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Brian Silverman1a675112016-02-20 20:42:49 -0500[diff] [blame]1<html lang="en">
2<head>
3 <meta http-equiv="content-type" content="text/html; charset=UTF-8" />
4 <link rel="stylesheet" type="text/css" href="javaguide.css"/>
5 <script src="https://google-code-prettify.googlecode.com/svn/loader/run_prettify.js"
6 type="text/javascript"></script>
7 <link href="http://www.google.com/favicon.ico"
8 type="image/x-icon" rel="shortcut icon" />
9 <title>WPILib Java Style</title>
10</head>
11<body>
12<h1>WPILib Java Style (Based on the
13 <a href=http://google-styleguide.googlecode.com/svn/trunk/javaguide.html>Google Java Style Guide</a>)</h1>
14 <div class="change">Last changed: June 19, 2015</div>
15<table border="0">
16<tr>
17<td>
18<dl>
19<br>
20<dt class="toc1">
21<a href="#s1-introduction">1 Introduction</a>
22</dt>
23<dd>
24<a href="#s1.1-terminology">1.1 Terminology notes</a>
25</dd>
26<dd>
27<a href="#s1.2-guide-notes">1.2 Guide notes</a>
28</dd>
29<br>
30<dt class="toc1">
31<a href="#s2-source-file-basics">2 Source file basics</a>
32</dt>
33<dd>
34<a href="#s2.1-file-name">2.1 File name</a>
35</dd>
36<dd>
37<a href="#s2.2-file-encoding">2.2 File encoding: UTF-8</a>
38</dd>
39<dd>
40<a href="#s2.3-special-characters">2.3 Special characters</a>
41</dd>
42<dd class="toc3">
43<a href="#s2.3.1-whitespace-characters">2.3.1 Whitespace characters</a>
44</dd>
45<dd class="toc3">
46<a href="#s2.3.2-special-escape-sequences">2.3.2 Special escape sequences</a>
47</dd>
48<dd class="toc3">
49<a href="#s2.3.3-non-ascii-characters">2.3.3 Non-ASCII characters</a>
50</dd>
51<br>
52<dt class="toc1">
53<a href="#s3-source-file-structure">3 Source file structure</a>
54</dt>
55<dd>
56<a href="#s3.1-copyright-statement">3.1 License or copyright information, if present</a>
57</dd>
58<dd>
59<a href="#s3.2-package-statement">3.2 Package statement</a>
60</dd>
61<dd>
62<a href="#s3.3-import-statements">3.3 Import statements</a>
63</dd>
64<dd class="toc3">
65<a href="#s3.3.1-wildcard-imports">3.3.1 No wildcard imports</a>
66</dd>
67<dd class="toc3">
68<a href="#s3.3.2-import-line-wrapping">3.3.2 No line-wrapping</a>
69</dd>
70<dd class="toc3">
71<a href="#s3.3.3-import-ordering-and-spacing">3.3.3 Ordering and spacing</a>
72</dd>
73<dd>
74<a href="#s3.4-class-declaration">3.4 Class declaration</a>
75</dd>
76<dd class="toc3">
77<a href="#s3.4.1-one-top-level-class">3.4.1 Exactly one top-level class declaration</a>
78</dd>
79<dd class="toc3">
80<a href="#s3.4.2-class-member-ordering">3.4.2 Class member ordering</a>
81</dd>
82</dl>
83</td><td>
84<dl>
85<br>
86<dt class="toc1">
87<a href="#s4-formatting">4 Formatting</a>
88</dt>
89<dd>
90<a href="#s4.1-braces">4.1 Braces</a>
91</dd>
92<dd class="toc3">
93<a href="#s4.1.1-braces-always-used">4.1.1 Braces are used where optional</a>
94</dd>
95<dd class="toc3">
96<a href="#s4.1.2-blocks-k-r-style">4.1.2 Nonempty blocks: K &amp; R style</a>
97</dd>
98<dd class="toc3">
99<a href="#s4.1.3-braces-empty-blocks">4.1.3 Empty blocks: may be concise</a>
100</dd>
101<dd>
102<a href="#s4.2-block-indentation">4.2 Block indentation: +2 spaces</a>
103</dd>
104<dd>
105<a href="#s4.3-one-statement-per-line">4.3 One statement per line</a>
106</dd>
107<dd>
108<a href="#s4.4-column-limit">4.4 Column limit: 80 or 100</a>
109</dd>
110<dd>
111<a href="#s4.5-line-wrapping">4.5 Line-wrapping</a>
112</dd>
113<dd class="toc3">
114<a href="#s4.5.1-line-wrapping-where-to-break">4.5.1 Where to break</a>
115</dd>
116<dd class="toc3">
117<a href="#s4.5.2-line-wrapping-indent">4.5.2 Indent continuation lines at least +4 spaces</a>
118</dd>
119<dd>
120<a href="#s4.6-whitespace">4.6 Whitespace</a>
121</dd>
122<dd class="toc3">
123<a href="#s4.6.1-vertical-whitespace">4.6.1 Vertical Whitespace</a>
124</dd>
125<dd class="toc3">
126<a href="#s4.6.2-horizontal-whitespace">4.6.2 Horizontal whitespace</a>
127</dd>
128<dd class="toc3">
129<a href="#s4.6.3-horizontal-alignment">4.6.3 Horizontal alignment: never required</a>
130</dd>
131<dd>
132<a href="#s4.7-grouping-parentheses">4.7 Grouping parentheses: recommended</a>
133</dd>
134<dd>
135<a href="#s4.8-specific-constructs">4.8 Specific constructs</a>
136</dd>
137<dd class="toc3">
138<a href="#s4.8.1-enum-classes">4.8.1 Enum classes</a>
139</dd>
140<dd class="toc3">
141<a href="#s4.8.2-variable-declarations">4.8.2 Variable declarations</a>
142</dd>
143<dd class="toc3">
144<a href="#s4.8.3-arrays">4.8.3 Arrays</a>
145</dd>
146<dd class="toc3">
147<a href="#s4.8.4-switch">4.8.4 Switch statements</a>
148</dd>
149<dd class="toc3">
150<a href="#s4.8.5-annotations">4.8.5 Annotations</a>
151</dd>
152<dd class="toc3">
153<a href="#s4.8.6-comments">4.8.6 Comments</a>
154</dd>
155<dd class="toc3">
156<a href="#s4.8.7-modifiers">4.8.7 Modifiers</a>
157</dd>
158<dd class="toc3">
159<a href="#s4.8.8-numeric-literals">4.8.8 Numeric Literals</a>
160</dd>
161</dl>
162</td><td>
163<dl>
164<br>
165<dt class="toc1">
166<a href="#s5-naming">5 Naming</a>
167</dt>
168<dd>
169<a href="#s5.1-identifier-names">5.1 Rules common to all identifiers</a>
170</dd>
171<dd>
172<a href="#s5.2-specific-identifier-names">5.2 Rules by identifier type</a>
173</dd>
174<dd class="toc3">
175<a href="#s5.2.1-package-names">5.2.1 Package names</a>
176</dd>
177<dd class="toc3">
178<a href="#s5.2.2-class-names">5.2.2 Class names</a>
179</dd>
180<dd class="toc3">
181<a href="#s5.2.3-method-names">5.2.3 Method names</a>
182</dd>
183<dd class="toc3">
184<a href="#s5.2.4-constant-names">5.2.4 Constant names</a>
185</dd>
186<dd class="toc3">
187<a href="#s5.2.5-non-constant-field-names">5.2.5 Non-constant field names</a>
188</dd>
189<dd class="toc3">
190<a href="#s5.2.6-parameter-names">5.2.6 Parameter names</a>
191</dd>
192<dd class="toc3">
193<a href="#s5.2.7-local-variable-names">5.2.7 Local variable names</a>
194</dd>
195<dd class="toc3">
196<a href="#s5.2.8-type-variable-names">5.2.8 Type variable names</a>
197</dd>
198<dd>
199<a href="#s5.3-camel-case">5.3 Camel case: defined</a>
200</dd>
201<br>
202<dt class="toc1">
203<a href="#s6-programming-practices">6 Programming Practices</a>
204</dt>
205<dd>
206<a href="#s6.1-override-annotation">6.1 @Override: always used</a>
207</dd>
208<dd>
209<a href="#s6.2-caught-exceptions">6.2 Caught exceptions: not ignored</a>
210</dd>
211<dd>
212<a href="#s6.3-static-members">6.3 Static members: qualified using class</a>
213</dd>
214<dd>
215<a href="#s6.4-finalizers">6.4 Finalizers: not used</a>
216</dd>
217<br>
218<dt class="toc1">
219<a href="#s7-javadoc">7 Javadoc</a>
220</dt>
221<dd>
222<a href="#s7.1-javadoc-formatting">7.1 Formatting</a>
223</dd>
224<dd class="toc3">
225<a href="#s7.1.1-javadoc-multi-line">7.1.1 General form</a>
226</dd>
227<dd class="toc3">
228<a href="#s7.1.2-javadoc-paragraphs">7.1.2 Paragraphs</a>
229</dd>
230<dd class="toc3">
231<a href="#s7.1.3-javadoc-at-clauses">7.1.3 At-clauses</a>
232</dd>
233<dd>
234<a href="#s7.2-summary-fragment">7.2 The summary fragment</a>
235</dd>
236<dd>
237<a href="#s7.3-javadoc-where-required">7.3 Where Javadoc is used</a>
238</dd>
239<dd class="toc3">
240<a href="#s7.3.1-javadoc-exception-self-explanatory">7.3.1 Exception: self-explanatory methods</a>
241</dd>
242<dd class="toc3">
