Class.asSubclass浅谈

<div id="article_content" class="article_content"><p><span style="font-family:Arial; font-size:14px; line-height:26px"></span></p><div class="dp-highlighter bg_java"><div class="bar"><div class="tools"><b>[java]</b> <a href="#" class="ViewSource" title="view plain" onclick="dp.sh.Toolbar.Command('ViewSource',this);return false;">view plain</a><span data-mod="popu_168"> <a href="#" class="CopyToClipboard" title="copy" onclick="dp.sh.Toolbar.Command('CopyToClipboard',this);return false;">copy</a><div style="position: absolute; left: 680px; top: 418px; width: 18px; height: 18px; z-index: 99;"><embed id="ZeroClipboardMovie_1" src="http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf" loop="false" menu="false" quality="best" bgcolor="#ffffff" width="18" height="18" name="ZeroClipboardMovie_1" align="middle" allowscriptaccess="always" allowfullscreen="false" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" flashvars="id=1&width=18&height=18" wmode="transparent"></div></span><span data-mod="popu_169"> <a href="#" class="PrintSource" title="print" onclick="dp.sh.Toolbar.Command('PrintSource',this);return false;">print</a></span><a href="#" class="About" title="?" onclick="dp.sh.Toolbar.Command('About',this);return false;">?</a><span class="tracking-ad" data-mod="popu_167"><a href="https://code.csdn.net/snippets/132604" target="_blank" title="在CODE上查看代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/CODE_ico.png" width="12" height="12" alt="在CODE上查看代码片" style="position:relative;top:1px;left:2px;"></a></span><span class="tracking-ad" data-mod="popu_170"><a href="https://code.csdn.net/snippets/132604/fork" target="_blank" title="派生到我的代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/ico_fork.svg" width="12" height="12" alt="派生到我的代码片" style="position:relative;top:2px;left:2px;"></a></span></div></div><ol start="1" class="dp-j"><li class="alt"><span><span class="keyword">public</span><span> <U> Class<? </span><span class="keyword">extends</span><span> U> asSubclass(Class<U> clazz)  </span></span></li></ol><div class="save_code tracking-ad" data-mod="popu_249" style="display: none;"><a href="javascript:;" target="_blank"><img src="http://static.blog.csdn.net/images/save_snippets_01.png"></a></div></div><pre code_snippet_id="132604" snippet_file_name="blog_20131228_1_1303865" name="code" class="java" style="display: none;">public <U> Class<? extends U> asSubclass(Class<U> clazz)</pre><p></p><p><span style="font-family:Arial; font-size:14px; line-height:26px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px">这是<a href="http://lib.csdn.net/base/javase" class="replace_word" title="Java SE知识库" target="_blank" style="color:#df3434; font-weight:bold;">Java</a>.lang.Class中的一个方法，作用是将调用这个方法的class对象转换成由clazz参数所表示的class对象的某个子类。举例来说，</span><br></span></p><p><span style="font-family:Arial; font-size:14px; line-height:26px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"></span></span></p><div class="dp-highlighter bg_java"><div class="bar"><div class="tools"><b>[java]</b> <a href="#" class="ViewSource" title="view plain" onclick="dp.sh.Toolbar.Command('ViewSource',this);return false;">view plain</a><span data-mod="popu_168"> <a href="#" class="CopyToClipboard" title="copy" onclick="dp.sh.Toolbar.Command('CopyToClipboard',this);return false;">copy</a><div style="position: absolute; left: 680px; top: 555px; width: 18px; height: 18px; z-index: 99;"><embed id="ZeroClipboardMovie_2" src="http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf" loop="false" menu="false" quality="best" bgcolor="#ffffff" width="18" height="18" name="ZeroClipboardMovie_2" align="middle" allowscriptaccess="always" allowfullscreen="false" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" flashvars="id=2&width=18&height=18" wmode="transparent"></div></span><span data-mod="popu_169"> <a href="#" class="PrintSource" title="print" onclick="dp.sh.Toolbar.Command('PrintSource',this);return false;">print</a></span><a href="#" class="About" title="?" onclick="dp.sh.Toolbar.Command('About',this);return false;">?