>The default case is present</h5>
  501       <p>Each switch statement includes a <code>default</code> statement
  502         group, even if it contains no code.</p>
  503         <a name="annotations"/>
  504       <h4>Annotations</h4>
  505       <p>Annotations applying to a class, method or constructor appear immediately after the
  506         documentation block, and each annotation is listed on a line of its own (that is, one annotation
  507         per line). These line breaks do not constitute line-wrapping (Section
  508         4.5, <a href="#line-wrapping">Line-wrapping</a>), so the indentation level is not
  509         increased. Example:</p><pre>
  510       @Override
  511       @Nullable
  512       public String getNameIfPresent() { ... }
  513     </pre><p class="exception"><strong>Exception:</strong> A <em>single</em> parameterless annotation
  514       <em>may</em> instead appear together with the first line of the signature, for example:</p><pre>
  515       @Override public int hashCode() { ... }
  516     </pre><p>Annotations applying to a field also appear immediately after the documentation block, but in
  517       this case, <em>multiple</em> annotations (possibly parameterized) may be listed on the same line;
  518       for example:</p><pre>
  519       @Partial @Mock DataLoader loader;
  520     </pre><p>There are no specific rules for formatting parameter and local variable annotations.</p>
  521         <a name="comments"/>
  522       <h4>Comments</h4>
  523       <a name="block-comment-style"/>
  524       <h5>Block comment style</h5>
  525       <p>Block comments are indented at the same level as the surrounding code. They may be in
  526         <code>/* ... */</code> style or
  527         <code>// ...</code> style. For multi-line
  528         <code>/* ... */</code> comments, subsequent lines must start with
  529         <code>*</code> aligned with the <code>*</code> on the previous line.</p><pre>
  530       /*
  531        * This is          // And so           /* Or you can
  532        * okay.            // is this.          * even do this. */
  533        */
  534     </pre>
  535         <p>Comments are not enclosed in boxes drawn with asterisks or other characters.</p>
  536         <p><strong>Tip:</strong> When writing multi-line comments, use the
  537       <code>/* ... */</code> style if you want automatic code formatters to
  538       re-wrap the lines when necessary (paragraph-style). Most formatters don't re-wrap lines in
  539       <code>// ...</code> style comment blocks.</p>
  540         <a name="modifiers"/>
  541       <h4>Modifiers</h4>
  542       <p>Class and member modifiers, when present, appear in the order
  543         recommended by the Java Language Specification:
  544       </p><pre>
  545       public protected private abstract static final transient volatile synchronized native strictfp
  546     </pre>
  547         <a name="numeric-literals"/>
  548       <h4>Numeric Literals</h4>
  549       <p><code>long</code>-valued integer literals use an uppercase <code>L</code> suffix, never
  550         lowercase (to avoid confusion with the digit <code>1</code>). For example, <code>3000000000L</code>
  551         rather than <code>3000000000l</code>.</p>
  552       </subsection>
  553       <a name="naming"/>
  554       <subsection name="Naming">
  555       <a name="identifier-names"/>
  556       <h3>Rules common to all identifiers</h3>
  557       <p>Identifiers use only ASCII letters and digits, and in two cases noted below, underscores. Thus
  558         each valid identifier name is matched by the regular expression <code>\w+</code> .</p>
  559         <p> In Google Style special prefixes or
  560       suffixes, like those seen in the examples <code>name_</code>,
  561       <code>mName</code>, <code>s_name</code> and
  562       <code>kName</code>, are <strong>not</strong> used.</p>
  563         <a name="specific-identifier-names"/>
  564       <h3>Rules by identifier type</h3>
  565       <a name="package-names"/>
  566       <h4>Package names</h4>
  567       <p>Package names are all lowercase, with consecutive words simply concatenated together (no
  568         underscores). For example, <code>com.example.deepspace</code>, not
  569         <code>com.example.deepSpace</code> or
  570         <code>com.example.deep_space</code>.</p>
  571         <a name="class-names"/>
  572       <h4>Class names</h4>
  573       <p>Class names are written in <a href="#camel-case">UpperCamelCase</a>.</p>
  574         <p>Class names are typically nouns or noun phrases. For example,
  575       <code>Character</code> or
  576       <code>ImmutableList</code>. Interface names may also be nouns or
  577       noun phrases (for example, <code>List</code>), but may sometimes be
