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11.20 多态性：动态方法查找</div>
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多态性使得超类中的一个变量能够保存子类对象，并且能够直接从超类变量中引用这些子类的方法。考虑一个例子，假设<i>Packet</i>对象的基类<i>BasePacket</i>通过虚拟函数定义了通常由它的子类使用的所有公开方法（例如<i>send</i>、<i>receive</i>、<i>print</i>等）。这时，尽管<i>BasePacket</i>是抽象的，它仍然可以用来声明一个变量： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00376"><pre class="Element12">BasePacket packets<span style="color: #800080">[</span><span style="color: #FF0000">100</span><span style="color: #800080">];</span></pre></div></div>
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现在，我们可以产生不同包对象的实例，并将它们放入到数组当中： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00377"><pre class="Element12">EtherPacket ep <span style="color: #800080">= </span><span style="color: #0000FF">new</span><span style="color: #800080">;</span> <span style="color: #008000">// extends BasePacket</span>
TokenPacket tp <span style="color: #800080">= </span><span style="color: #0000FF">new</span><span style="color: #800080">;</span> <span style="color: #008000">// extends BasePacket</span>
GPSSPacket gp <span style="color: #800080">= </span><span style="color: #0000FF">new</span><span style="color: #800080">;</span> <span style="color: #008000">// extends EtherPacket</span>
packets<span style="color: #800080">[</span><span style="color: #FF0000">0</span><span style="color: #800080">] =</span> ep<span style="color: #800080">;</span>
packets<span style="color: #800080">[</span><span style="color: #FF0000">1</span><span style="color: #800080">]</span> = tp<span style="color: #800080">;</span>
packets<span style="color: #800080">[</span><span style="color: #FF0000">2</span><span style="color: #800080">]</span> = gp<span style="color: #800080">;</span></pre></div></div>
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如果数据类型为诸如整数、位和字符串子类的数据类型，那么所有的这些类型都不能被存储到一个单一数组中。然而，使用多态性，这个功能就能够实现。在这个例子中，由于方法被声明成虚拟的，所以可以从超类变量中访问恰当的子类方法，即使编译器在编译的时候并不知道它将要加载哪一个方法。 </p>
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例如，对于<i>packets<span style="color: #800080">[</span><span style="color: #FF0000">1</span><span style="color: #800080">]</span></i>： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00378"><pre class="Element12">packets<span style="color: #800080">[</span><span style="color: #FF0000">1</span><span style="color: #800080">].</span>send<span style="color: #800080">();</span></pre></div></div>
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应该调用与<i>TokenPacket</i>类相关联的<i>send</i>方法。在运行的时候，系统能够从合适的类中正确地绑定方法。 </p>
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这就是多态性工作的一个典型例子，它比非面向对象的框架提供更为强大的能力。</p></div>
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