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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>有限制类型</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">到目前为止，已经看到了一种类型限制 &mdash;&mdash;<i>上限</i>。指定上限会将类型参数约束为一个给定类型限制的父类型（或者等于这个类型限制），如 <code>Collection&lt;? extends Number&gt;</code> 中所示。尽管不太常见，但是也可以指定<i>下限</i>，下限写为 <code>Collection&lt;? super Foo&gt;</code>。只有通配符可以具有下限。</p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">除了在类型参数上指定类型约束之外，指定限制还有另一个重要的作用。如果知道一个类型 <code>T</code> 扩展 <code>Number</code>，那么 <code>Number</code> 的方法和域就可以通过类型 <code>T</code> 的变量被访问到。可能到编译时还不知道 <code>T</code> 的值是什么，但是至少知道是一个 <code>Number</code>。</p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">关于哪些类可以充当类型限制存在一些限制。基本类型和数组类型不能用作类型限制（但是数组类型可以用作通配符限制）。任何引用类型（包括参数化类型）都可以用作类型限制。</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
class C &lt;T extends int&gt; // illegal
class C &lt;T extends Foo[]&gt; // illegal
class C &lt;T extends Foo&gt; //legal
class C &lt;T extends Foo&lt;? extends Moo&lt;T&gt;&gt;&gt; //legal
class C &lt;T, V extends T&gt; // legal 
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
一个可能使用下限的地方是在一个方法中，该方法从一个集合选择元素，并把所选元素放到另一个集合中。例如：
</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
class Bunch&lt;V&gt; {
  public void add(V value) { ... }
  public void copyTo(Collection&lt;? super V&gt;) { ... }
  ...
}
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<code>copyTo()</code> 方法将 <code>Bunch</code> 的所有值复制到指定的集合中。不是指定它必须是一个 <code>Collection&lt;V&gt;</code>，而可以指定它是一个 <code>Collection&lt;?  super V&gt;</code>，这意味着 <code>copyTo()</code> 可以将 <code>Bunch&lt;String&gt;</code> 的内容复制到 <code>Collection&lt;Object&gt;</code> 或 <code>Collection&lt;String&gt;</code>，而不只是可以复制到 <code>Collection&lt;String&gt;</code>。 </p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">下限的另一个常见情况是用于 <code>Comparable</code> 接口。不是指定： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public static &lt;T extends Comparable&lt;T&gt;&gt; T max(Collection&lt;T&gt; c) { ... }
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
关于接受什么类型，您可以更灵活一些：
</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public static &lt;T extends Comparable&lt;? super T&gt;&gt; T max(Collection&lt;T&gt; c) { ... }
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
这样，出于一些附加的灵活性，您除了可以传递可与自己比较的类型之外，还可以传递可与它的父类型比较的类型。这对于那些扩展已经 <code>Comparable</code> 的类的类是有价值的： 
</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public class Base implements Comparable&lt;Base&gt; { ... }
public class Child extends Base { }
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
因为 <code>Child</code> 已经实现 <code>Comparable&lt;Base&gt;</code>（继承自父类 <code>Base</code>），所以您可以将它传递给上面 <code>max()</code> 的第二个例子，但是不能传递给第一个例子。
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