243<a href="#s7.3.2-javadoc-exception-overrides">7.3.2 Exception: overrides</a>
244</dd>
245</dl>
246</td>
247</tr>
248</table>
249<div><a name="s1-introduction"/>
250 <h2>1 Introduction&nbsp;<a href="#s1-introduction"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
251
252<p><strong>This guide is a work in progress.</strong>
253We are currently working on getting this guide updated to
254a point where it is useful for WPILib developers to use.</p>
255
256 <p>This document serves as the style guide for WPILib. It is <em>heavily</em>
257 based on the Google Java Style Guide and copies pretty much word-for-word
258 the formatting/style components of the guide while cutting a couple
259 of the programming practices. As this guide evolves, we will likely
260 introduce more suggested/mandated programming practices specific
261 to WPILib.</p>
262 <p>It is encouraged that anyone working on the Java WPILib also
263 read the corresponding C++ guide, as we generally try to develop
264 the C++ and Java components of the library in parallel and many
265 programming practices true in one language will be true in the other
266 (although this is not universally true).</p>
267</p><a name="s1.1-terminology"/>
268 <h3>1.1 Terminology notes&nbsp;<a href="#s1.1-terminology"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
269 <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,
270 interface or annotation type (<code class="prettyprint lang-java">@interface</code>).</li><li>The term <em>comment</em> always refers to <em>implementation</em> comments. We do not
271 use the phrase "documentation comments", instead using the common term "Javadoc."</li></ol><p>Other "terminology notes" will appear occasionally throughout the document.</p><a name="s1.2-guide-notes"/>
272 <h3>1.2 Guide notes&nbsp;<a href="#s1.2-guide-notes"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
273 <p>Example code in this document is <strong>non-normative</strong>. That is, while the examples
274are in Google Style, they may not illustrate the <em>only</em> stylish way to represent the
275code. Optional formatting choices made in examples should not be enforced as rules.</p><a name="s2-source-file-basics"/>
276 <h2>2 Source file basics&nbsp;<a href="#s2-source-file-basics"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
277 <a name="s2.1-file-name"/>
278 <h3>2.1 File name&nbsp;<a href="#s2.1-file-name"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
279 <p>The source file name consists of the case-sensitive name of the top-level class it contains,
280plus the <code>.java</code> extension.</p><a name="s2.2-file-encoding"/>
281 <h3>2.2 File encoding: UTF-8&nbsp;<a href="#s2.2-file-encoding"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
282 <p>Source files are encoded in <strong>UTF-8</strong>.</p><a name="s2.3-special-characters"/>
283 <h3>2.3 Special characters&nbsp;<a href="#s2.3-special-characters"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
284 <a name="s2.3.1-whitespace-characters"/>
285 <h4>2.3.1 Whitespace characters&nbsp;<a href="#s2.3.1-whitespace-characters"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
286 <p>Aside from the line terminator sequence, the <strong>ASCII horizontal space
287character</strong> (<strong>0x20</strong>) is the only whitespace character that appears
288anywhere in a source file. This implies that:</p><ol><li>All other whitespace characters in string and character literals are escaped.</li><li>Tab characters are <strong>not</strong> used for indentation.</li></ol><a name="s2.3.2-special-escape-sequences"/>
289 <h4>2.3.2 Special escape sequences&nbsp;<a href="#s2.3.2-special-escape-sequences"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
290 <p>For any character that has a special escape sequence
291(<code class="prettyprint lang-java">\b</code>,
292<code class="prettyprint lang-java">\t</code>,
293<code class="prettyprint lang-java">\n</code>,
294<code class="prettyprint lang-java">\f</code>,
295<code class="prettyprint lang-java">\r</code>,
296<code class="prettyprint lang-java">\"</code>,
297<code class="prettyprint lang-java">\'</code> and
298<code class="prettyprint lang-java">\\</code>), that sequence
299is used rather than the corresponding octal
300(e.g. <code class="badcode">\012</code>) or Unicode
301(e.g. <code class="badcode">\u000a</code>) escape.</p><a name="s2.3.3-non-ascii-characters"/>
302 <h4>2.3.3 Non-ASCII characters&nbsp;<a href="#s2.3.3-non-ascii-characters"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
303 <p>For the remaining non-ASCII characters, either the actual Unicode character
304(e.g. <code class="prettyprint lang-java">∞</code>) or the equivalent Unicode escape
305(e.g. <code class="prettyprint lang-java">\u221e</code>) is used, depending only on which
306makes the code <strong>easier to read and understand</strong>.</p><p class="tip"><strong>Tip:</strong> In the Unicode escape case, and occasionally even when actual
307Unicode characters are used, an explanatory comment can be very helpful.</p><p>Examples:</p><table><tr><th>Example</th><th>Discussion</th></tr><tr><td><code class="prettyprint lang-java">String unitAbbrev = "μs";</code></td><td>Best: perfectly clear even without a comment.</td></tr><tr><td><code class="prettyprint lang-java">String unitAbbrev = "\u03bcs"; // "μs"</code></td><td>Allowed, but there's no reason to do this.</td></tr><tr><td><code class="prettyprint lang-java">String unitAbbrev = "\u03bcs";
308 // Greek letter mu, "s"</code></td><td>Allowed, but awkward and prone to mistakes.</td></tr><tr><td><code class="badcode">String unitAbbrev = "\u03bcs";</code></td><td>Poor: the reader has no idea what this is.</td></tr><tr><td><code class="prettyprint lang-java">return '\ufeff' + content;
309 // byte order mark</code></td><td>Good: use escapes for non-printable characters, and comment if necessary.</td></tr></table><p class="tip"><strong>Tip:</strong> Never make your code less readable simply out of fear that
310some programs might not handle non-ASCII characters properly. If that should happen, those
311programs are <strong>broken</strong> and they must be <strong>fixed</strong>.</p><a name="filestructure"/><a name="s3-source-file-structure"/>
312 <h2>3 Source file structure&nbsp;<a href="#s3-source-file-structure"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
313 <div><p>A source file consists of, <strong>in order</strong>:</p><ol><li>License or copyright information, if present</li><li>Package statement</li><li>Import statements</li><li>Exactly one top-level class</li></ol></div><p><strong>Exactly one blank line</strong> separates each section that is present.</p><a name="s3.1-copyright-statement"/>
314 <h3>3.1 License or copyright information, if present&nbsp;<a href="#s3.1-copyright-statement"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
315 <p>If license or copyright information belongs in a file, it belongs here.</p><a name="s3.2-package-statement"/>
316 <h3>3.2 Package statement&nbsp;<a href="#s3.2-package-statement"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
317 <p>The package statement is <strong>not line-wrapped</strong>. The column limit (Section 4.4,