  578       adjectives or adjective phrases instead (for example,
  579       <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
  580       with <code>Test</code>. For example,
  581       <code>HashTest</code> or
  582       <code>HashIntegrationTest</code>.</p>
  583         <a name="method-names"/>
  584       <h4>Method names</h4>
  585       <p>Method names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p>
  586         <p>Method names are typically verbs or verb phrases. For example,
  587       <code>sendMessage</code> or
  588       <code>stop</code>.</p><p>Underscores may appear in JUnit <em>test</em> method names to separate logical components of the
  589       name. One typical pattern is <code>test<i>&lt;MethodUnderTest&gt;</i>_<i>&lt;state&gt;</i></code>,
  590       for example <code>testPop_emptyStack</code>. There is no One Correct
  591       Way to name test methods.</p>
  592         <a name="constants"/>
  593         <a name="constant-names"/>
  594       <h4>Constant names</h4>
  595       <p>Constant names use <code>CONSTANT_CASE</code>: all uppercase
  596         letters, with words separated by underscores. But what <em>is</em> a constant, exactly?</p>
  597         <p>Every constant is a static final field, but not all static final fields are constants. Before
  598       choosing constant case, consider whether the field really <em>feels like</em> a constant. For
  599       example, if any of that instance's observable state can change, it is almost certainly not a
  600       constant. Merely <em>intending</em> to never mutate the object is generally not
  601       enough. Examples:</p><pre>
  602       // Constants
  603       static final int NUMBER = 5;
  604       static final ImmutableList&lt;String&gt; NAMES = ImmutableList.of("Ed", "Ann");
  605       static final Joiner COMMA_JOINER = Joiner.on(',');  // because Joiner is immutable
  606       static final SomeMutableType[] EMPTY_ARRAY = {};
  607       enum SomeEnum { ENUM_CONSTANT }
  609       // Not constants
  610       static String nonFinal = "non-final";
  611       final String nonStatic = "non-static";
  612       static final Set&lt;String&gt; mutableCollection = new HashSet&lt;String&gt;();
  613       static final ImmutableSet&lt;SomeMutableType&gt; mutableElements = ImmutableSet.of(mutable);
  614       static final Logger logger = Logger.getLogger(MyClass.getName());
  615       static final String[] nonEmptyArray = {"these", "can", "change"};
  616     </pre>
  617         <p>These names are typically nouns or noun phrases.</p>
  618         <a name="non-constant-field-names"/>
  619       <h4>Non-constant field names</h4>
  620       <p>Non-constant field names (static or otherwise) are written
  621         in <a href="#camel-case">lowerCamelCase</a>.</p>
  622         <p>These names are typically nouns or noun phrases.  For example,
  623       <code>computedValues</code> or
  624       <code>index</code>.</p>
  625         <a name="parameter-names"/>
  626       <h4>Parameter names</h4>
  627       <p>Parameter names are written in <a href="#camel-case">lowerCamelCase</a>.</p>
  628         <p>One-character parameter names should be avoided.</p>
  629         <a name="local-variable-names"/>
  630       <h4>Local variable names</h4>
  631       <p>Local variable names are written in <a href="#camel-case">lowerCamelCase</a>, and can be
  632         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
  633       be styled as constants.</p>
  634         <a name="type-variable-names"/>
  635       <h4>Type variable names</h4>
  636       <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
  637       <code>E</code>, <code>T</code>,
  638       <code>X</code>, <code>T2</code>)
  639     </li><li>A name in the form used for classes (see
  640       <a href="#class-names">Class names</a>), followed by the capital letter
  641       <code>T</code> (examples:
  642       <code>RequestT</code>,
  643       <code>FooBarT</code>).</li></ul><a name="acronyms"/>
  644         <a name="camelcase"/>
  645         <a name="camel-case"/>
  646       <h3>Camel case: defined</h3>
  647       <p>Sometimes there is more than one reasonable way to convert an English phrase into camel case,
  648         such as when acronyms or unusual constructs like "IPv6" or "iOS" are present. To improve
  649         predictability, Google Style specifies the following (nearly) deterministic scheme.</p>
  650         <p>Beginning with the prose form of the name:</p>
  651         <ol>
  652           <li>Convert the phrase to plain ASCII and remove any apostrophes. For example, "Müller's
  653       algorithm" might become "Muellers algorithm".</li>
  654           <li>Divide this result into words, splitting on spaces and any remaining punctuation (typically
  655       hyphens).
