forked from ashishjohn1908/Java
-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathannotations.html
More file actions
615 lines (572 loc) · 23 KB
/
Copy pathannotations.html
File metadata and controls
615 lines (572 loc) · 23 KB
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
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html lang="en-US" xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Annotations (The Java™ Tutorials >
Learning the Java Language > Classes and Objects)
</title>
<meta name="description" content="This beginner Java tutorial describes fundamentals of programming in the Java programming language" />
<meta name="keywords" content="java programming, learn java, java sample code, java objects, java classes, java inheritance, interfaces, variables, arrays, data types, operators, control flow, number, string" />
<style type="text/css">
.FigureCaption {
margin-left: 1in;
margin-right: 1in;
font-family: sans-serif;
font-size: smaller;
text-align: justify;
}
#TopBar_bl {
background: url(../../images/java_bar_bl.gif) 0 100% no-repeat;
width: 100%;
height: 60px;
}
#TopBar_br {
background: url(../../images/java_bar_br.gif) 100% 100% no-repeat;
width: 100%;
height: 60px;
}
#TopBar_tl {
background: url(../../images/java_bar_tl.gif) 0 0 no-repeat;
width: 100%;
height: 60px;
}
#TopBar_tr {
background: url(../../images/java_bar_tr.gif) 100% 0 no-repeat;
width: 100%;
height: 60px;
}
#TopBar {
background: #35556B url(../../images/java_bar.gif);
margin: 10px 10px 0 10px;
height:60px;
min-width:700px;
color: white;
font-family: sans-serif;
font-weight: bold;
}
@media print {
#BreadCrumbs, #Download {
display: none;
}
}
#TopBar_right {
line-height: 14px;
float: right;
padding-top: 2px;
padding-right: 30px;
text-align: center;
}
@media print {
#TopBar_right {
display: none;
}
}
#TopBar_right a {
font-size: 12px;
margin: 3px;
padding: 0;
}
#TopBar a:visited, #TopBar a:link {
color: white;
text-decoration: none;
}
#TopBar a:hover, #TopBar a:active {
background-color: white;
color: #35556B;
}
#BreadCrumbs {
padding: 4px 5px 0.5em 0;
font-family: sans-serif;
float: right;
}
#BreadCrumbs a {
color: blue;
}
#BreadCrumbs a:visited, #BreadCrumbs a:link {
text-decoration: none;
}
#BreadCrumbs a:hover, #BreadCrumbs a:active {
text-decoration: underline;
}
#PageTitle {
margin: 0 5px 0.5em 0;
color: #F90000;
}
#PageContent{
margin: 0 5px 0 20px;
}
.LeftBar_shown {
width: 13em;
float: left;
margin-left: 10px;
margin-top: 4px;
margin-bottom: 2em;
margin-right: 10px;
}
@media print {
.LeftBar_shown {
display: none;
}
}
.LeftBar_hidden {
display: none;
}
#Footer {
padding-top: 10px;
padding-left: 10px;
margin-right: 10px;
}
.footertext {
font-size: 10px;
font-family: sans-serif;
margin-top: 1px;
}
#Footer2 {
padding-top: 10px;
padding-left: 10px;
margin-right: 10px;
}
.NavBit {
padding: 4px 5px 0.5em 0;
font-family: sans-serif;
}
@media print {
.NavBit {
display: none;
}
}
#TagNotes {
text-align: right;
}
@media print {
#TagNotes a:visited, #TagNotes a:link {
color: #35556B;
text-decoration: none;
}
}
#Contents a, .NavBit a, #TagNotes a {
color: blue
}
#TagNotes a:visited, #TagNotes a:link,
#Contents a:visited, #Contents a:link,
.NavBit a:visited, .NavBit a:link {
text-decoration: none;
}
#TagNotes a:hover, #TagNotes a:active,
#Contents a:hover, #Contents a:active,
.NavBit a:hover, .NavBit a:active {
text-decoration: underline;