318<a href="#s4.4-column-limit">Column limit: 80</a>) does not apply to package statements.</p><a name="imports"/><a name="s3.3-import-statements"/>
319 <h3>3.3 Import statements&nbsp;<a href="#s3.3-import-statements"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
320 <a name="s3.3.1-wildcard-imports"/>
321 <h4>3.3.1 No wildcard imports&nbsp;<a href="#s3.3.1-wildcard-imports"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
322 <p><strong>Wildcard imports</strong>, static or otherwise, <strong>are not used</strong>.</p><a name="s3.3.2-import-line-wrapping"/>
323 <h4>3.3.2 No line-wrapping&nbsp;<a href="#s3.3.2-import-line-wrapping"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
324 <p>Import statements are <strong>not line-wrapped</strong>. The column limit (Section 4.4,
325<a href="#s4.4-column-limit">Column limit: 80</a>) does not apply to import
326statements.</p><a name="s3.3.3-import-ordering-and-spacing"/>
327 <h4>3.3.3 Ordering and spacing&nbsp;<a href="#s3.3.3-import-ordering-and-spacing"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
328 <p>Import statements are divided into the following groups, in this order, with each group
329separated by a single blank line:</p><ol><li>All static imports in a single group</li><li><code>com.google</code> imports
330 (only if this source file is in the <code>com.google</code> package
331 space)</li><li>Third-party imports, one group per top-level package, in ASCII sort order
332 <ul><li>for example: <code>android</code>, <code>com</code>, <code>junit</code>, <code>org</code>,
333 <code>sun</code></li></ul></li><li><code>java</code> imports</li><li><code>javax</code> imports</li></ol><p>Within a group there are no blank lines, and the imported names appear in ASCII sort
334order. (<strong>Note:</strong> this is not the same as the import <em>statements</em> being in
335ASCII sort order; the presence of semicolons warps the result.)</p><a name="s3.4-class-declaration"/>
336 <h3>3.4 Class declaration&nbsp;<a href="#s3.4-class-declaration"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
337 <a name="oneclassperfile"/><a name="s3.4.1-one-top-level-class"/>
338 <h4>3.4.1 Exactly one top-level class declaration&nbsp;<a href="#s3.4.1-one-top-level-class"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
339 <p>Each top-level class resides in a source file of its own.</p><a name="s3.4.2-class-member-ordering"/>
340 <h4>3.4.2 Class member ordering&nbsp;<a href="#s3.4.2-class-member-ordering"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
341 <p>The ordering of the members of a class can have a great effect on learnability, but there is
342no single correct recipe for how to do it. Different classes may order their members
343differently.</p><p>What is important is that each class order its members in <strong><em>some</em> logical
344order</strong>, which its maintainer could explain if asked. For example, new methods are not
345just habitually added to the end of the class, as that would yield "chronological by date
346added" ordering, which is not a logical ordering.</p><a name="overloads"/><a name="s3.4.2.1-overloads-never-split"/>
347 <h5>3.4.2.1 Overloads: never split&nbsp;<a href="#s3.4.2.1-overloads-never-split"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
348 <p>When a class has multiple constructors, or multiple methods with the same name, these appear
349sequentially, with no intervening members.</p><a name="s4-formatting"/>
350 <h2>4 Formatting&nbsp;<a href="#s4-formatting"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
351 <p class="terminology"><strong>Terminology Note:</strong> <em>block-like construct</em> refers to
352the body of a class, method or constructor. Note that, by Section 4.8.3.1 on
353<a href="#s4.8.3.1-array-initializers">array initializers</a>, any array initializer
354<em>may</em> optionally be treated as if it were a block-like construct.</p><a name="braces"/><a name="s4.1-braces"/>
355 <h3>4.1 Braces&nbsp;<a href="#s4.1-braces"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
356 <a name="s4.1.1-braces-always-used"/>
357 <h4>4.1.1 Braces are used where optional&nbsp;<a href="#s4.1.1-braces-always-used"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
358 <p>Braces are used with
359<code class="prettyprint lang-java">if</code>,
360<code class="prettyprint lang-java">else</code>,
361<code class="prettyprint lang-java">for</code>,
362<code class="prettyprint lang-java">do</code> and
363<code class="prettyprint lang-java">while</code> statements, even when the
364body is empty or contains only a single statement.</p><a name="s4.1.2-blocks-k-r-style"/>
365 <h4>4.1.2 Nonempty blocks: K &amp; R style&nbsp;<a href="#s4.1.2-blocks-k-r-style"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
366 <p>Braces follow the Kernighan and Ritchie style
367("<a href="http://www.codinghorror.com/blog/2012/07/new-programming-jargon.html">Egyptian brackets</a>")
368for <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
369 of a method, constructor or <em>named</em> class. For example, there is <em>no</em> line break
370 after the brace if it is followed by <code class="prettyprint lang-java">else</code> or a
371 comma.</li></ul><p>Example:</p><pre class="prettyprint lang-java">
372return new MyClass() {
373 @Override public void method() {
374 if (condition()) {
375 try {
376 something();
377 } catch (ProblemException e) {
378 recover();
379 }
380 }
381 }
382};
383</pre><p>A few exceptions for enum classes are given in Section 4.8.1,
384<a href="#s4.8.1-enum-classes">Enum classes</a>.</p><a name="emptyblocks"/><a name="s4.1.3-braces-empty-blocks"/>
385 <h4>4.1.3 Empty blocks: may be concise&nbsp;<a href="#s4.1.3-braces-empty-blocks"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
386 <p>An empty block or block-like construct <em>may</em> be closed immediately after it is
387opened, with no characters or line break in between
388(<code class="prettyprint lang-java">{}</code>), <strong>unless</strong> it is part of a
389<em>multi-block statement</em> (one that directly contains multiple blocks:
390<code class="prettyprint lang-java">if/else-if/else</code> or
391<code class="prettyprint lang-java">try/catch/finally</code>).</p><p>Example:</p><pre class="prettyprint lang-java">
392 void doNothing() {}
393</pre><a name="s4.2-block-indentation"/>
394 <h3>4.2 Block indentation: +2 spaces&nbsp;<a href="#s4.2-block-indentation"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
395 <p>Each time a new block or block-like construct is opened, the indent increases by two
396spaces. When the block ends, the indent returns to the previous indent level. The indent level
397applies to both code and comments throughout the block. (See the example in Section 4.1.2,
398<a href="#s4.1.2-blocks-k-r-style">Nonempty blocks: K &amp; R Style</a>.)</p><a name="s4.3-one-statement-per-line"/>
399 <h3>4.3 One statement per line&nbsp;<a href="#s4.3-one-statement-per-line"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
400 <p>Each statement is followed by a line-break.</p><a name="columnlimit"/><a name="s4.4-column-limit"/>
401 <h3>4.4 Column limit: 80&nbsp;<a href="#s4.4-column-limit"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