</a><span class="tracking-ad" data-mod="popu_167"><a href="https://code.csdn.net/snippets/132604" target="_blank" title="在CODE上查看代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/CODE_ico.png" width="12" height="12" alt="在CODE上查看代码片" style="position:relative;top:1px;left:2px;"></a></span><span class="tracking-ad" data-mod="popu_170"><a href="https://code.csdn.net/snippets/132604/fork" target="_blank" title="派生到我的代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/ico_fork.svg" width="12" height="12" alt="派生到我的代码片" style="position:relative;top:2px;left:2px;"></a></span></div></div><ol start="1" class="dp-j"><li class="alt"><span><span>List<String> strList = </span><span class="keyword">new</span><span> ArrayList<String>();  </span></span></li><li class=""><span>Class<? <span class="keyword">extends</span><span> List> strList_cast = strList.getClass().asSubclass(List.</span><span class="keyword">class</span><span>);  </span></span></li></ol><div class="save_code tracking-ad" data-mod="popu_249" style="display: none;"><a href="javascript:;" target="_blank"><img src="http://static.blog.csdn.net/images/save_snippets_01.png"></a></div></div><pre code_snippet_id="132604" snippet_file_name="blog_20131228_2_4826658" name="code" class="java" style="display: none;">List<String> strList = new ArrayList<String>();Class<? extends List> strList_cast = strList.getClass().asSubclass(List.class);</pre><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px">上面的代码将strList.getClass()获取的class对象转换成Class<? extends List>，这么做似乎没有什么意义，因为我们很清楚strList.getClass()获取的class对象就是ArrayList，它当然是List.class的一个子类；<span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px">但有些情况下，我们并不能确知一个class对象的类型，典型的情况是Class.forName()获取的class对象：<span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px">Class.forName()的返回类型是Class<?>，但这显然太宽泛了，假设我们需要List.class类型的class对象，但我们传递给Class.forName的参数是未知的（可能是"java.lang.String"，也可能是"java.util.ArrayList"），这时我们就可以用到asSubclass()这个方法了，如下：</span></span></span><br><p></p><p><span style="font-family:Arial; font-size:14px; line-height:26px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"></span></span></span></span></span></p><div class="dp-highlighter bg_java"><div class="bar"><div class="tools"><b>[java]</b> <a href="#" class="ViewSource" title="view plain" onclick="dp.sh.Toolbar.Command('ViewSource',this);return false;">view plain</a><span data-mod="popu_168"> <a href="#" class="CopyToClipboard" title="copy" onclick="dp.sh.Toolbar.Command('CopyToClipboard',this);return false;">copy</a><div style="position: absolute; left: 680px; top: 810px; width: 18px; height: 18px; z-index: 99;"><embed id="ZeroClipboardMovie_3" src="http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf" loop="false" menu="false" quality="best" bgcolor="#ffffff" width="18" height="18" name="ZeroClipboardMovie_3" align="middle" allowscriptaccess="always" allowfullscreen="false" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" flashvars="id=3&width=18&height=18" wmode="transparent"></div></span><span data-mod="popu_169"> <a href="#" class="PrintSource" title="print" onclick="dp.sh.Toolbar.Command('PrintSource',this);return false;">print</a></span><a href="#" class="About" title="?" onclick="dp.sh.Toolbar.Command('About',this);return false;">?</a><span class="tracking-ad" data-mod="popu_167"><a href="https://code.csdn.net/snippets/132604" target="_blank" title="在CODE上查看代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/CODE_ico.png" width="12" height="12" alt="在CODE上查看代码片" style="position:relative;top:1px;left:2px;"></a></span><span class="tracking-ad" data-mod="popu_170"><a href="https://code.csdn.net/snippets/132604/fork" target="_blank" title="派生到我的代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/ico_fork.svg" width="12" height="12" alt="派生到我的代码片" style="position:relative;top:2px;left:2px;"></a></span></div></div><ol start="1" class="dp-j"><li class="alt"><span><span>Class.forName(</span><span class="string">"xxx.xxx.xxx"</span><span>).asSubclass(List.