}
#Contents {
float: left;
font-family: sans-serif;
}
@media print {
#Contents {
display: none;
}
}
@media screen {
div.PrintHeaders {
display: none;
}
}
.linkLESSON, .nolinkLESSON {
margin-left: 0.5em;
text-indent: -0.5em
}
.linkAHEAD, .nolinkAHEAD, .linkQUESTIONS, .nolinkQUESTIONS {
margin-left: 1.5em;
text-indent: -0.5em
}
.linkBHEAD, .nolinkBHEAD {
margin-left: 2.5em;
text-indent: -0.5em
}
.linkCHEAD, .nolinkCHEAD {
margin-left: 3.5em;
text-indent: -0.5em
}
.nolinkLESSON, .nolinkAHEAD, .nolinkBHEAD, .nolinkCHEAD,
.nolinkQUESTIONS {
font-weight: bold;
color: #F90000;
}
.MainFlow_indented {
margin-right: 10px;
margin-left: 15em;
margin-bottom: 2em;
}
.MainFlow_wide {
margin-right: 10px;
margin-left: 10px;
margin-bottom: 2em;
}
@media print {
.MainFlow_indented, .MainFlow_wide {
padding-top: 0;
margin-top: 10px;
margin-right: 10px;
margin-left: 0;
}
}
h1, h2, h3, h4, h5 {
color: #F90000;
font-family: sans-serif;
}
h1 {
font-weight: bold;
font-size: 20px;
}
h2 {
font-weight: bold;
font-size: 17px;
}
h3 {
font-weight: bold;
font-size: 14px;
}
h4 {
font-size: 15px;
}
h5 {
font-size: 12px;
}
#ToggleLeft {
display: none;
}
.note {
margin: 0 30px 0px 30px;
}
.codeblock {
margin: 0 30px 0px 30px;
}
.tocli {
list-style-type:none;
}
</style>
<script type="text/javascript">
/* <![CDATA[ */
function leftBar() {
var nameq = 'tutorial_showLeftBar='
var cookies = document.cookie.split(';');
for (var i = 0; i < cookies.length; i++) {
var cookieString = cookies[i];
while (cookieString.charAt(0) == ' ') {
cookieString = cookieString.substring(1, cookieString.length);
}
if (cookieString.indexOf(nameq) == 0) {
cookieValue = cookieString.substring(nameq.length,
cookieString.length);
return cookieValue == 'yes';
}
}
return true;
}
function showLeft(b) {
var contents = document.getElementById("LeftBar");
var main = document.getElementById("MainFlow");
var toggle = document.getElementById("ToggleLeft");
if (b) {
contents.className = "LeftBar_shown";
main.className = "MainFlow_indented";
toggle.innerHTML = "Hide the TOC";
document.cookie = 'tutorial_showLeftBar=yes; path=/';
} else {
contents.className = "LeftBar_hidden";
main.className = "MainFlow_wide";
toggle.innerHTML = "Show the TOC";
document.cookie = 'tutorial_showLeftBar=no; path=/';
}
}
function toggleLeft() {
showLeft(document.getElementById("LeftBar").className ==
"LeftBar_hidden");
document.getElementById("ToggleLeft").blur();
}
function load() {
showLeft(leftBar());
document.getElementById("ToggleLeft").style.display="inline";
}
function showCode(displayCodePage, codePath) {
var codePathEls = codePath.split("/");
var currDocPathEls = location.href.split("/");
//alert ("codePathEls = " + codePathEls + "\n" + "currDocPathEls = " + currDocPathEls);
currDocPathEls.pop(); // remove file name at the end
while (codePathEls.length > 0) {
if (codePathEls[0] == "..") {
codePathEls.shift();
currDocPathEls.pop();
} else {
break;
}
}
var fullCodePath = currDocPathEls.join("/") + "/" + codePathEls.join("/");
//alert ("fullCodePath = " + fullCodePath );
if (codePath.indexOf(".java") != -1 || codePath.indexOf(".jnlp") != -1) {
window.location.href = displayCodePage + "?code=" + encodeURI(fullCodePath);
} else {
window.location.href = fullCodePath;
}
}
/* ]]> */
</script>
</head>
<body onload="load()">
<noscript>
A browser with JavaScript enabled is required for this page to operate properly.
