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📁 这是jdk1.5的泛型教程
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b><code style="font-family: Courier New, Courier, monospace; font-size: 18">Enum&lt;E&gt;</code></b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 7 页（共9 页）</nobr></font></td>
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<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">JDK 5.0 中 Java 语言另一个增加的特性是枚举。当您使用 <code>enum</code> 关键字声明一个枚举时，编译器就会在内部为您生成一个类，用于扩展 <code>Enum</code> 并为枚举的每个值声明静态实例。所以如果您说： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public enum Suit {HEART, DIAMOND, CLUB, SPADE};
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">编译器就会在内部生成一个叫做 <code>Suit</code> 的类，该类扩展 <code>java.lang.Enum&lt;Suit&gt;</code> 并具有叫做 <code>HEART</code>、<code>DIAMOND</code>、<code>CLUB</code> 和 <code>SPADE</code> 的常量（<code>public static final</code>）成员，每个成员都是 <code>Suit</code> 类。 </p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">与 <code>Class</code> 一样，<code>Enum</code> 也是一个泛型类。但是与 <code>Class</code> 不同，它的签名稍微更复杂一些： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
class Enum&lt;E extends Enum&lt;E&gt;&gt; { . . . }
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
这究竟是什么意思？这难道不会导致无限递归？
</p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">我们逐步来分析。类型参数 <code>E</code> 用于 <code>Enum</code> 的各种方法中，比如 <code>compareTo()</code> 或 <code>getDeclaringClass()</code>。为了这些方法的类型安全，<code>Enum</code> 类必须在枚举的类上泛型化。 </p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">所以 <code>extends Enum&lt;E&gt;</code> 部分如何理解？该部分又具有两个部分。第一部分指出，作为 <code>Enum</code> 的类型参数的类本身必须是 <code>Enum</code> 的子类型，所以您不能声明一个类 X 扩展 <code>Enum&lt;Integer&gt;</code>。第二部分指出，任何扩展 <code>Enum</code> 的类必须传递<i>它本身</i> 作为类型参数。您不能声明 X 扩展 <code>Enum&lt;Y&gt;</code>，即使 Y 扩展 <code>Enum</code>。 </p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">总之，<code>Enum</code> 是一个参数化的类型，只可以为它的子类型实例化，并且这些子类型然后将根据子类型来继承方法。幸运的是，在 <code>Enum</code> 情况下，编译器为您做这些工作，一切都很好。 </p>
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