402 <p>
403 Projects should have a column limit of 80 characters.
404
405Except as noted below, any line that would exceed this limit must be line-wrapped, as explained in
406Section 4.5, <a href="#s4.5-line-wrapping">Line-wrapping</a>.
407</p><p><strong>Exceptions:</strong></p><ol><li>Lines where obeying the column limit is not possible (for example, a long URL in Javadoc,
408 or a long JSNI method reference).</li><li><code class="prettyprint lang-java">package</code> and
409 <code class="prettyprint lang-java">import</code> statements (see Sections
410 3.2 <a href="#s3.2-package-statement">Package statement</a> and
411 3.3 <a href="#s3.3-import-statements">Import statements</a>).</li><li>Command lines in a comment that may be cut-and-pasted into a shell.</li></ol><a name="s4.5-line-wrapping"/>
412 <h3>4.5 Line-wrapping&nbsp;<a href="#s4.5-line-wrapping"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
413 <p class="terminology"><strong>Terminology Note:</strong> When code that might otherwise legally
414occupy a single line is divided into multiple lines, typically to avoid overflowing the column
415limit, this activity is called
416<em>line-wrapping</em>.</p><p>There is no comprehensive, deterministic formula showing <em>exactly</em> how to line-wrap in
417every 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
418without the need to line-wrap.</p><a name="s4.5.1-line-wrapping-where-to-break"/>
419 <h4>4.5.1 Where to break&nbsp;<a href="#s4.5.1-line-wrapping-where-to-break"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
420 <p>The prime directive of line-wrapping is: prefer to break at a
421<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>
422 the symbol. (Note that this is not the same practice used in Google style for other languages,
423 such as C++ and JavaScript.)
424 <ul><li>This also applies to the following "operator-like" symbols: the dot separator
425 (<code class="prettyprint lang-java">.</code>), the ampersand in type bounds
426 (<code class="prettyprint lang-java">&lt;T extends Foo &amp; Bar&gt;</code>), and the pipe in
427 catch blocks
428 (<code class="prettyprint lang-java">catch (FooException | BarException e)</code>).</li></ul></li><li>When a line is broken at an <em>assignment</em> operator the break typically comes
429 <em>after</em> the symbol, but either way is acceptable.
430 <ul><li>This also applies to the "assignment-operator-like" colon in an enhanced
431 <code class="prettyprint lang-java">for</code> ("foreach") statement.</li></ul></li><li>A method or constructor name stays attached to the open parenthesis
432 (<code class="prettyprint lang-java">(</code>) that follows it.</li><li>A comma (<code class="prettyprint lang-java">,</code>) stays attached to the token that
433 precedes it.</li></ol><a name="indentation"/><a name="s4.5.2-line-wrapping-indent"/>
434 <h4>4.5.2 Indent continuation lines at least +4 spaces&nbsp;<a href="#s4.5.2-line-wrapping-indent"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
435 <p>When line-wrapping, each line after the first (each <em>continuation line</em>) is indented
436at least +4 from the original line.</p><p>When there are multiple continuation lines, indentation may be varied beyond +4 as
437desired. In general, two continuation lines use the same indentation level if and only if they
438begin with syntactically parallel elements.</p><p>Section 4.6.3 on <a href="#s4.6.3-horizontal-alignment">Horizontal alignment</a> addresses
439the discouraged practice of using a variable number of spaces to align certain tokens with
440previous lines.</p><a name="s4.6-whitespace"/>
441 <h3>4.6 Whitespace&nbsp;<a href="#s4.6-whitespace"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
442 <a name="s4.6.1-vertical-whitespace"/>
443 <h4>4.6.1 Vertical Whitespace&nbsp;<a href="#s4.6.1-vertical-whitespace"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
444 <p>A single blank line appears:</p><ol><li><em>Between</em> consecutive members (or initializers) of a class: fields, constructors,
445 methods, nested classes, static initializers, instance initializers.
