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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>泛型用法的例子</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 3 页（共4 页）</nobr></font></td>
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<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">泛型的许多最佳例子都来自集合框架，因为泛型让您在保存在集合中的元素上指定类型约束。考虑这个使用 <code>Map</code> 类的例子，其中涉及一定程度的优化，即 <code>Map.get()</code> 返回的结果将确实是一个 <code>String</code>：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Map m = new HashMap();
m.put("key", "blarg");
String s = (String) m.get("key");
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">如果有人已经在映射中放置了不是 <code>String</code> 的其他东西，上面的代码将会抛出 <code>ClassCastException</code>。泛型允许您表达这样的类型约束，即 <code>m</code> 是一个将 <code>String</code> 键映射到 <code>String</code> 值的 <code>Map</code>。这可以消除代码中的强制类型转换，同时获得一个附加的类型检查层，这个检查层可以防止有人将错误类型的键或值保存在集合中。 </p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
下面的代码示例展示了 JDK 5.0 中集合框架中的 <code>Map</code> 接口的定义的一部分：
</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public interface Map&lt;K, V&gt; {
  public void put(K key, V value);
  public V get(K key);
}
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">注意该接口的两个附加物： 
<ul>
<li>
<i>类型参数</i> <code>K</code> 和 <code>V</code> 在类级别的规格说明，表示在声明一个 <code>Map</code> 类型的变量时指定的类型的占位符。<br>
<br>
</li>
<li>在 <code>get()</code>、<code>put()</code> 和其他方法的方法签名中使用的 <code>K</code> 和 <code>V</code>。</li>
</ul>
</p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">为了赢得使用泛型的好处，必须在定义或实例化 <code>Map</code> 类型的变量时为 <code>K</code> 和 <code>V</code> 提供具体的值。以一种相对直观的方式做这件事：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Map&lt;String, String&gt; m = new HashMap&lt;String, String&gt;();
m.put("key", "blarg");
String s = m.get("key");
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">当使用 <code>Map</code> 的泛型化版本时，您不再需要将 <code>Map.get()</code> 的结果强制类型转换为 <code>String</code>，因为编译器知道 <code>get()</code> 将返回一个 <code>String</code>。</p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">在使用泛型的版本中并没有减少键盘录入；实际上，比使用强制类型转换的版本需要做更多键入。使用泛型只是带来了附加的类型安全。因为编译器知道关于您将放进 <code>Map</code> 中的键和值的类型的更多信息，所以类型检查从执行时挪到了编译时，这会提高可靠性并加快开发速度。 </p>
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