forked from google/styleguide
-
Notifications
You must be signed in to change notification settings - Fork 0
/
javaguide.html
1177 lines (860 loc) · 47.8 KB
/
javaguide.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<title>Google Java Style Guide</title>
<link rel="stylesheet" type="text/css" href="javaguide.css">
<script language="javascript" src="include/styleguide.js"></script>
<link rel="shortcut icon" type="image/x-icon" href="https://www.google.com/favicon.ico" />
<script src="https://cdn.rawgit.com/google/code-prettify/master/loader/run_prettify.js"></script>
</head>
<body onload="initStyleGuide();">
<div id="content">
<h1>Google Java Style Guide</h1>
<div class="vertical_toc" id="tocDiv"></div>
<div class="main_body">
<h2 id="s1-introduction">1 Introduction</h2>
<p>This document serves as the <strong>complete</strong> definition of Google's coding standards for
source code in the Java™ Programming Language. A Java source file is described as being <em>in
Google Style</em> if and only if it adheres to the rules herein.</p>
<p>Like other programming style guides, the issues covered span not only aesthetic issues of
formatting, but other types of conventions or coding standards as well. However, this document
focuses primarily on the <strong>hard-and-fast rules</strong> that we follow universally, and
avoids giving <em>advice</em> that isn't clearly enforceable (whether by human or tool).
</p>
<h3 id="s1.1-terminology">1.1 Terminology notes</h3>
<p>In this document, unless otherwise clarified:</p>
<ol>
<li>The term <em>class</em> is used inclusively to mean an "ordinary" class, enum class,
interface or annotation type (<code class="prettyprint lang-java">@interface</code>).</li>
<li>The term <em>member</em> (of a class) is used inclusively to mean a nested class, field,
method, <em>or constructor</em>; that is, all top-level contents of a class except initializers
and comments.
</li><li>The term <em>comment</em> always refers to <em>implementation</em> comments. We do not
use the phrase "documentation comments", instead using the common term "Javadoc."</li>
</ol>
<p>Other "terminology notes" will appear occasionally throughout the document.</p>
<h3 id="s1.2-guide-notes">1.2 Guide notes</h3>
<p>Example code in this document is <strong>non-normative</strong>. That is, while the examples
are in Google Style, they may not illustrate the <em>only</em> stylish way to represent the
code. Optional formatting choices made in examples should not be enforced as rules.</p>
<h2 id="s2-source-file-basics">2 Source file basics</h2>
<h3 id="s2.1-file-name">2.1 File name</h3>
<p>The source file name consists of the case-sensitive name of the top-level class it contains
(of which there is <a href="#s3.4.1-one-top-level-class">exactly one</a>), plus the
<code>.java</code> extension.</p>
<h3 id="s2.2-file-encoding">2.2 File encoding: UTF-8</h3>
<p>Source files are encoded in <strong>UTF-8</strong>.</p>
<h3 id="s2.3-special-characters">2.3 Special characters</h3>
<h4 id="s2.3.1-whitespace-characters">2.3.1 Whitespace characters</h4>
<p>Aside from the line terminator sequence, the <strong>ASCII horizontal space
character</strong> (<strong>0x20</strong>) is the only whitespace character that appears
anywhere in a source file. This implies that:</p>
<ol>
<li>All other whitespace characters in string and character literals are escaped.</li>
<li>Tab characters are <strong>not</strong> used for indentation.</li>
</ol>
<h4 id="s2.3.2-special-escape-sequences">2.3.2 Special escape sequences</h4>
<p>For any character that has a
<a href="http://docs.oracle.com/javase/tutorial/java/data/characters.html">
special escape sequence</a>
(<code class="prettyprint lang-java">\b</code>,
<code class="prettyprint lang-java">\t</code>,
<code class="prettyprint lang-java">\n</code>,
<code class="prettyprint lang-java">\f</code>,
<code class="prettyprint lang-java">\r</code>,
<code class="prettyprint lang-java">\"</code>,
<code class="prettyprint lang-java">\'</code> and
<code class="prettyprint lang-java">\\</code>), that sequence
is used rather than the corresponding octal
(e.g. <code class="badcode">\012</code>) or Unicode
(e.g. <code class="badcode">\u000a</code>) escape.</p>
<h4 id="s2.3.3-non-ascii-characters">2.3.3 Non-ASCII characters</h4>
<p>For the remaining non-ASCII characters, either the actual Unicode character
(e.g. <code class="prettyprint lang-java">∞</code>) or the equivalent Unicode escape
(e.g. <code class="prettyprint lang-java">\u221e</code>) is used. The choice depends only on
which makes the code <strong>easier to read and understand</strong>, although Unicode escapes
outside string literals and comments are strongly discouraged.</p>
<p class="tip"><strong>Tip:</strong> In the Unicode escape case, and occasionally even when actual
Unicode characters are used, an explanatory comment can be very helpful.</p>
<p>Examples:</p>
<table>
<tbody><tr>
<th>Example</th>
<th>Discussion</th>
</tr>
<tr>
<td><code class="prettyprint lang-java">String unitAbbrev = "μs";</code></td>
<td>Best: perfectly clear even without a comment.</td>
</tr>
<tr>
<td><code class="prettyprint lang-java">String unitAbbrev = "\u03bcs"; // "μs"</code></td>
<td>Allowed, but there's no reason to do this.</td>