446 <ul><li><span class="exception"><strong>Exception:</strong> A blank line between two consecutive
447 fields (having no other code between them) is optional. Such blank lines are used as needed to
448 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
449 encouraged nor discouraged).</li><li>As required by other sections of this document (such as Section 3.3,
450 <a href="#s3.3-import-statements">Import statements</a>).</li></ol><p><em>Multiple</em> consecutive blank lines are permitted, but never required (or encouraged).</p><a name="s4.6.2-horizontal-whitespace"/>
451 <h4>4.6.2 Horizontal whitespace&nbsp;<a href="#s4.6.2-horizontal-whitespace"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
452 <p>Beyond where required by the language or other style rules, and apart from literals, comments and
453Javadoc, a single ASCII space also appears in the following places <strong>only</strong>.</p><ol><li>Separating any reserved word, such as
454 <code class="prettyprint lang-java">if</code>,
455 <code class="prettyprint lang-java">for</code> or
456 <code class="prettyprint lang-java">catch</code>, from an open parenthesis
457 (<code class="prettyprint lang-java">(</code>)
458 that follows it on that line</li><li>Separating any reserved word, such as
459 <code class="prettyprint lang-java">else</code> or
460 <code class="prettyprint lang-java">catch</code>, from a closing curly brace
461 (<code class="prettyprint lang-java">}</code>) that precedes it on that line</li><li>Before any open curly brace
462 (<code class="prettyprint lang-java">{</code>), with two exceptions:
463 <ul><li><code class="prettyprint lang-java">@SomeAnnotation({a, b})</code> (no space is used)</li><li><code class="prettyprint lang-java">String[][] x = {{"foo"}};</code> (no space is required
464 between <code class="prettyprint lang-java">{{</code>, by item 8 below)</li></ul></li><li>On both sides of any binary or ternary operator. This also applies to the following
465 "operator-like" symbols:
466 <ul><li>the ampersand in a conjunctive type bound:
467 <code class="prettyprint lang-java">&lt;T extends Foo &amp; Bar&gt;</code></li><li>the pipe for a catch block that handles multiple exceptions:
468 <code class="prettyprint lang-java">catch (FooException | BarException e)</code></li><li>the colon (<code class="prettyprint lang-java">:</code>) in an enhanced
469 <code class="prettyprint lang-java">for</code> ("foreach") statement</li></ul></li><li>After <code class="prettyprint lang-java">,:;</code> or the closing parenthesis
470 (<code class="prettyprint lang-java">)</code>) of a cast</li><li>On both sides of the double slash (<code class="prettyprint lang-java">//</code>) that
471 begins an end-of-line comment. Here, multiple spaces are allowed, but not required.</li><li>Between the type and variable of a declaration:
472 <code class="prettyprint lang-java">List&lt;String&gt; list</code></li><li><em>Optional</em> just inside both braces of an array initializer
473 <ul><li><code class="prettyprint lang-java">new int[] {5, 6}</code> and
474 <code class="prettyprint lang-java">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
475start or end of a line, only <em>interior</em> space.</p><a name="s4.6.3-horizontal-alignment"/>
476 <h4>4.6.3 Horizontal alignment: never required&nbsp;<a href="#s4.6.3-horizontal-alignment"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
477 <p class="terminology"><strong>Terminology Note:</strong> <em>Horizontal alignment</em> is the
478practice of adding a variable number of additional spaces in your code with the goal of making
479certain 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
480even required to <em>maintain</em> horizontal alignment in places where it was already used.</p><p>Here is an example without alignment, then using alignment:</p><pre class="prettyprint lang-java">
481private int x; // this is fine
482private Color color; // this too
483
484private int x; // permitted, but future edits
485private Color color; // may leave it unaligned
486</pre><p class="tip"><strong>Tip:</strong> Alignment can aid readability, but it creates problems for
487future maintenance. Consider a future change that needs to touch just one line. This change may
488leave the formerly-pleasing formatting mangled, and that is <strong>allowed</strong>. More often
489it prompts the coder (perhaps you) to adjust whitespace on nearby lines as well, possibly
490triggering a cascading series of reformattings. That one-line change now has a "blast radius."
491This can at worst result in pointless busywork, but at best it still corrupts version history
492information, slows down reviewers and exacerbates merge conflicts.</p><a name="parentheses"/><a name="s4.7-grouping-parentheses"/>
493 <h3>4.7 Grouping parentheses: recommended&nbsp;<a href="#s4.7-grouping-parentheses"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
494 <p>Optional grouping parentheses are omitted only when author and reviewer agree that there is no
495reasonable chance the code will be misinterpreted without them, nor would they have made the code
496easier to read. It is <em>not</em> reasonable to assume that every reader has the entire Java
497operator precedence table memorized.</p><a name="s4.8-specific-constructs"/>
498 <h3>4.8 Specific constructs&nbsp;<a href="#s4.8-specific-constructs"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
499 <a name="s4.8.1-enum-classes"/>
500 <h4>4.8.1 Enum classes&nbsp;<a href="#s4.8.1-enum-classes"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
501 <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
502as if it were an array initializer (see Section 4.8.3.1 on
503<a href="#s4.8.3.1-array-initializers">array initializers</a>).</p><pre class="prettyprint lang-java">
504private enum Suit { CLUBS, HEARTS, SPADES, DIAMONDS }
505</pre><p>Since enum classes <em>are classes</em>, all other rules for formatting classes apply.</p><a name="localvariables"/><a name="s4.8.2-variable-declarations"/>
506 <h4>4.8.2 Variable declarations&nbsp;<a href="#s4.8.2-variable-declarations"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
507 <a name="s4.8.2.1-variables-per-declaration"/>
508 <h5>4.8.2.1 One variable per declaration&nbsp;<a href="#s4.8.2.1-variables-per-declaration"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
509 <p>Every variable declaration (field or local) declares only one variable: declarations such as
510<code class="badcode">int a, b;</code> are not used.</p><a name="s4.8.2.2-variables-limited-scope"/>
511 <h5>4.8.2.2 Declared when needed, initialized as soon as
512possible&nbsp;<a href="#s4.8.2.2-variables-limited-scope"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
513 <p>Local variables are <strong>not</strong> habitually declared at the start of their containing
514block or block-like construct. Instead, local variables are declared close to the point they are
515first used (within reason), to minimize their scope. Local variable declarations typically have
516initializers, or are initialized immediately after declaration.</p><a name="s4.8.3-arrays"/>
517 <h4>4.8.3 Arrays&nbsp;<a href="#s4.8.3-arrays"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
518 <a name="s4.8.3.1-array-initializers"/>