</span><span class="keyword">class</span><span>).newInstance();  </span></span></li></ol><div class="save_code tracking-ad" data-mod="popu_249"><a href="javascript:;" target="_blank"><img src="http://static.blog.csdn.net/images/save_snippets.png"></a></div></div><pre code_snippet_id="132604" snippet_file_name="blog_20131228_3_5524769" name="code" class="java" style="display: none;">Class.forName("xxx.xxx.xxx").asSubclass(List.class).newInstance();</pre><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px">当xxx.xxx.xxx是List的子类时，正常执行，当不是List的子类时，抛出ClassCastException，这时我们可以做些别的处理；<span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px">如果我们查看Class.asSubclass()在JDK中的中的引用的话，会发现这种用法有很多。</span></span><br><br><p></p><p><span style="font-family:Arial; font-size:14px; line-height:26px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px">这里有必要说下 instanceof 关键字.</span><br></span></span></span></span></span></p><p><span style="font-family:Arial; font-size:14px; line-height:26px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px"></span></span></span></span></span></span></p><div class="dp-highlighter bg_java"><div class="bar"><div class="tools"><b>[java]</b> <a href="#" class="ViewSource" title="view plain" onclick="dp.sh.Toolbar.Command('ViewSource',this);return false;">view plain</a><span data-mod="popu_168"> <a href="#" class="CopyToClipboard" title="copy" onclick="dp.sh.Toolbar.Command('CopyToClipboard',this);return false;">copy</a><div style="position: absolute; left: 680px; top: 1012px; width: 18px; height: 18px; z-index: 99;"><embed id="ZeroClipboardMovie_4" src="http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf" loop="false" menu="false" quality="best" bgcolor="#ffffff" width="18" height="18" name="ZeroClipboardMovie_4" align="middle" allowscriptaccess="always" allowfullscreen="false" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" flashvars="id=4&width=18&height=18" wmode="transparent"></div></span><span data-mod="popu_169"> <a href="#" class="PrintSource" title="print" onclick="dp.sh.Toolbar.Command('PrintSource',this);return false;">print</a></span><a href="#" class="About" title="?" onclick="dp.sh.Toolbar.Command('About',this);return false;">?</a><span class="tracking-ad" data-mod="popu_167"><a href="https://code.csdn.net/snippets/132604" target="_blank" title="在CODE上查看代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/CODE_ico.png" width="12" height="12" alt="在CODE上查看代码片" style="position:relative;top:1px;left:2px;"></a></span><span class="tracking-ad" data-mod="popu_170"><a href="https://code.csdn.net/snippets/132604/fork" target="_blank" title="派生到我的代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/ico_fork.svg" width="12" height="12" alt="派生到我的代码片" style="position:relative;top:2px;left:2px;"></a></span></div></div><ol start="1" class="dp-j"><li class="alt"><span><span><<object reference>> </span><span class="keyword">instanceof</span><span> <<</span><span class="keyword">class</span><span>/</span><span class="keyword">interface</span><span> name>>  </span></span></li></ol><div class="save_code tracking-ad" data-mod="popu_249" style="display: none;"><a href="javascript:;" target="_blank"><img src="http://static.blog.csdn.net/images/save_snippets_01.png"></a></div></div><pre code_snippet_id="132604" snippet_file_name="blog_20131228_4_6538525" name="code" class="java" style="display: none;"><<object reference>> instanceof <<class/interface name>></pre><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px">它的用法用于判断一个Object是否是某个超类或者接口的一个子类或者实现类（不一定是直接超类或者直接接口），如下</span><br><div class="dp-highlighter bg_java"><div class="bar"><div class="tools"><b>[java]</b> <a href="#" class="ViewSource" title="view plain" onclick="dp.sh.Toolbar.Command('ViewSource',this);return false;">view plain</a><span data-mod="popu_168"> <a href="#" class="CopyToClipboard" title="copy" onclick="dp.sh.Toolbar.Command('CopyToClipboard',this);return false;">copy</a><div style="position: absolute; left: 680px; top: 1149px; width: 18px; height: 18px; z-index: 99;"><embed id="ZeroClipboardMovie_5" src="http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf" loop="false" menu="false" quality="best" bgcolor="#ffffff" width="18" height="18" name="ZeroClipboardMovie_5" align="middle" allowscriptaccess="always" allowfullscreen="false" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" flashvars="id=5&width=18&height=18" wmode="transparent"></div></span><span data-mod="popu_169"> <a href="#" class="PrintSource" title="print" onclick="dp.sh.Toolbar.Command('PrintSource',this);return false;">print</a></span><a href="#" class="About" title="?" onclick="dp.sh.Toolbar.Command('About',this);return false;">?