</tr>
<tr>
<td><code class="prettyprint lang-java">String unitAbbrev = "\u03bcs";
// Greek letter mu, "s"</code></td>
<td>Allowed, but awkward and prone to mistakes.</td>
</tr>
<tr>
<td><code class="badcode">String unitAbbrev = "\u03bcs";</code></td>
<td>Poor: the reader has no idea what this is.</td>
</tr>
<tr>
<td><code class="prettyprint lang-java">return '\ufeff' + content;
// byte order mark</code></td>
<td>Good: use escapes for non-printable characters, and comment if necessary.</td>
</tr>
</tbody></table>
<p class="tip"><strong>Tip:</strong> Never make your code less readable simply out of fear that
some programs might not handle non-ASCII characters properly. If that should happen, those
programs are <strong>broken</strong> and they must be <strong>fixed</strong>.</p>
<a name="filestructure"></a>
<h2 id="s3-source-file-structure">3 Source file structure</h2>
<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>
<h3 id="s3.1-copyright-statement">3.1 License or copyright information, if present</h3>
<p>If license or copyright information belongs in a file, it belongs here.</p>
<h3 id="s3.2-package-statement">3.2 Package statement</h3>
<p>The package statement is <strong>not line-wrapped</strong>. The column limit (Section 4.4,
<a href="#s4.4-column-limit">Column limit: 100</a>) does not apply to package statements.</p>
<a name="imports"></a>
<h3 id="s3.3-import-statements">3.3 Import statements</h3>
<h4 id="s3.3.1-wildcard-imports">3.3.1 No wildcard imports</h4>
<p><strong>Wildcard imports</strong>, static or otherwise, <strong>are not used</strong>.</p>
<h4 id="s3.3.2-import-line-wrapping">3.3.2 No line-wrapping</h4>
<p>Import statements are <strong>not line-wrapped</strong>. The column limit (Section 4.4,
<a href="#s4.4-column-limit">Column limit: 100</a>) does not apply to import
statements.</p>
<h4 id="s3.3.3-import-ordering-and-spacing">3.3.3 Ordering and spacing</h4>
<p>Imports are ordered as follows:</p>
<ol>
<li>All static imports in a single block.</li>
<li>All non-static imports in a single block.</li>
</ol>
<p>If there are both static and non-static imports, a single blank line separates the two
blocks. There are no other blank lines between import statements.</p>
<p>Within each block the imported names appear in ASCII sort order. (<strong>Note:</strong>
this is not the same as the import <em>statements</em> being in ASCII sort order, since '.'
sorts before ';'.)</p>
<h4 id="s3.3.4-import-class-not-static">3.3.4 No static import for classes</h4>
<p>Static import is not used for static nested classes. They are imported with
normal imports.</p>
<h3 id="s3.4-class-declaration">3.4 Class declaration</h3>
<a name="oneclassperfile"></a>
<h4 id="s3.4.1-one-top-level-class">3.4.1 Exactly one top-level class declaration</h4>
<p>Each top-level class resides in a source file of its own.</p>
<a name="s3.4.2-class-member-ordering"></a>
<h4 id="s3.4.2-ordering-class-contents">3.4.2 Ordering of class contents</h4>
<p>The order you choose for the members and initializers of your class can have a great effect on
learnability. However, there's no single correct recipe for how to do it; different classes may
order their contents in different ways.</p>
<p>What is important is that each class uses <strong><em>some</em> logical order</strong>, which its
maintainer could explain if asked. For example, new methods are not just habitually added to the end
of the class, as that would yield "chronological by date added" ordering, which is not a logical
ordering.</p>
<a name="overloads"></a>
<h5 id="s3.4.2.1-overloads-never-split">3.4.2.1 Overloads: never split</h5>
<p>When a class has multiple constructors, or multiple methods with the same name, these appear
sequentially, with no other code in between (not even private members).</p>
<h2 id="s4-formatting">4 Formatting</h2>
<p class="terminology"><strong>Terminology Note:</strong> <em>block-like construct</em> refers to
the body of a class, method or constructor. Note that, by Section 4.8.3.1 on
<a href="#s4.8.3.1-array-initializers">array initializers</a>, any array initializer
<em>may</em> optionally be treated as if it were a block-like construct.</p>
<a name="braces"></a>
<h3 id="s4.1-braces">4.1 Braces</h3>
<h4 id="s4.1.1-braces-always-used">4.1.1 Braces are used where optional</h4>
<p>Braces are used with
<code class="prettyprint lang-java">if</code>,
<code class="prettyprint lang-java">else</code>,
<code class="prettyprint lang-java">for</code>,
<code class="prettyprint lang-java">do</code> and
<code class="prettyprint lang-java">while</code> statements, even when the
body is empty or contains only a single statement.</p>
<h4 id="s4.1.2-blocks-k-r-style">4.1.2 Nonempty blocks: K & R style</h4>
<p>Braces follow the Kernighan and Ritchie style
("<a href="http://www.codinghorror.com/blog/2012/07/new-programming-jargon.html">Egyptian brackets</a>")
for <em>nonempty</em> blocks and block-like constructs:</p>
<ul>
<li>No line break before the opening brace.</li>
<li>Line break after the opening brace.</li>
<li>Line break before the closing brace.</li>
<li>Line break after the closing brace, <em>only if</em> that brace terminates a statement or
terminates the body of a method, constructor, or <em>named</em> class.