</noscript>
<div id="TopBar"> <div id="TopBar_tr"> <div id="TopBar_tl"> <div id="TopBar_br"> <div id="TopBar_bl">
<div id="TopBar_right">
<a target="_blank"
href="http://www.oracle.com/technetwork/java/javase/downloads/index.html">Download
the JDK</a>
<br />
<a href="../../search.html" target="_blank">Search the
Tutorials</a>
<br />
<a href="javascript:toggleLeft()"
id="ToggleLeft">Hide the TOC</a>
</div>
</div> </div> </div> </div> </div>
<div class="PrintHeaders">
<b>Trail:</b> Learning the Java Language
<br /><b>Lesson:</b> Classes and Objects
</div>
<div id="LeftBar" class="LeftBar_shown">
<div id="Contents">
<div class="linkLESSON"><a href="index.html">Classes and Objects</a></div>
<div class="linkAHEAD"><a href="classes.html">Classes</a></div>
<div class="linkBHEAD"><a href="classdecl.html">Declaring Classes</a></div>
<div class="linkBHEAD"><a href="variables.html">Declaring Member Variables</a></div>
<div class="linkBHEAD"><a href="methods.html">Defining Methods</a></div>
<div class="linkBHEAD"><a href="constructors.html">Providing Constructors for Your Classes</a></div>
<div class="linkBHEAD"><a href="arguments.html">Passing Information to a Method or a Constructor</a></div>
<div class="linkAHEAD"><a href="objects.html">Objects</a></div>
<div class="linkBHEAD"><a href="objectcreation.html">Creating Objects</a></div>
<div class="linkBHEAD"><a href="usingobject.html">Using Objects</a></div>
<div class="linkAHEAD"><a href="more.html">More on Classes</a></div>
<div class="linkBHEAD"><a href="returnvalue.html">Returning a Value from a Method</a></div>
<div class="linkBHEAD"><a href="thiskey.html">Using the this Keyword</a></div>
<div class="linkBHEAD"><a href="accesscontrol.html">Controlling Access to Members of a Class</a></div>
<div class="linkBHEAD"><a href="classvars.html">Understanding Instance and Class Members</a></div>
<div class="linkBHEAD"><a href="initial.html">Initializing Fields</a></div>
<div class="linkBHEAD"><a href="summaryclasses.html">Summary of Creating and Using Classes and Objects</a></div>
<div class="linkQUESTIONS"><a href="QandE/creating-questions.html">Questions and Exercises</a></div>
<div class="linkQUESTIONS"><a href="QandE/objects-questions.html">Questions and Exercises</a></div>
<div class="linkAHEAD"><a href="nested.html">Nested Classes</a></div>
<div class="linkBHEAD"><a href="innerclasses.html">Inner Class Example</a></div>
<div class="linkBHEAD"><a href="summarynested.html">Summary of Nested Classes</a></div>
<div class="linkQUESTIONS"><a href="QandE/nested-questions.html">Questions and Exercises</a></div>
<div class="linkAHEAD"><a href="enum.html">Enum Types</a></div>
<div class="linkQUESTIONS"><a href="QandE/enum-questions.html">Questions and Exercises</a></div>
<div class="nolinkAHEAD">Annotations</div>
<div class="linkQUESTIONS"><a href="QandE/annotations-questions.html">Questions and Exercises</a></div>
</div>
</div>
<div id="MainFlow" class="MainFlow_indented">
<span id="BreadCrumbs">
<a href="../../index.html" target="_top">Home Page</a>
>
<a href="../index.html" target="_top">Learning the Java Language</a>
>
<a href="index.html" target="_top">Classes and Objects</a>
</span>
<div class="NavBit">
<a target="_top" href="QandE/enum-questions.html">« Previous</a> • <a target="_top" href="../TOC.html">Trail</a> • <a target="_top" href="QandE/annotations-questions.html">Next »</a>
</div>
<div id="PageTitle"><h1>Annotations</h1></div>
<div id="PageContent">
<p><em>Annotations</em> provide data about a program that is not part of the program itself. They have no direct effect on the operation of the code they annotate.</p>
<p>Annotations have a number of uses, among them:</p>
<ul>
<li><b>Information for the compiler</b> — Annotations can be used by the compiler to detect errors or suppress warnings.</li>
<li><b>Compiler-time and deployment-time processing</b> — Software tools can process annotation information to generate code, XML files, and so forth.</li>
<li><b>Runtime processing</b> — Some annotations are available to be examined at runtime.</li>
</ul>
<p>Annotations can be applied to a program's declarations of classes, fields, methods, and other program elements.</p>
<p>The annotation appears first, often (by convention) on its own line, and may include <i>elements</i> with named or unnamed values:</p>
<div class="codeblock"><pre>
@Author(
name = "Benjamin Franklin",
date = "3/27/2003"
)
class MyClass() { }