519 <h5>4.8.3.1 Array initializers: can be "block-like"&nbsp;<a href="#s4.8.3.1-array-initializers"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
520 <p>Any array initializer may <em>optionally</em> be formatted as if it were a "block-like
521construct." For example, the following are all valid (<strong>not</strong> an exhaustive
522list):</p><pre class="prettyprint lang-java">
523new int[] { new int[] {
524 0, 1, 2, 3 0,
525} 1,
526 2,
527new int[] { 3,
528 0, 1, }
529 2, 3
530} new int[]
531 {0, 1, 2, 3}
532</pre><a name="s4.8.3.2-array-declarations"/>
533 <h5>4.8.3.2 No C-style array declarations&nbsp;<a href="#s4.8.3.2-array-declarations"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
534 <p>The square brackets form a part of the <em>type</em>, not the variable:
535<code class="prettyprint lang-java">String[] args</code>, not
536<code class="badcode">String args[]</code>.</p><a name="s4.8.4-switch"/>
537 <h4>4.8.4 Switch statements&nbsp;<a href="#s4.8.4-switch"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
538 <p class="terminology"><strong>Terminology Note:</strong> Inside the braces of a
539<em>switch block</em> are one or more <em>statement groups</em>. Each statement group consists of
540one or more <em>switch labels</em> (either <code class="prettyprint lang-java">case FOO:</code> or
541<code class="prettyprint lang-java">default:</code>), followed by one or more statements.</p><a name="s4.8.4.1-switch-indentation"/>
542 <h5>4.8.4.1 Indentation&nbsp;<a href="#s4.8.4.1-switch-indentation"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
543 <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
544if a block were being opened. The following switch label returns to the previous indentation
545level, as if a block had been closed.</p><a name="fallthrough"/><a name="s4.8.4.2-switch-fall-through"/>
546 <h5>4.8.4.2 Fall-through: commented&nbsp;<a href="#s4.8.4.2-switch-fall-through"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
547 <p>Within a switch block, each statement group either terminates abruptly (with a
548<code class="prettyprint lang-java">break</code>,
549<code class="prettyprint lang-java">continue</code>,
550<code class="prettyprint lang-java">return</code> or thrown exception), or is marked with a comment
551to indicate that execution will or <em>might</em> continue into the next statement group. Any
552comment that communicates the idea of fall-through is sufficient (typically
553<code class="prettyprint lang-java">// fall through</code>). This special comment is not required in
554the last statement group of the switch block. Example:</p><pre class="prettyprint lang-java">
555switch (input) {
556 case 1:
557 case 2:
558 prepareOneOrTwo();
559 // fall through
560 case 3:
561 handleOneTwoOrThree();
562 break;
563 default:
564 handleLargeNumber(input);
565}
566</pre><a name="s4.8.4.3-switch-default"/>
567 <h5>4.8.4.3 The default case is present&nbsp;<a href="#s4.8.4.3-switch-default"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
568 <p>Each switch statement includes a <code class="prettyprint lang-java">default</code> statement
569group, even if it contains no code.</p><a name="annotations"/><a name="s4.8.5-annotations"/>
570 <h4>4.8.5 Annotations&nbsp;<a href="#s4.8.5-annotations"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
571 <p>Annotations applying to a class, method or constructor appear immediately after the
572documentation block, and each annotation is listed on a line of its own (that is, one annotation
573per line). These line breaks do not constitute line-wrapping (Section
5744.5, <a href="#s4.5-line-wrapping">Line-wrapping</a>), so the indentation level is not
575increased. Example:</p><pre class="prettyprint lang-java">
576@Override
577@Nullable
578public String getNameIfPresent() { ... }
579</pre><p class="exception"><strong>Exception:</strong> A <em>single</em> parameterless annotation
580<em>may</em> instead appear together with the first line of the signature, for example:</p><pre class="prettyprint lang-java">
581@Override public int hashCode() { ... }
582</pre><p>Annotations applying to a field also appear immediately after the documentation block, but in
583this case, <em>multiple</em> annotations (possibly parameterized) may be listed on the same line;
584for example:</p><pre class="prettyprint lang-java">
585@Partial @Mock DataLoader loader;
586</pre><p>There are no specific rules for formatting parameter and local variable annotations.</p><a name="comments"/><a name="s4.8.6-comments"/>
587 <h4>4.8.6 Comments&nbsp;<a href="#s4.8.6-comments"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
588 <a name="s4.8.6.1-block-comment-style"/>
589 <h5>4.8.6.1 Block comment style&nbsp;<a href="#s4.8.6.1-block-comment-style"><img height="21" width="21" src="javaguidelink.png"/></a></h5>
590 <p>Block comments are indented at the same level as the surrounding code. They may be in
591<code class="prettyprint lang-java">/* ... */</code> style or
592<code class="prettyprint lang-java">// ...</code> style. For multi-line
593<code class="prettyprint lang-java">/* ... */</code> comments, subsequent lines must start with
594<code>*</code> aligned with the <code>*</code> on the previous line.</p><pre class="prettyprint lang-java">
595/*
596 * This is // And so /* Or you can
597 * okay. // is this. * even do this. */
598 */
599</pre><p>Comments are not enclosed in boxes drawn with asterisks or other characters.</p><p class="tip"><strong>Tip:</strong> When writing multi-line comments, use the
600<code class="prettyprint lang-java">/* ... */</code> style if you want automatic code formatters to
601re-wrap the lines when necessary (paragraph-style). Most formatters don't re-wrap lines in
602<code class="prettyprint lang-java">// ...</code> style comment blocks.</p><a name="modifiers"/><a name="s4.8.7-modifiers"/>
603 <h4>4.8.7 Modifiers&nbsp;<a href="#s4.8.7-modifiers"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
604 <p>Class and member modifiers, when present, appear in the order
605recommended by the Java Language Specification:
606</p><pre>
607public protected private abstract static final transient volatile synchronized native strictfp
608</pre><a name="s4.8.8-numeric-literals"/>
609 <h4>4.8.8 Numeric Literals&nbsp;<a href="#s4.8.8-numeric-literals"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
610 <p><code>long</code>-valued integer literals use an uppercase <code>L</code> suffix, never
611lowercase (to avoid confusion with the digit <code>1</code>). For example, <code>3000000000L</code>
612rather than <code class="badcode">3000000000l</code>.</p><a name="naming"/><a name="s5-naming"/>
613 <h2>5 Naming&nbsp;<a href="#s5-naming"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
614 <a name="s5.1-identifier-names"/>
615 <h3>5.1 Rules common to all identifiers&nbsp;<a href="#s5.1-identifier-names"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
616 <p>Identifiers use only ASCII letters and digits, and in two cases noted below, underscores. Thus
617each valid identifier name is matched by the regular expression <code>\w+</code> .</p><p> In Google Style special prefixes or
618suffixes, like those seen in the examples <code class="badcode">name_</code>,
619<code class="badcode">mName</code>, <code class="badcode">s_name</code> and
620<code class="badcode">kName</code>, are <strong>not</strong> used.