For example, there is <em>no</em> line break after the brace if it is followed by
<code class="prettyprint lang-java">else</code> or a comma.</li>
</ul>
<p>Examples:</p>
<pre class="prettyprint lang-java">return () -> {
while (condition()) {
method();
}
};
return new MyClass() {
@Override public void method() {
if (condition()) {
try {
something();
} catch (ProblemException e) {
recover();
}
} else if (otherCondition()) {
somethingElse();
} else {
lastThing();
}
}
};
</pre>
<p>A few exceptions for enum classes are given in Section 4.8.1,
<a href="#s4.8.1-enum-classes">Enum classes</a>.</p>
<a name="emptyblocks"></a>
<h4 id="s4.1.3-braces-empty-blocks">4.1.3 Empty blocks: may be concise</h4>
<p>An empty block or block-like construct may be in K & R style (as described in
<a href="#s4.1.2-blocks-k-r-style">Section 4.1.2</a>). Alternatively, it may be closed immediately
after it is opened, with no characters or line break in between
(<code class="prettyprint lang-java">{}</code>), <strong>unless</strong> it is part of a
<em>multi-block statement</em> (one that directly contains multiple blocks:
<code class="prettyprint lang-java">if/else</code> or
<code class="prettyprint lang-java">try/catch/finally</code>).</p>
<p>Examples:</p>
<pre class="prettyprint lang-java"> // This is acceptable
void doNothing() {}
// This is equally acceptable
void doNothingElse() {
}
</pre>
<pre class="prettyprint lang-java badcode"> // This is not acceptable: No concise empty blocks in a multi-block statement
try {
doSomething();
} catch (Exception e) {}
</pre>
<h3 id="s4.2-block-indentation">4.2 Block indentation: +2 spaces</h3>
<p>Each time a new block or block-like construct is opened, the indent increases by two
spaces. When the block ends, the indent returns to the previous indent level. The indent level
applies to both code and comments throughout the block. (See the example in Section 4.1.2,
<a href="#s4.1.2-blocks-k-r-style">Nonempty blocks: K & R Style</a>.)</p>
<h3 id="s4.3-one-statement-per-line">4.3 One statement per line</h3>
<p>Each statement is followed by a line break.</p>
<a name="columnlimit"></a>
<h3 id="s4.4-column-limit">4.4 Column limit: 100</h3>
<p>Java code has a column limit of 100 characters.
Except as noted below, any line that would exceed this limit must be line-wrapped, as explained in
Section 4.5, <a href="#s4.5-line-wrapping">Line-wrapping</a>.
</p>
<p><strong>Exceptions:</strong></p>
<ol>
<li>Lines where obeying the column limit is not possible (for example, a long URL in Javadoc,
or a long JSNI method reference).</li>
<li><code class="prettyprint lang-java">package</code> and
<code class="prettyprint lang-java">import</code> statements (see Sections
3.2 <a href="#s3.2-package-statement">Package statement</a> and
3.3 <a href="#s3.3-import-statements">Import statements</a>).</li>
<li>Command lines in a comment that may be cut-and-pasted into a shell.</li>
</ol>
<h3 id="s4.5-line-wrapping">4.5 Line-wrapping</h3>
<p class="terminology"><strong>Terminology Note:</strong> When code that might otherwise legally
occupy a single line is divided into multiple lines, this activity is called
<em>line-wrapping</em>.</p>
<p>There is no comprehensive, deterministic formula showing <em>exactly</em> how to line-wrap in
every situation. Very often there are several valid ways to line-wrap the same piece of code.</p>
<p class="note"><strong>Note:</strong> While the typical reason for line-wrapping is to avoid
overflowing the column limit, even code that would in fact fit within the column limit <em>may</em>
be line-wrapped at the author's discretion.</p>
<p class="tip"><strong>Tip:</strong> Extracting a method or local variable may solve the problem
without the need to line-wrap.</p>
<h4 id="s4.5.1-line-wrapping-where-to-break">4.5.1 Where to break</h4>
<p>The prime directive of line-wrapping is: prefer to break at a
<strong>higher syntactic level</strong>. Also:</p>
<ol>
<li>When a line is broken at a <em>non-assignment</em> operator the break comes <em>before</em>
the symbol. (Note that this is not the same practice used in Google style for other languages,
such as C++ and JavaScript.)