  657             <ul>
  658               <li><em>Recommended:</em> if any word already has a conventional camel-case appearance in common
  659         usage, split this into its constituent parts (e.g., "AdWords" becomes "ad words"). Note
  660         that a word such as "iOS" is not really in camel case <em>per se</em>; it defies <em>any</em>
  661         convention, so this recommendation does not apply.</li>
  662             </ul>
  663           </li>
  664           <li>Now lowercase <em>everything</em> (including acronyms), then uppercase only the first
  665       character of:
  666             <ul><li>... each word, to yield <em>upper camel case</em>, or</li>
  667               <li>... each word except the first, to yield <em>lower camel case</em></li>
  668             </ul>
  669           </li>
  670           <li>Finally, join all the words into a single identifier.</li>
  671         </ol>
  672         <p>Note that the casing of the original words is almost entirely disregarded. Examples:</p>
  673         <table>
  674           <tr><th>Prose form</th><th>Correct</th><th>Incorrect</th></tr>
  675           <tr><td>"XML HTTP request"</td><td><code>XmlHttpRequest</code></td><td><code>XMLHTTPRequest</code></td></tr>
  676           <tr><td>"new customer ID"</td><td><code>newCustomerId</code></td><td><code>newCustomerID</code></td></tr>
  677           <tr><td>"inner stopwatch"</td><td><code>innerStopwatch</code></td><td><code>innerStopWatch</code></td></tr>
  678           <tr><td>"supports IPv6 on iOS?"</td><td><code>supportsIpv6OnIos</code></td><td><code>supportsIPv6OnIOS</code></td></tr>
  679           <tr><td>"YouTube importer"</td><td><code>YouTubeImporter</code><br/><code>YoutubeImporter</code>*</td><td/></tr>
  680         </table>
  681         <p>*Acceptable, but not recommended.</p>
  682         <p><strong>Note:</strong> Some words are ambiguously hyphenated in the English
  683       language: for example "nonempty" and "non-empty" are both correct, so the method names
  684       <code>checkNonempty</code> and
  685       <code>checkNonEmpty</code> are likewise both correct.</p>
  686       </subsection>
  687       <subsection name="Programming Practices">
  688       <a name="programming-practices"/>
  689       <a name="override-annotation"/>
  690       <h3>@Override: always used</h3>
  691       <p>A method is marked with the <code>@Override</code> annotation
  692         whenever it is legal.  This includes a class method overriding a superclass method, a class method
  693         implementing an interface method, and an interface method respecifying a superinterface
  694         method.</p>
  695       <p class="exception"><strong>Exception:</strong><code>@Override</code> may be omitted when the parent method is
  696       <code>@Deprecated</code>.</p>
  697       <a name="caughtexceptions"/>
  698       <a name="caught-exceptions"/>
  699       <h3>Caught exceptions: not ignored</h3>
  700       <p>Except as noted below, it is very rarely correct to do nothing in response to a caught
  701         exception. (Typical responses are to log it, or if it is considered "impossible", rethrow it as an
  702         <code>AssertionError</code>.)</p>
  703         <p>When it truly is appropriate to take no action whatsoever in a catch block, the reason this is
  704       justified is explained in a comment.</p><pre>
  705       try {
  706           int i = Integer.parseInt(response);
  707           return handleNumericResponse(i);
  708       } catch (NumberFormatException ok) {
  709           // it's not numeric; that's fine, just continue
  710       }
  711       return handleTextResponse(response);
  712     </pre><p><strong>Exception:</strong> In tests, a caught exception may be ignored
  713       without comment <em>if</em> it is named <code>expected</code>. The
  714       following is a very common idiom for ensuring that the method under test <em>does</em> throw an
  715       exception of the expected type, so a comment is unnecessary here.</p><pre>
  716       try {
  717           emptyStack.pop();
  718           fail();
  719       } catch (NoSuchElementException expected) {
  720       }
  721     </pre><a name="static-members"/>
  722       <h3>Static members: qualified using class</h3>
  723       <p>When a reference to a static class member must be qualified, it is qualified with that class's
  724         name, not with a reference or expression of that class's type.</p><pre>
  725       Foo aFoo = ...;
  726       Foo.aStaticMethod(); // good
  727       <span>aFoo.aStaticMethod();</span> // bad
  728       <span>somethingThatYieldsAFoo().aStaticMethod();</span> // very bad
  729     </pre>
  730         <a name="finalizers"/>
  731       <h3>Finalizers: not used</h3>
  732       <p>It is <strong>extremely rare</strong> to override <code>Object.finalize</code>.</p>
  733       <p><strong>Tip:</strong> Don't do it. If you absolutely must, first read and understand
  734       <a href="http://books.google.com/books?isbn=8131726592"><em>Effective Java</em></a>
  735       Item 7, "Avoid Finalizers," very carefully, and <em>then</em> don't do it.</p>
  736       </subsection>
  737       <a name="javadoc"/>
  738       <subsection name="Javadoc">
  739       <a name="javadoc-formatting"/>
  740       <h3>Formatting</h3>
  741       <a name="javadoc-multi-line"/>
  742       <h4>General form</h4>
  743       <p>The <em>basic</em> formatting of Javadoc blocks is as seen in this example:</p><pre>
  744       /**
  745        * Multiple lines of Javadoc text are written here,
  746        * wrapped normally...
