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<b>Trail:</b> Learning the Java Language
<br /><b>Lesson:</b> Annotations
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<div class="linkLESSON"><a href="index.html">Annotations</a></div>
<div class="linkAHEAD"><a href="basics.html">Annotations Basics</a></div>
<div class="nolinkAHEAD">Declaring an Annotation Type</div>
<div class="linkAHEAD"><a href="predefined.html">Predefined Annotation Types</a></div>
<div class="linkAHEAD"><a href="type_annotations.html">Type Annotations and Pluggable Type Systems</a></div>
<div class="linkAHEAD"><a href="repeating.html">Repeating Annotations</a></div>
<div class="linkQUESTIONS"><a href="QandE/questions.html">Questions and Exercises</a></div>
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<div id="PageTitle"><h1>Declaring an Annotation Type</h1></div>
<div id="PageContent">
<p>Many annotations replace comments in code.</p>
<p>Suppose that a software group traditionally starts 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 similar to an interface definition where the keyword <code>interface</code> is preceded by the at sign (<code>@</code>) (@ = AT, as in annotation type). Annotation types are 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 previous annotation definition contains <i>annotation type element</i> declarations, which look a lot like methods. Note that they can define optional default values.</p>
<p>After the annotation type is 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 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>
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