<ul>
<li>This also applies to the following "operator-like" symbols:
<ul>
<li>the dot separator (<code class="prettyprint lang-java">.</code>)</li>
<li>the two colons of a method reference
(<code class="prettyprint lang-java">::</code>)</li>
<li>an ampersand in a type bound
(<code class="prettyprint lang-java"><T extends Foo & Bar></code>)</li>
<li>a pipe in a catch block
(<code class="prettyprint lang-java">catch (FooException | BarException e)</code>).</li>
</ul>
</li>
</ul>
</li>
<li>When a line is broken at an <em>assignment</em> operator the break typically comes
<em>after</em> the symbol, but either way is acceptable.
<ul>
<li>This also applies to the "assignment-operator-like" colon in an enhanced
<code class="prettyprint lang-java">for</code> ("foreach") statement.</li>
</ul>
</li>
<li>A method or constructor name stays attached to the open parenthesis
(<code class="prettyprint lang-java">(</code>) that follows it.</li>
<li>A comma (<code class="prettyprint lang-java">,</code>) stays attached to the token that
precedes it.</li>
<li>A line is never broken adjacent to the arrow in a lambda, except that a
break may come immediately after the arrow if the body of the lambda consists
of a single unbraced expression. Examples:
<pre class="prettyprint lang-java">MyLambda<String, Long, Object> lambda =
(String label, Long value, Object obj) -> {
...
};
Predicate<String> predicate = str ->
longExpressionInvolving(str);
</pre>
</li>
</ol>
<p class="note"><strong>Note:</strong> The primary goal for line wrapping is to have clear
code, <em>not necessarily</em> code that fits in the smallest number of lines.</p>
<a name="indentation"></a>
<h4 id="s4.5.2-line-wrapping-indent">4.5.2 Indent continuation lines at least +4 spaces</h4>
<p>When line-wrapping, each line after the first (each <em>continuation line</em>) is indented
at least +4 from the original line.</p>
<p>When there are multiple continuation lines, indentation may be varied beyond +4 as
desired. In general, two continuation lines use the same indentation level if and only if they
begin with syntactically parallel elements.</p>
<p>Section 4.6.3 on <a href="#s4.6.3-horizontal-alignment">Horizontal alignment</a> addresses
the discouraged practice of using a variable number of spaces to align certain tokens with
previous lines.</p>
<h3 id="s4.6-whitespace">4.6 Whitespace</h3>
<h4 id="s4.6.1-vertical-whitespace">4.6.1 Vertical Whitespace</h4>
<p>A single blank line appears:</p>
<ol>
<li><em>Between</em> consecutive members or initializers of a class: fields, constructors,
methods, nested classes, static initializers, and instance initializers.
<ul>
<li><span class="exception"><strong>Exception:</strong> A blank line between two consecutive
fields (having no other code between them) is optional. Such blank lines are used as needed to
create <em>logical groupings</em> of fields.</span></li>
<li><span class="exception"><strong>Exception:</strong> Blank lines between enum constants are
covered in <a href="#s4.8.1-enum-classes">Section 4.8.1</a>.</span></li>
</ul>
</li>
<li>Between statements, <em>as needed</em> to organize the code into logical subsections.
</li><li><em>Optionally</em> before the first member or initializer, or after the last member or
initializer of the class (neither encouraged nor discouraged).</li>
<li>As required by other sections of this document (such as Section 3,
<a href="#s3-source-file-structure">Source file structure</a>, and Section 3.3,
<a href="#s3.3-import-statements">Import statements</a>).</li>
</ol>
<p><em>Multiple</em> consecutive blank lines are permitted, but never required (or encouraged).</p>
<h4 id="s4.6.2-horizontal-whitespace">4.6.2 Horizontal whitespace</h4>
<p>Beyond where required by the language or other style rules, and apart from literals, comments and
Javadoc, a single ASCII space also appears in the following places <strong>only</strong>.</p>
<ol>
<li>Separating any reserved word, such as
<code class="prettyprint lang-java">if</code>,
<code class="prettyprint lang-java">for</code> or
<code class="prettyprint lang-java">catch</code>, from an open parenthesis
(<code class="prettyprint lang-java">(</code>)
that follows it on that line</li>
<li>Separating any reserved word, such as
<code class="prettyprint lang-java">else</code> or
<code class="prettyprint lang-java">catch</code>, from a closing curly brace
(<code class="prettyprint lang-java">}</code>) that precedes it on that line</li>
<li>Before any open curly brace
(<code class="prettyprint lang-java">{</code>), with two exceptions:
<ul>
<li><code class="prettyprint lang-java">@SomeAnnotation({a, b})</code> (no space is used)</li>
<li><code class="prettyprint lang-java">String[][] x = {{"foo"}};</code> (no space is required
between <code class="prettyprint lang-java">{{</code>, by item 8 below)</li>
</ul>
</li>
<li>On both sides of any binary or ternary operator. This also applies to the following
"operator-like" symbols:
<ul>