</pre></div>
<p>or</p>
<div class="codeblock"><pre>
@SuppressWarnings(value = "unchecked")
void myMethod() { }
</pre></div>
<p>If there is just one element named "value," then the name may be omitted, as in:</p>
<div class="codeblock"><pre>
@SuppressWarnings("unchecked")
void myMethod() { }
</pre></div>
<p>Also, if an annotation has no elements, the parentheses may be omitted, as in:</p>
<div class="codeblock"><pre>
@Override
void mySuperMethod() { }
</pre></div>
<h2>Documentation</h2>
<p>Many annotations replace what would otherwise have been comments in code.</p>
<p>Suppose that a software group has traditionally begun the body of every class with comments providing important information:</p>
<div class="codeblock"><pre>
public class Generation3List extends Generation2List {
// Author: John Doe
// Date: 3/17/2002
// Current revision: 6
// Last modified: 4/12/2004
// By: Jane Doe
// Reviewers: Alice, Bill, Cindy
// class code goes here
}
</pre></div>
<p>To add this same metadata with an annotation, you must first define the <i>annotation type</i>. The syntax for doing this is:</p>
<div class="codeblock"><pre>
@interface ClassPreamble {
String author();
String date();
int currentRevision() default 1;
String lastModified() default "N/A";
String lastModifiedBy() default "N/A";
// Note use of array
String[] reviewers();
}
</pre></div>
<p>The annotation type definition looks somewhat like an interface definition where the keyword <code>interface</code> is preceded by the @ character (@ = "AT" as in Annotation Type). Annotation types are, in fact, a form of <i>interface</i>, which will be covered in a later
lesson.
For the moment, you do not need to understand interfaces.</p>
<p>The body of the annotation definition above contains <i>annotation type element</i> declarations, which look a lot like methods. Note that they may define optional default values.</p>
<p>Once the annotation type has been defined, you can use annotations of that type, with the values filled in, like this:</p>
<div class="codeblock"><pre>
@ClassPreamble (
author = "John Doe",
date = "3/17/2002",
currentRevision = 6,
lastModified = "4/12/2004",
lastModifiedBy = "Jane Doe",
// Note array notation
reviewers = {"Alice", "Bob", "Cindy"}
)
public class Generation3List extends Generation2List {
// class code goes here
}
</pre></div>
<div class="note"><hr /><strong>Note:</strong> To make the information in <code>@ClassPreamble</code> appear in Javadoc-generated documentation, you must annotate the <code>@ClassPreamble</code> definition itself with the <code>@Documented</code> annotation:
<div class="codeblock"><pre>
// import this to use <code>@Documented</code>
import java.lang.annotation.*;
@Documented
@interface ClassPreamble {
// Annotation element definitions
}
</pre></div>
<hr /></div>
<h2>Annotations Used by the Compiler</h2>
<p>There are three annotation types that are predefined by the language specification itself: <code>@Deprecated</code>, <code>@Override</code>, and <code>@SuppressWarnings</code>.</p>
<p><b>@Deprecated</b>—the
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/7/docs/api/java/lang/Deprecated.html"><code>@Deprecated</code></a> annotation indicates that the marked element is <i>deprecated</i> and should no longer be used. The compiler generates a warning whenever a program uses a method, class, or field with the <code>@Deprecated</code> annotation. When an element is deprecated, it should also be documented using the Javadoc <code>@deprecated</code> tag, as shown in the following example. The use of the "@" symbol in both Javadoc comments and in annotations is not coincidental — they are related conceptually. Also, note that the Javadoc tag starts with a lowercase "d" and the annotation starts with an uppercase "D".</p>
<div class="codeblock"><pre>
// Javadoc comment follows
/**
* <i>@deprecated</i>
* <i>explanation of why it</i>
* <i>was deprecated</i>
*/
<b>@Deprecated</b>
static void deprecatedMethod() { }
}
</pre></div>
<p><b>@Override</b>—the
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/7/docs/api/java/lang/Override.html"><code>@Override</code></a> annotation informs the compiler that the element is meant to override an element declared in a superclass (overriding methods will be discussed in the
the lesson titled
"Interfaces and Inheritance").</p>
<div class="codeblock"><pre>
// <i>mark method as a superclass method</i>
// <i>that has been overridden</i>
