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这是jdk1.5的泛型教程

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<a name="navskip"></a><td height="25" colspan="4"><img alt="2.泛型简介" src="imagemaster/titlebar2.jpg" border="0" height="25" width="562"></td>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>泛型的好处</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 2 页（共4 页）</nobr></font></td>
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<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">Java 语言中引入泛型是一个较大的功能增强。不仅语言、类型系统和编译器有了较大的变化，以支持泛型，而且类库也进行了大翻修，所以许多重要的类，比如集合框架，都已经成为泛型化的了。这带来了很多好处：</p>
<ul>
<li>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<b>类型安全。</b>  泛型的主要目标是提高 Java 程序的类型安全。通过知道使用泛型定义的变量的类型限制，编译器可以在一个高得多的程度上验证类型假设。没有泛型，这些假设就只存在于程序员的头脑中（或者如果幸运的话，还存在于代码注释中）。</p>


<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">Java 程序中的一种流行技术是定义这样的集合，即它的元素或键是公共类型的，比如&ldquo;<code>String</code> 列表&rdquo;或者&ldquo;<code>String</code> 到 <code>String</code> 的映射&rdquo;。通过在变量声明中捕获这一附加的类型信息，泛型允许编译器实施这些附加的类型约束。类型错误现在就可以在编译时被捕获了，而不是在运行时当作 <code>ClassCastException</code> 展示出来。将类型检查从运行时挪到编译时有助于您更容易找到错误，并可提高程序的可靠性。 </p>
</li>
<li>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<b>消除强制类型转换。</b>  泛型的一个附带好处是，消除源代码中的许多强制类型转换。这使得代码更加可读，并且减少了出错机会。</p>


<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">尽管减少强制类型转换可以降低使用泛型类的代码的罗嗦程度，但是声明泛型变量会带来相应的罗嗦。比较下面两个代码例子。</p>


<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">该代码不使用泛型： </p>


<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
List li = new ArrayList();
li.put(new Integer(3));
Integer i =  (Integer) li.get(0);
</code>
</pre>


<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
该代码使用泛型：
</p>


<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
List&lt;Integer&gt; li = new ArrayList&lt;Integer&gt;();
li.put(new Integer(3));
Integer i =  li.get(0);
</code>
</pre>


<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">在简单的程序中使用一次泛型变量不会降低罗嗦程度。但是对于多次使用泛型变量的大型程序来说，则可以累积起来降低罗嗦程度。</p>
</li>
<li>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<b>潜在的性能收益。</b> 泛型为较大的优化带来可能。在泛型的初始实现中，编译器将强制类型转换（没有泛型的话，程序员会指定这些强制类型转换）插入生成的字节码中。但是更多类型信息可用于编译器这一事实，为未来版本的 JVM 的优化带来可能。</p>

</li>
</ul>
<p xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">由于泛型的实现方式，支持泛型（几乎）不需要 JVM 或类文件更改。所有工作都在编译器中完成，编译器生成类似于没有泛型（和强制类型转换）时所写的代码，只是更能确保类型安全而已。 </p>
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