再论泛型
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<h1><span lang="en-us">why</span><span>，java为什么需要泛型？</span></h1>
<h1><span lang="en-us">how</span><span>，擦除，一种折中方案</span></h1><p name="code">list<string > list = new arrarylist<string>();list.add(“cantellow”);int integer = list.get(0);//compile error?
<span>然后，将参数化类型中的类型参数</span><span lang="en-us">"</span><span>擦除</span><span lang="en-us">"</span><span>（</span><span lang="en-us">erasure</span><span>）掉，并且将类型变量用</span><span lang="en-us">"</span><span>上限（</span><span lang="en-us">upper bound</span><span>）</span><span lang="en-us">"</span><span>取代，通常情况下这些上限是</span><span lang="en-us"> object</span><span>。这里的类型变量是指实例域，本地方法域，方法参数以及方法返回值中用来标记类型信息的</span><span lang="en-us">"</span><span>变量</span><span lang="en-us">"</span><span>，例如：实例域中的变量声明</span><span lang="en-us">?a elem;</span><span>，方法声明</span><span lang="en-us">?node (a elem){};</span><span>，其中，</span><span lang="en-us">a </span><span>用来标记</span><span lang="en-us"> elem </span><span>的类型，它就是类型变量。</span>
<p name="code">list<integer> li = new arraylist<integer>(); list<number> ln = li; // illegal ln.add(new float(3.1415)); ?
<span>因为</span><span lang="en-us">?ln</span><span>是</span><span lang="en-us">?list<number></span><span>，所以向其添加</span><span lang="en-us">?float</span><span>似乎是完全合法的。但是如果</span><span lang="en-us">?ln</span><span>是</span><span lang="en-us">?li</span><span>的别名，那么这就破坏了蕴含在</span><span lang="en-us">?li</span><span>定义中的类型安全承诺</span><span lang="en-us"> —— </span><span>它是一个整数列表，这就是泛型类型不能协变的原因。</span>
<h2><span>获取泛型类型</span></h2><p class="msonormal"><span>更多信息可以参考讨论：</span><span lang="en-us">http://www.iteye.com/topic/585900</span>
