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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>泛型方法</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 6 页（共7 页）</nobr></font></td>
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<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">（在 <a href="j-generics-3-1.html">类型参数</a> 一节中）您已经看到，通过在类的定义中添加一个形式类型参数列表，可以将类泛型化。方法也可以被泛型化，不管它们定义在其中的类是不是泛型化的。</p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">泛型类在多个方法签名间实施类型约束。在 <code>List&lt;V&gt;</code> 中，类型参数 <code>V</code> 出现在 <code>get()</code>、<code>add()</code>、<code>contains()</code> 等方法的签名中。当创建一个 <code>Map&lt;K, V&gt;</code> 类型的变量时，您就在方法之间宣称一个类型约束。您传递给 <code>add()</code> 的值将与 <code>get()</code> 返回的值的类型相同。</p>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">类似地，之所以声明泛型方法，一般是因为您想要在该方法的多个参数之间宣称一个类型约束。例如，下面代码中的 <code>ifThenElse()</code> 方法，根据它的第一个参数的布尔值，它将返回第二个或第三个参数：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public &lt;T&gt; T ifThenElse(boolean b, T first, T second) {
  return b ? first : second;
}
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">注意，您可以调用 <code>ifThenElse()</code>，而不用显式地告诉编译器，您想要 <code>T</code> 的什么值。编译器不必显式地被告知 T 将具有什么值；它只知道这些值都必须相同。编译器允许您调用下面的代码，因为编译器可以使用类型推理来推断出，替代 <code>T</code> 的 <code>String</code> 满足所有的类型约束：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
String s = ifThenElse(b, "a", "b");
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">类似地，您可以调用： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Integer i = ifThenElse(b, new Integer(1), new Integer(2));
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
但是，编译器不允许下面的代码，因为没有类型会满足所需的类型约束：
</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
String s = ifThenElse(b, "pi", new Float(3.14));
</code>
</pre>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">为什么您选择使用泛型方法，而不是将类型 <code>T</code> 添加到类定义呢？（至少）有两种情况应该这样做：
<ul>
<li>当泛型方法是静态的时，这种情况下不能使用类类型参数。<br>
<br>
</li>
<li>当 <code>T</code> 上的类型约束对于方法真正是局部的时，这意味着没有在相同类的<i>另一个</i> 方法签名中使用<i>相同</i> 类型 <code>T</code> 的约束。通过使得泛型方法的类型参数对于方法是局部的，可以简化封闭类型的签名。</li>
</ul> 
</p>
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