<li>the ampersand in a conjunctive type bound:
<code class="prettyprint lang-java"><T extends Foo & Bar></code></li>
<li>the pipe for a catch block that handles multiple exceptions:
<code class="prettyprint lang-java">catch (FooException | BarException e)</code></li>
<li>the colon (<code class="prettyprint lang-java">:</code>) in an enhanced
<code class="prettyprint lang-java">for</code> ("foreach") statement</li>
<li>the arrow in a lambda expression:
<code class="prettyprint lang-java">(String str) -> str.length()</code></li>
</ul>
but not
<ul>
<li>the two colons (<code class="prettyprint lang-java">::</code>) of a method reference, which
is written like <code class="prettyprint lang-java">Object::toString</code></li>
<li>the dot separator (<code class="prettyprint lang-java">.</code>), which is written like
<code class="prettyprint lang-java">object.toString()</code></li>
</ul>
</li>
<li>After <code class="prettyprint lang-java">,:;</code> or the closing parenthesis
(<code class="prettyprint lang-java">)</code>) of a cast</li>
<li>On both sides of the double slash (<code class="prettyprint lang-java">//</code>) that
begins an end-of-line comment. Here, multiple spaces are allowed, but not required.</li>
<li>Between the type and variable of a declaration:
<code class="prettyprint lang-java">List<String> list</code></li>
<li><em>Optional</em> just inside both braces of an array initializer
<ul>
<li><code class="prettyprint lang-java">new int[] {5, 6}</code> and
<code class="prettyprint lang-java">new int[] { 5, 6 }</code> are both valid</li>
</ul>
</li>
</ol>
This rule is never interpreted as requiring or forbidding additional space at the start or
end of a line; it addresses only <em>interior</em> space.<p></p>
<h4 id="s4.6.3-horizontal-alignment">4.6.3 Horizontal alignment: never required</h4>
<p class="terminology"><strong>Terminology Note:</strong> <em>Horizontal alignment</em> is the
practice of adding a variable number of additional spaces in your code with the goal of making
certain tokens appear directly below certain other tokens on previous lines.</p>
<p>This practice is permitted, but is <strong>never required</strong> by Google Style. It is not
even required to <em>maintain</em> horizontal alignment in places where it was already used.</p>
<p>Here is an example without alignment, then using alignment:</p>
<pre class="prettyprint lang-java">private int x; // this is fine
private Color color; // this too
private int x; // permitted, but future edits
private Color color; // may leave it unaligned
</pre>
<p class="tip"><strong>Tip:</strong> Alignment can aid readability, but it creates problems for
future maintenance. Consider a future change that needs to touch just one line. This change may
leave the formerly-pleasing formatting mangled, and that is <strong>allowed</strong>. More often
it prompts the coder (perhaps you) to adjust whitespace on nearby lines as well, possibly
triggering a cascading series of reformattings. That one-line change now has a "blast radius."
This can at worst result in pointless busywork, but at best it still corrupts version history
information, slows down reviewers and exacerbates merge conflicts.</p>
<a name="parentheses"></a>
<h3 id="s4.7-grouping-parentheses">4.7 Grouping parentheses: recommended</h3>
<p>Optional grouping parentheses are omitted only when author and reviewer agree that there is no
reasonable chance the code will be misinterpreted without them, nor would they have made the code
easier to read. It is <em>not</em> reasonable to assume that every reader has the entire Java
operator precedence table memorized.</p>
<h3 id="s4.8-specific-constructs">4.8 Specific constructs</h3>
<h4 id="s4.8.1-enum-classes">4.8.1 Enum classes</h4>
<p>After each comma that follows an enum constant, a line break is optional. Additional blank
lines (usually just one) are also allowed. This is one possibility:
</p><pre class="prettyprint lang-java">private enum Answer {
YES {
@Override public String toString() {
return "yes";
}
},
NO,
MAYBE
}
</pre>
<p>An enum class with no methods and no documentation on its constants may optionally be formatted
as if it were an array initializer (see Section 4.8.3.1 on
<a href="#s4.8.3.1-array-initializers">array initializers</a>).</p>
<pre class="prettyprint lang-java">private enum Suit { CLUBS, HEARTS, SPADES, DIAMONDS }
</pre>
<p>Since enum classes <em>are classes</em>, all other rules for formatting classes apply.</p>
<a name="localvariables"></a>
<h4 id="s4.8.2-variable-declarations">4.8.2 Variable declarations</h4>
<h5 id="s4.8.2.1-variables-per-declaration">4.8.2.1 One variable per declaration</h5>
<p>Every variable declaration (field or local) declares only one variable: declarations such as
<code class="badcode">int a, b;</code> are not used.</p>
<h5 id="s4.8.2.2-variables-limited-scope">4.8.2.2 Declared when needed</h5>
<p>Local variables are <strong>not</strong> habitually declared at the start of their containing