<b>@Override</b>
int overriddenMethod() { }
</pre></div>
<p>While it's not required to use this annotation when overriding a method, it helps to prevent errors. If a method marked with <code>@Override</code> fails to correctly override a method in one of its superclasses, the compiler generates an error.</p>
<p><b>@SuppressWarnings</b>—the
<a class="APILink" target="_blank" href="http://docs.oracle.com/javase/7/docs/api/java/lang/SuppressWarnings.html"><code>@SuppressWarnings</code></a> annotation tells the compiler to suppress specific warnings that it would otherwise generate. In the example below, a deprecated method is used and the compiler would normally generate a warning. In this case, however, the annotation causes the warning to be suppressed.</p>
<div class="codeblock"><pre>
// <i>use a deprecated method and tell</i>
// <i>compiler not to generate a warning</i>
<b>@SuppressWarnings("deprecation")</b>
void useDeprecatedMethod() {
// deprecation warning
// - suppressed
objectOne.deprecatedMethod();
}
</pre></div>
<p>Every compiler warning belongs to a category. The Java Language Specification lists two categories: "deprecation" and "unchecked." The "unchecked" warning can occur when interfacing with legacy code written before the advent of generics (discussed in
the lesson titled
"Generics"). To suppress more than one category of warnings, use the following syntax:</p>
<div class="codeblock"><pre>
@SuppressWarnings({"unchecked", "deprecation"})
</pre></div>
<h2>Annotation Processing</h2>
<p>The more advanced uses of annotations include writing an <i>annotation processor</i> that can read a Java program and take actions based on its annotations. It might, for example, generate auxiliary source code, relieving the programmer of having to create boilerplate code that always follows predictable patterns. To facilitate this task, release 5.0 of the JDK includes an annotation processing tool, called <code>apt</code>. In release 6 of the JDK, the functionality of <code>apt</code> is a standard part of the Java compiler.</p>
<p>To make annotation information available at runtime, the annotation type itself must be annotated with <code>@Retention(RetentionPolicy.RUNTIME)</code>, as follows:</p>
<div class="codeblock"><pre>
import java.lang.annotation.*;
@Retention(RetentionPolicy.RUNTIME)
@interface AnnotationForRuntime {
// Elements that give information
// for runtime processing
}
</pre></div>
</div>
<div class="NavBit">
<a target="_top" href="QandE/enum-questions.html">« Previous</a>
•
<a target="_top" href="../TOC.html">Trail</a>
•
<a target="_top" href="QandE/annotations-questions.html">Next »</a>
</div>
</div>
<div id="Footer2">
<hr />
<div id="TagNotes">
<p class="footertext">Problems with the examples? Try <a target="_blank"
href="../../information/run-examples.html">Compiling and Running
the Examples: FAQs</a>.
<br />
Complaints? Compliments? Suggestions? <a target="_blank"
href="http://docs.oracle.com/javase/feedback.html">Give
us your feedback</a>.
</p>
</div>
<div id="Footer">
<p class="footertext"><a name="license_info">Your use of this</a> page and all the material on pages under "The Java Tutorials" banner
is subject to these <a href="../../information/cpyr.html">legal notices</a>.
</p>
<table border="0" cellspacing="0" cellpadding="5" summary="">
<tr>
<td headers="h1" width="20%">
<table width="100%" border="0" cellspacing="0" cellpadding="5">
<tr>
<td headers="h1" align="center"><img id="duke" src="../../images/DukeWave.gif" width="55" height="55" alt="duke image" /></td>
<td headers="h2" align="left" valign="middle"><img id="oracle" src="../../images/logo_oracle_footer.gif" width="100" height="29" alt="Oracle logo" /></td>
</tr>
</table>
</td>
<td width="55%" valign="middle" align="center">
<p class="footertext"><a href="http://www.oracle.com/us/corporate/index.html">About Oracle</a> | <a href="http://www.oracle.com/technology/index.html">Oracle Technology Network</a> | <a href="https://www.samplecode.oracle.com/servlets/CompulsoryClickThrough?type=TermsOfService">Terms of Service</a></p>
</td>
<td width="25%" valign="middle" align="right">
<p class="footertext">Copyright © 1995, 2012 Oracle and/or its affiliates. All rights reserved.</p>
</td>
</tr>
</table>
</div>
</div>
<div class="PrintHeaders">
<b>Previous page:</b> Questions and Exercises: Enum Types
<br /><b>Next page:</b> Questions and Exercises: Annotations
</div>
</body>
</html>