621For WPILib, we do make one exception to this rule for non-constant field name.</p><a name="s5.2-specific-identifier-names"/>
622 <h3>5.2 Rules by identifier type&nbsp;<a href="#s5.2-specific-identifier-names"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
623 <a name="s5.2.1-package-names"/>
624 <h4>5.2.1 Package names&nbsp;<a href="#s5.2.1-package-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
625 <p>Package names are all lowercase, with consecutive words simply concatenated together (no
626underscores). For example, <code>com.example.deepspace</code>, not
627<code class="badcode">com.example.deepSpace</code> or
628<code class="badcode">com.example.deep_space</code>.</p><a name="s5.2.2-class-names"/>
629 <h4>5.2.2 Class names&nbsp;<a href="#s5.2.2-class-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
630 <p>Class names are written in <a href="#s5.3-camel-case">UpperCamelCase</a>.</p><p>Class names are typically nouns or noun phrases. For example,
631<code class="prettyprint lang-java">Character</code> or
632<code class="prettyprint lang-java">ImmutableList</code>. Interface names may also be nouns or
633noun phrases (for example, <code class="prettyprint lang-java">List</code>), but may sometimes be
634adjectives or adjective phrases instead (for example,
635<code class="prettyprint lang-java">Readable</code>).</p><p>There are no specific rules or even well-established conventions for naming annotation types.</p><p><em>Test</em> classes are named starting with the name of the class they are testing, and ending
636with <code class="prettyprint lang-java">Test</code>. For example,
637<code class="prettyprint lang-java">HashTest</code> or
638<code class="prettyprint lang-java">HashIntegrationTest</code>.</p><a name="s5.2.3-method-names"/>
639 <h4>5.2.3 Method names&nbsp;<a href="#s5.2.3-method-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
640 <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,
641<code class="prettyprint lang-java">sendMessage</code> or
642<code class="prettyprint lang-java">stop</code>.</p><p>Underscores may appear in JUnit <em>test</em> method names to separate logical components of the
643name. One typical pattern is <code>test<i>&lt;MethodUnderTest&gt;</i>_<i>&lt;state&gt;</i></code>,
644for example <code class="prettyprint lang-java">testPop_emptyStack</code>. There is no One Correct
645Way to name test methods.</p><a name="constants"/><a name="s5.2.4-constant-names"/>
646 <h4>5.2.4 Constant names&nbsp;<a href="#s5.2.4-constant-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
647 <p>Constant names use <code class="prettyprint lang-java">CONSTANT_CASE</code>: all uppercase
648letters, 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
649choosing constant case, consider whether the field really <em>feels like</em> a constant. For
650example, if any of that instance's observable state can change, it is almost certainly not a
651constant. Merely <em>intending</em> to never mutate the object is generally not
652enough. Examples:</p><pre class="prettyprint lang-java">
653// Constants
654static final int NUMBER = 5;
655static final ImmutableList&lt;String&gt; NAMES = ImmutableList.of("Ed", "Ann");
656static final Joiner COMMA_JOINER = Joiner.on(','); // because Joiner is immutable
657static final SomeMutableType[] EMPTY_ARRAY = {};
658enum SomeEnum { ENUM_CONSTANT }
659
660// Not constants
661static String nonFinal = "non-final";
662final String nonStatic = "non-static";
663static final Set&lt;String&gt; mutableCollection = new HashSet&lt;String&gt;();
664static final ImmutableSet&lt;SomeMutableType&gt; mutableElements = ImmutableSet.of(mutable);
665static final Logger logger = Logger.getLogger(MyClass.getName());
666static final String[] nonEmptyArray = {"these", "can", "change"};
667</pre><p>These names are typically nouns or noun phrases.</p><a name="s5.2.5-non-constant-field-names"/>
668 <h4>5.2.5 Non-constant field names&nbsp;<a href="#s5.2.5-non-constant-field-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
669 <p>Non-constant field names (static or otherwise) are written
670in <a href="#s5.3-camel-case">lowerCamelCase</a> with a preceding <code class="prettyprint lang-java">m_</code>.</p>
671<p>These names are typically nouns or noun phrases. For example,
672<code class="prettyprint lang-java">m_computedValues</code> or
673<code class="prettyprint lang-java">m_index</code>.</p><a name="s5.2.6-parameter-names"/>
674 <h4>5.2.6 Parameter names&nbsp;<a href="#s5.2.6-parameter-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
675 <p>Parameter names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p><p>One-character parameter names should be avoided.</p><a name="s5.2.7-local-variable-names"/>
676 <h4>5.2.7 Local variable names&nbsp;<a href="#s5.2.7-local-variable-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
677 <p>Local variable names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>, and can be
678abbreviated 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
679be styled as constants.</p><a name="s5.2.8-type-variable-names"/>
680 <h4>5.2.8 Type variable names&nbsp;<a href="#s5.2.8-type-variable-names"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
681 <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
682 <code class="prettyprint lang-java">E</code>, <code class="prettyprint lang-java">T</code>,
683 <code class="prettyprint lang-java">X</code>, <code class="prettyprint lang-java">T2</code>)
684 </li><li>A name in the form used for classes (see Section 5.2.2,
685 <a href="#s5.2.2-class-names">Class names</a>), followed by the capital letter
686 <code class="prettyprint lang-java">T</code> (examples:
687 <code class="prettyprint lang-java">RequestT</code>,
688 <code class="prettyprint lang-java">FooBarT</code>).</li></ul><a name="acronyms"/><a name="camelcase"/><a name="s5.3-camel-case"/>
689 <h3>5.3 Camel case: defined&nbsp;<a href="#s5.3-camel-case"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
690 <p>Sometimes there is more than one reasonable way to convert an English phrase into camel case,
691such as when acronyms or unusual constructs like "IPv6" or "iOS" are present. To improve
692predictability, 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
693 algorithm" might become "Muellers algorithm".</li><li>Divide this result into words, splitting on spaces and any remaining punctuation (typically
694 hyphens).
695
696 <ul><li><em>Recommended:</em> if any word already has a conventional camel-case appearance in common
697 usage, split this into its constituent parts (e.g., "AdWords" becomes "ad words"). Note
698 that a word such as "iOS" is not really in camel case <em>per se</em>; it defies <em>any</em>
699 convention, so this recommendation does not apply.</li></ul></li><li>Now lowercase <em>everything</em> (including acronyms), then uppercase only the first
700 character of:
701 <ul><li>... each word, to yield <em>upper camel case</em>, or</li><li>... each word except the first, to yield <em>lower camel case</em></li></ul></li><li>Finally, join all the words into a single identifier.</li></ol><p>Note that the casing of the original words is almost entirely disregarded. Examples:</p><table><tr><th>Prose form</th><th>Correct</th><th>Incorrect</th></tr><tr><td>"XML HTTP request"</td><td><code class="prettyprint lang-java">XmlHttpRequest</code></td><td><code class="badcode">XMLHTTPRequest</code></td></tr><tr><td>"new customer ID"</td><td><code class="prettyprint lang-java">newCustomerId</code></td><td><code class="badcode">newCustomerID</code></td></tr><tr><td>"inner stopwatch"</td><td><code class="prettyprint lang-java">innerStopwatch</code></td><td><code class="badcode">innerStopWatch</code></td></tr><tr><td>"supports IPv6 on iOS?"</td><td><code class="prettyprint lang-java">supportsIpv6OnIos</code></td><td><code class="badcode">supportsIPv6OnIOS</code></td></tr><tr><td>"YouTube importer"</td><td><code class="prettyprint lang-java">YouTubeImporter</code><br/><code class="prettyprint lang-java">YoutubeImporter</code>*</td><td/></tr></table><p>*Acceptable, but not recommended.</p><p class="note"><strong>Note:</strong> Some words are ambiguously hyphenated in the English
702language: for example "nonempty" and "non-empty" are both correct, so the method names
703<code class="prettyprint lang-java">checkNonempty</code> and
704<code class="prettyprint lang-java">checkNonEmpty</code> are likewise both correct.</p><a name="s6-programming-practices"/>