block or block-like construct. Instead, local variables are declared close to the point they are
first used (within reason), to minimize their scope. Local variable declarations typically have
initializers, or are initialized immediately after declaration.</p>
<h4 id="s4.8.3-arrays">4.8.3 Arrays</h4>
<h5 id="s4.8.3.1-array-initializers">4.8.3.1 Array initializers: can be "block-like"</h5>
<p>Any array initializer may <em>optionally</em> be formatted as if it were a "block-like
construct." For example, the following are all valid (<strong>not</strong> an exhaustive
list):</p>
<pre class="prettyprint lang-java">new int[] { new int[] {
0, 1, 2, 3 0,
} 1,
2,
new int[] { 3,
0, 1, }
2, 3
} new int[]
{0, 1, 2, 3}
</pre>
<h5 id="s4.8.3.2-array-declarations">4.8.3.2 No C-style array declarations</h5>
<p>The square brackets form a part of the <em>type</em>, not the variable:
<code class="prettyprint lang-java">String[] args</code>, not
<code class="badcode">String args[]</code>.</p>
<h4 id="s4.8.4-switch">4.8.4 Switch statements</h4>
<p class="terminology"><strong>Terminology Note:</strong> Inside the braces of a
<em>switch block</em> are one or more <em>statement groups</em>. Each statement group consists of
one or more <em>switch labels</em> (either <code class="prettyprint lang-java">case FOO:</code> or
<code class="prettyprint lang-java">default:</code>), followed by one or more statements (or, for
the <em>last</em> statement group, <em>zero</em> or more statements).</p>
<h5 id="s4.8.4.1-switch-indentation">4.8.4.1 Indentation</h5>
<p>As with any other block, the contents of a switch block are indented +2.</p>
<p>After a switch label, there is a line break, and the indentation level is increased +2, exactly
as if a block were being opened. The following switch label returns to the previous indentation
level, as if a block had been closed.</p>
<a name="fallthrough"></a>
<h5 id="s4.8.4.2-switch-fall-through">4.8.4.2 Fall-through: commented</h5>
<p>Within a switch block, each statement group either terminates abruptly (with a
<code class="prettyprint lang-java">break</code>,
<code class="prettyprint lang-java">continue</code>,
<code class="prettyprint lang-java">return</code> or thrown exception), or is marked with a comment
to indicate that execution will or <em>might</em> continue into the next statement group. Any
comment that communicates the idea of fall-through is sufficient (typically
<code class="prettyprint lang-java">// fall through</code>). This special comment is not required in
the last statement group of the switch block. Example:</p>
<pre class="prettyprint lang-java">switch (input) {
case 1:
case 2:
prepareOneOrTwo();
// fall through
case 3:
handleOneTwoOrThree();
break;
default:
handleLargeNumber(input);
}
</pre>
<p>Notice that no comment is needed after <code class="prettyprint lang-java">case 1:</code>, only
at the end of the statement group.</p>
<h5 id="s4.8.4.3-switch-default">4.8.4.3 The <code>default</code> case is present</h5>
<p>Each switch statement includes a <code class="prettyprint lang-java">default</code> statement
group, even if it contains no code.</p>
<p><strong>Exception:</strong> A switch statement for an <code>enum</code> type <em>may</em> omit
the <code class="prettyprint lang-java">default</code> statement group, <em>if</em> it includes
explicit cases covering <em>all</em> possible values of that type. This enables IDEs or other static
analysis tools to issue a warning if any cases were missed.
</p>
<a name="annotations"></a>
<h4 id="s4.8.5-annotations">4.8.5 Annotations</h4>
<p>Annotations applying to a class, method or constructor appear immediately after the
documentation block, and each annotation is listed on a line of its own (that is, one annotation
per line). These line breaks do not constitute line-wrapping (Section
4.5, <a href="#s4.5-line-wrapping">Line-wrapping</a>), so the indentation level is not
increased. Example:</p>
<pre class="prettyprint lang-java">@Override
@Nullable
public String getNameIfPresent() { ... }
</pre>
<p class="exception"><strong>Exception:</strong> A <em>single</em> parameterless annotation
<em>may</em> instead appear together with the first line of the signature, for example:</p>
<pre class="prettyprint lang-java">@Override public int hashCode() { ... }
</pre>
<p>Annotations applying to a field also appear immediately after the documentation block, but in
this case, <em>multiple</em> annotations (possibly parameterized) may be listed on the same line;
for example:</p>
<pre class="prettyprint lang-java">@Partial @Mock DataLoader loader;
</pre>
<p>There are no specific rules for formatting annotations on parameters, local variables, or types.
</p>
<a name="comments"></a>
<h4 id="s4.8.6-comments">4.8.6 Comments</h4>
<p>This section addresses <em>implementation comments</em>. Javadoc is addressed separately in
Section 7, <a href="#s7-javadoc">Javadoc</a>.</p>
<p>Any line break may be preceded by arbitrary whitespace followed by an implementation comment.