  747        */
  748       public int method(String p1) { ... }
  749     </pre><p>... or in this single-line example:</p><pre>
  750       /** An especially short bit of Javadoc. */
  751     </pre><p>The basic form is always acceptable. The single-line form may be substituted when there are no
  752       at-clauses present, and the entirety of the Javadoc block (including comment markers) can fit on a
  753       single line.</p>
  754       <a name="javadoc-paragraphs"/>
  755       <h4>Paragraphs</h4>
  756       <p>One blank line—that is, a line containing only the aligned leading asterisk
  757         (<code>*</code>)—appears between paragraphs, and before the group of "at-clauses" if
  758         present. Each paragraph but the first has <code>&lt;p&gt;</code> immediately before the first word,
  759         with no space after.</p>
  760       <a name="javadoc-at-clauses"/>
  761       <h4>At-clauses</h4>
  762       <p>Any of the standard "at-clauses" that are used appear in the order <code>@param</code>,
  763         <code>@return</code>, <code>@throws</code>, <code>@deprecated</code>, and these four types never
  764         appear with an empty description. When an at-clause doesn't fit on a single line, continuation lines
  765         are indented four (or more) spaces from the position of the <code>@</code>.
  766       </p>
  767         <a name="summary-fragment"/>
  768       <h3>The summary fragment</h3>
  769       <p>The Javadoc for each class and member begins with a brief <strong>summary fragment</strong>. This
  770         fragment is very important: it is the only part of the text that appears in certain contexts such as
  771         class and method indexes.</p><p>This is a fragment—a noun phrase or verb phrase, not a complete sentence. It does
  772       <strong>not</strong> begin with <code>A {@code Foo} is a...</code>, or
  773       <code>This method returns...</code>, nor does it form a complete imperative sentence
  774       like <code>Save the record.</code>. However, the fragment is capitalized and
  775       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
  776       <code>/** @return the customer ID */</code>. This is incorrect, and should be
  777       changed to <code>/** Returns the customer ID. */</code>.</p>
  778       <a name="javadoc-optional"/>
  779       <a name="javadoc-where-required"/>
  780       <h3>Where Javadoc is used</h3>
  781       <p>At the <em>minimum</em>, Javadoc is present for every
  782         <code>public</code> class, and every
  783         <code>public</code> or
  784         <code>protected</code> member of such a class, with a few exceptions
  785         noted below.</p><p>Other classes and members still have Javadoc <em>as needed</em>.  Whenever an implementation
  786       comment would be used to define the overall purpose or behavior of a class, method or field, that
  787       comment is written as Javadoc instead. (It's more uniform, and more tool-friendly.)</p>
  788       <a name="javadoc-exception-self-explanatory"/>
  789       <h4>Exception: self-explanatory methods</h4>
  790       <p>Javadoc is optional for "simple, obvious" methods like
  791         <code>getFoo</code>, in cases where there <em>really and truly</em> is
  792         nothing else worthwhile to say but "Returns the foo".</p>
  793         <p class="note"><strong>Important:</strong> it is not appropriate to cite this exception to justify
  794       omitting relevant information that a typical reader might need to know. For example, for a method
  795       named <code>getCanonicalName</code>, don't omit its documentation
  796       (with the rationale that it would say only
  797       <code>/** Returns the canonical name. */</code>) if a typical reader may have no idea
  798       what the term "canonical name" means!</p>
  799         <a name="javadoc-exception-overrides"/>
  800       <h4>Exception: overrides</h4>
  801       <p>Javadoc is not always present on a method that overrides a supertype method.
  802       </p>
  803       </subsection>
  804     </section>
  805   </body>
  806 </document>