705 <h2>6 Programming Practices&nbsp;<a href="#s6-programming-practices"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
706 <a name="s6.1-override-annotation"/>
707 <h3>6.1 @Override: always used&nbsp;<a href="#s6.1-override-annotation"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
708 <p>A method is marked with the <code class="prettyprint lang-java">@Override</code> annotation
709whenever it is legal. This includes a class method overriding a superclass method, a class method
710implementing an interface method, and an interface method respecifying a superinterface
711method.</p><p class="exception"><strong>Exception:</strong><code class="prettyprint lang-java">@Override</code> may be omitted when the parent method is
712<code class="prettyprint lang-java">@Deprecated</code>.</p><a name="caughtexceptions"/><a name="s6.2-caught-exceptions"/>
713 <h3>6.2 Caught exceptions: not ignored&nbsp;<a href="#s6.2-caught-exceptions"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
714 <p>Except as noted below, it is very rarely correct to do nothing in response to a caught
715exception. (Typical responses are to log it, or if it is considered "impossible", rethrow it as an
716<code class="prettyprint lang-java">AssertionError</code>.)</p><p>When it truly is appropriate to take no action whatsoever in a catch block, the reason this is
717justified is explained in a comment.</p><pre class="prettyprint lang-java">
718try {
719 int i = Integer.parseInt(response);
720 return handleNumericResponse(i);
721} catch (NumberFormatException ok) {
722 // it's not numeric; that's fine, just continue
723}
724return handleTextResponse(response);
725</pre><p class="exception"><strong>Exception:</strong> In tests, a caught exception may be ignored
726without comment <em>if</em> it is named <code class="prettyprint lang-java">expected</code>. The
727following is a very common idiom for ensuring that the method under test <em>does</em> throw an
728exception of the expected type, so a comment is unnecessary here.</p><pre class="prettyprint lang-java">
729try {
730 emptyStack.pop();
731 fail();
732} catch (NoSuchElementException expected) {
733}
734</pre><a name="s6.3-static-members"/>
735 <h3>6.3 Static members: qualified using class&nbsp;<a href="#s6.3-static-members"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
736 <p>When a reference to a static class member must be qualified, it is qualified with that class's
737name, not with a reference or expression of that class's type.</p><pre class="prettyprint lang-java">
738Foo aFoo = ...;
739Foo.aStaticMethod(); // good
740<span class="badcode">aFoo.aStaticMethod();</span> // bad
741<span class="badcode">somethingThatYieldsAFoo().aStaticMethod();</span> // very bad
742</pre><a name="finalizers"/><a name="s6.4-finalizers"/>
743 <h3>6.4 Finalizers: not used&nbsp;<a href="#s6.4-finalizers"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
744 <p>It is <strong>extremely rare</strong> to override <code class="prettyprint lang-java">Object.finalize</code>.</p><p class="tip"><strong>Tip:</strong> Don't do it. If you absolutely must, first read and understand
745<a href="http://books.google.com/books?isbn=8131726592"><em>Effective Java</em></a>
746Item 7, "Avoid Finalizers," very carefully, and <em>then</em> don't do it.</p><a name="javadoc"/><a name="s7-javadoc"/>
747 <h2>7 Javadoc&nbsp;<a href="#s7-javadoc"><img height="21" width="21" src="javaguidelink.png"/></a></h2>
748 <a name="s7.1-javadoc-formatting"/>
749 <h3>7.1 Formatting&nbsp;<a href="#s7.1-javadoc-formatting"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
750 <a name="s7.1.1-javadoc-multi-line"/>
751 <h4>7.1.1 General form&nbsp;<a href="#s7.1.1-javadoc-multi-line"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
752 <p>The <em>basic</em> formatting of Javadoc blocks is as seen in this example:</p><pre class="prettyprint lang-java">
753/**
754 * Multiple lines of Javadoc text are written here,
755 * wrapped normally...
756 */
757public int method(String p1) { ... }
758</pre><p>... or in this single-line example:</p><pre class="prettyprint lang-java">
759/** An especially short bit of Javadoc. */
760</pre><p>The basic form is always acceptable. The single-line form may be substituted when there are no
761at-clauses present, and the entirety of the Javadoc block (including comment markers) can fit on a
762single line.</p><a name="s7.1.2-javadoc-paragraphs"/>
763 <h4>7.1.2 Paragraphs&nbsp;<a href="#s7.1.2-javadoc-paragraphs"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
764 <p>One blank line—that is, a line containing only the aligned leading asterisk
765(<code>*</code>)—appears between paragraphs, and before the group of "at-clauses" if
766present. Each paragraph but the first has <code>&lt;p&gt;</code> immediately before the first word,
767with no space after.</p><a name="s7.1.3-javadoc-at-clauses"/>
768 <h4>7.1.3 At-clauses&nbsp;<a href="#s7.1.3-javadoc-at-clauses"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
769 <p>Any of the standard "at-clauses" that are used appear in the order <code>@param</code>,
770<code>@return</code>, <code>@throws</code>, <code>@deprecated</code>, and these four types never
771appear with an empty description. When an at-clause doesn't fit on a single line, continuation lines
772are indented four (or more) spaces from the position of the <code>@</code>.
773</p><a name="s7.2-summary-fragment"/>
774 <h3>7.2 The summary fragment&nbsp;<a href="#s7.2-summary-fragment"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
775 <p>The Javadoc for each class and member begins with a brief <strong>summary fragment</strong>. This
776fragment is very important: it is the only part of the text that appears in certain contexts such as
777class and method indexes.</p><p>This is a fragment—a noun phrase or verb phrase, not a complete sentence. It does
778<strong>not</strong> begin with <code class="badcode">A {@code Foo} is a...</code>, or
779<code class="badcode">This method returns...</code>, nor does it form a complete imperative sentence
780like <code class="badcode">Save the record.</code>. However, the fragment is capitalized and
781punctuated 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
782<code class="badcode">/** @return the customer ID */</code>. This is incorrect, and should be
783changed to <code class="prettyprint lang-java">/** Returns the customer ID. */</code>.</p><a name="s7.3.3-javadoc-optional"/><a name="s7.3-javadoc-where-required"/>
784 <h3>7.3 Where Javadoc is used&nbsp;<a href="#s7.3-javadoc-where-required"><img height="21" width="21" src="javaguidelink.png"/></a></h3>
785 <p>At the <em>minimum</em>, Javadoc is present for every
786<code class="prettyprint lang-java">public</code> class, and every
787<code class="prettyprint lang-java">public</code> or
788<code class="prettyprint lang-java">protected</code> member of such a class, with a few exceptions
789noted below.</p><p>Other classes and members still have Javadoc <em>as needed</em>. Whenever an implementation
790comment would be used to define the overall purpose or behavior of a class, method or field, that
791comment is written as Javadoc instead. (It's more uniform, and more tool-friendly.)</p><a name="s7.3.1-javadoc-exception-self-explanatory"/>
792 <h4>7.3.1 Exception: self-explanatory methods&nbsp;<a href="#s7.3.1-javadoc-exception-self-explanatory"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
793 <p>Javadoc is optional for "simple, obvious" methods like
794<code class="prettyprint lang-java">getFoo</code>, in cases where there <em>really and truly</em> is
795nothing 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
796omitting relevant information that a typical reader might need to know. For example, for a method
797named <code class="prettyprint lang-java">getCanonicalName</code>, don't omit its documentation
798(with the rationale that it would say only
799<code class="badcode">/** Returns the canonical name. */</code>) if a typical reader may have no idea
800what the term "canonical name" means!</p><a name="s7.3.2-javadoc-exception-overrides"/>
801 <h4>7.3.2 Exception: overrides&nbsp;<a href="#s7.3.2-javadoc-exception-overrides"><img height="21" width="21" src="javaguidelink.png"/></a></h4>
802 <p>Javadoc is not always present on a method that overrides a supertype method.
803</p></div> <hr/>
804 <div class="change">Last changed: March 21, 2014</div>
805</body>
806</html>