Such a comment renders the line non-blank.</p>
<h5 id="s4.8.6.1-block-comment-style">4.8.6.1 Block comment style</h5>
<p>Block comments are indented at the same level as the surrounding code. They may be in
<code class="prettyprint lang-java">/* ... */</code> style or
<code class="prettyprint lang-java">// ...</code> style. For multi-line
<code class="prettyprint lang-java">/* ... */</code> comments, subsequent lines must start with
<code>*</code> aligned with the <code>*</code> on the previous line.</p>
<pre class="prettyprint lang-java">/*
* This is // And so /* Or you can
* okay. // is this. * even do this. */
*/
</pre>
<p>Comments are not enclosed in boxes drawn with asterisks or other characters.</p>
<p class="tip"><strong>Tip:</strong> When writing multi-line comments, use the
<code class="prettyprint lang-java">/* ... */</code> style if you want automatic code formatters to
re-wrap the lines when necessary (paragraph-style). Most formatters don't re-wrap lines in
<code class="prettyprint lang-java">// ...</code> style comment blocks.</p>
<a name="modifiers"></a>
<h4 id="s4.8.7-modifiers">4.8.7 Modifiers</h4>
<p>Class and member modifiers, when present, appear in the order
recommended by the Java Language Specification:
</p>
<pre>public protected private abstract default static final transient volatile synchronized native strictfp
</pre>
<h4 id="s4.8.8-numeric-literals">4.8.8 Numeric Literals</h4>
<p><code>long</code>-valued integer literals use an uppercase <code>L</code> suffix, never
lowercase (to avoid confusion with the digit <code>1</code>). For example, <code>3000000000L</code>
rather than <code class="badcode">3000000000l</code>.</p>
<a name="naming"></a>
<h2 id="s5-naming">5 Naming</h2>
<h3 id="s5.1-identifier-names">5.1 Rules common to all identifiers</h3>
<p>Identifiers use only ASCII letters and digits, and, in a small number of cases noted below,
underscores. Thus each valid identifier name is matched by the regular expression
<code>\w+</code> .</p>
<p>In Google Style special prefixes or
suffixes, like those seen in the examples <code class="badcode">name_</code>,
<code class="badcode">mName</code>, <code class="badcode">s_name</code> and
<code class="badcode">kName</code>, are <strong>not</strong> used.</p>
<h3 id="s5.2-specific-identifier-names">5.2 Rules by identifier type</h3>
<h4 id="s5.2.1-package-names">5.2.1 Package names</h4>
<p>Package names are all lowercase, with consecutive words simply concatenated together (no
underscores). For example, <code>com.example.deepspace</code>, not
<code class="badcode">com.example.deepSpace</code> or
<code class="badcode">com.example.deep_space</code>.</p>
<h4 id="s5.2.2-class-names">5.2.2 Class names</h4>
<p>Class names are written in <a href="#s5.3-camel-case">UpperCamelCase</a>.</p>
<p>Class names are typically nouns or noun phrases. For example,
<code class="prettyprint lang-java">Character</code> or
<code class="prettyprint lang-java">ImmutableList</code>. Interface names may also be nouns or
noun phrases (for example, <code class="prettyprint lang-java">List</code>), but may sometimes be
adjectives or adjective phrases instead (for example,
<code class="prettyprint lang-java">Readable</code>).</p>
<p>There are no specific rules or even well-established conventions for naming annotation types.</p>
<p><em>Test</em> classes are named starting with the name of the class they are testing, and ending
with <code class="prettyprint lang-java">Test</code>. For example,
<code class="prettyprint lang-java">HashTest</code> or
<code class="prettyprint lang-java">HashIntegrationTest</code>.</p>
<h4 id="s5.2.3-method-names">5.2.3 Method names</h4>
<p>Method names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p>
<p>Method names are typically verbs or verb phrases. For example,
<code class="prettyprint lang-java">sendMessage</code> or
<code class="prettyprint lang-java">stop</code>.</p>
<p>Underscores may appear in JUnit <em>test</em> method names to separate logical components of the
name. One typical pattern is <code>test<i><MethodUnderTest></i>_<i><state></i></code>,
for example <code class="prettyprint lang-java">testPop_emptyStack</code>. There is no One Correct
Way to name test methods.</p>
<a name="constants"></a>
<h4 id="s5.2.4-constant-names">5.2.4 Constant names</h4>
<p>Constant names use <code class="prettyprint lang-java">CONSTANT_CASE</code>: all uppercase
letters, with words separated by underscores. But what <em>is</em> a constant, exactly?</p>
<p>Constants are static final fields whose contents are deeply immutable and whose methods have no
detectable side effects. This includes primitives, Strings, immutable types, and immutable
collections of immutable types. If any of the instance's observable state can change, it is not a
constant. Merely <em>intending</em> to never mutate the object is not enough. Examples:</p>
<pre class="prettyprint lang-java">// Constants
static final int NUMBER = 5;
static final ImmutableList<String> NAMES = ImmutableList.of("Ed", "Ann");
static final ImmutableMap<String, Integer> AGES = ImmutableMap.of("Ed", 35, "Ann", 32);
static final Joiner COMMA_JOINER = Joiner.on(','); // because Joiner is immutable
static final SomeMutableType[] EMPTY_ARRAY = {};
enum SomeEnum { ENUM_CONSTANT }
// Not constants
static String nonFinal = "non-final";
final String nonStatic = "non-static";
static final Set<String> mutableCollection = new HashSet<String>();
static final ImmutableSet<SomeMutableType> mutableElements = ImmutableSet.of(mutable);
static final ImmutableMap<String, SomeMutableType> mutableValues =
ImmutableMap.of("Ed", mutableInstance, "Ann", mutableInstance2);
static final Logger logger = Logger.getLogger(MyClass.getName());
static final String[] nonEmptyArray = {"these", "can", "change"};
</pre>
<p>These names are typically nouns or noun phrases.</p>
<h4 id="s5.2.5-non-constant-field-names">5.2.5 Non-constant field names</h4>
<p>Non-constant field names (static or otherwise) are written
in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p>
<p>These names are typically nouns or noun phrases. For example,
<code class="prettyprint lang-java">computedValues</code> or
<code class="prettyprint lang-java">index</code>.</p>
<h4 id="s5.2.6-parameter-names">5.2.6 Parameter names</h4>
<p>Parameter names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p>
<p>One-character parameter names in public methods should be avoided.</p>
<h4 id="s5.2.7-local-variable-names">5.2.7 Local variable names</h4>
<p>Local variable names are written in <a href="#s5.3-camel-case">lowerCamelCase</a>.</p>
<p>Even when final and immutable, local variables are not considered to be constants, and should not
be styled as constants.</p>
<h4 id="s5.2.8-type-variable-names">5.2.8 Type variable names</h4>
<p>Each type variable is named in one of two styles:</p>
<ul>
<li>A single capital letter, optionally followed by a single numeral (such as
<code class="prettyprint lang-java">E</code>, <code class="prettyprint lang-java">T</code>,
<code class="prettyprint lang-java">X</code>, <code class="prettyprint lang-java">T2</code>)
</li>
<li>A name in the form used for classes (see Section 5.2.2,
<a href="#s5.2.2-class-names">Class names</a>), followed by the capital letter
<code class="prettyprint lang-java">T</code> (examples:
<code class="prettyprint lang-java">RequestT</code>,
<code class="prettyprint lang-java">FooBarT</code>).</li>
</ul>
<a name="acronyms"></a>
<a name="camelcase"></a>
<h3 id="s5.3-camel-case">5.3 Camel case: defined</h3>
<p>Sometimes there is more than one reasonable way to convert an English phrase into camel case,
such as when acronyms or unusual constructs like "IPv6" or "iOS" are present. To improve
predictability, Google Style specifies the following (nearly) deterministic scheme.</p>
<p>Beginning with the prose form of the name:</p>
<ol>
<li>Convert the phrase to plain ASCII and remove any apostrophes. For example, "Müller's
algorithm" might become "Muellers algorithm".</li>
<li>Divide this result into words, splitting on spaces and any remaining punctuation (typically
hyphens).
<ul>
<li><em>Recommended:</em> if any word already has a conventional camel-case appearance in common
usage, split this into its constituent parts (e.g., "AdWords" becomes "ad words"). Note
that a word such as "iOS" is not really in camel case <em>per se</em>; it defies <em>any</em>
convention, so this recommendation does not apply.</li>
</ul>
</li>
<li>Now lowercase <em>everything</em> (including acronyms), then uppercase only the first
character of:
<ul>
<li>... each word, to yield <em>upper camel case</em>, or</li>
<li>... each word except the first, to yield <em>lower camel case</em></li>
</ul>
</li>
<li>Finally, join all the words into a single identifier.</li>
</ol>
<p>Note that the casing of the original words is almost entirely disregarded. Examples:</p>
<table>
<tbody><tr>
<th>Prose form</th>
<th>Correct</th>
<th>Incorrect</th>
</tr>
<tr>
<td>"XML HTTP request"</td>
<td><code class="prettyprint lang-java">XmlHttpRequest</code></td>
<td><code class="badcode">XMLHTTPRequest</code></td>
</tr>
<tr>
<td>"new customer ID"</td>
<td><code class="prettyprint lang-java">newCustomerId</code></td>
<td><code class="badcode">newCustomerID</code></td>
</tr>
<tr>
<td>"inner stopwatch"</td>
<td><code class="prettyprint lang-java">innerStopwatch</code></td>
<td><code class="badcode">innerStopWatch</code></td>
</tr>
<tr>
<td>"supports IPv6 on iOS?"</td>
<td><code class="prettyprint lang-java">supportsIpv6OnIos</code></td>
<td><code class="badcode">supportsIPv6OnIOS</code></td>
</tr>
<tr>
<td>"YouTube importer"</td>
<td><code class="prettyprint lang-java">YouTubeImporter</code><br>
<code class="prettyprint lang-java">YoutubeImporter</code>*</td>
<td></td>
</tr>
</tbody></table>
<p>*Acceptable, but not recommended.</p>
<p class="note"><strong>Note:</strong> Some words are ambiguously hyphenated in the English
language: for example "nonempty" and "non-empty" are both correct, so the method names
<code class="prettyprint lang-java">checkNonempty</code> and
<code class="prettyprint lang-java">checkNonEmpty</code> are likewise both correct.</p>
<h2 id="s6-programming-practices">6 Programming Practices</h2>