</a><span class="tracking-ad" data-mod="popu_167"><a href="https://code.csdn.net/snippets/132604" target="_blank" title="在CODE上查看代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/CODE_ico.png" width="12" height="12" alt="在CODE上查看代码片" style="position:relative;top:1px;left:2px;"></a></span><span class="tracking-ad" data-mod="popu_170"><a href="https://code.csdn.net/snippets/132604/fork" target="_blank" title="派生到我的代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/ico_fork.svg" width="12" height="12" alt="派生到我的代码片" style="position:relative;top:2px;left:2px;"></a></span></div></div><ol start="1" class="dp-j"><li class="alt"><span><span class="keyword">interface</span><span> IFace {  </span></span></li><li class=""><span>}  </span></li><li class="alt"><span>  </span></li><li class=""><span><span class="keyword">class</span><span> IFaceImpl </span><span class="keyword">implements</span><span> IFace {  </span></span></li><li class="alt"><span>}  </span></li><li class=""><span>  </span></li><li class="alt"><span><span class="keyword">class</span><span> IFaceImplExt </span><span class="keyword">extends</span><span> IFaceImpl {  </span></span></li><li class=""><span>}  </span></li><li class="alt"><span>  </span></li><li class=""><span>System.out.println(<span class="keyword">new</span><span> IFaceImpl() </span><span class="keyword">instanceof</span><span> IFace); </span><span class="comment">// true</span><span>  </span></span></li><li class="alt"><span>System.out.println(<span class="keyword">new</span><span> IFaceImplExt() </span><span class="keyword">instanceof</span><span> IFace); </span><span class="comment">// true</span><span>  </span></span></li></ol><div class="save_code tracking-ad" data-mod="popu_249" style="display: none;"><a href="javascript:;" target="_blank"><img src="http://static.blog.csdn.net/images/save_snippets.png"></a></div></div><pre code_snippet_id="132604" snippet_file_name="blog_20131228_5_6156841" name="code" class="java" style="display: none;">interface IFace {}class IFaceImpl implements IFace {}class IFaceImplExt extends IFaceImpl {}System.out.println(new IFaceImpl() instanceof IFace); // trueSystem.out.println(new IFaceImplExt() instanceof IFace); // true</pre><span style="font-family:Helvetica,Tahoma,Arial,sans-serif; font-size:14px; line-height:25.1875px">asSubclass用于窄化未知的Class类型的范围，而instanceof用于判断一个对象引用是否是一个超类或者接口的子类/实现类，如果试图将instanceof用于Class类型的判断会引起编译错误。</span><div class="dp-highlighter bg_java"><div class="bar"><div class="tools"><b>[java]</b> <a href="#" class="ViewSource" title="view plain" onclick="dp.sh.Toolbar.Command('ViewSource',this);return false;">view plain</a><span data-mod="popu_168"> <a href="#" class="CopyToClipboard" title="copy" onclick="dp.sh.Toolbar.Command('CopyToClipboard',this);return false;">copy</a><div style="position: absolute; left: 680px; top: 1466px; width: 18px; height: 18px; z-index: 99;"><embed id="ZeroClipboardMovie_6" src="http://static.blog.csdn.net/scripts/ZeroClipboard/ZeroClipboard.swf" loop="false" menu="false" quality="best" bgcolor="#ffffff" width="18" height="18" name="ZeroClipboardMovie_6" align="middle" allowscriptaccess="always" allowfullscreen="false" type="application/x-shockwave-flash" pluginspage="http://www.macromedia.com/go/getflashplayer" flashvars="id=6&width=18&height=18" wmode="transparent"></div></span><span data-mod="popu_169"> <a href="#" class="PrintSource" title="print" onclick="dp.sh.Toolbar.Command('PrintSource',this);return false;">print</a></span><a href="#" class="About" title="?" onclick="dp.sh.Toolbar.Command('About',this);return false;">?</a><span class="tracking-ad" data-mod="popu_167"><a href="https://code.csdn.net/snippets/132604" target="_blank" title="在CODE上查看代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/CODE_ico.png" width="12" height="12" alt="在CODE上查看代码片" style="position:relative;top:1px;left:2px;"></a></span><span class="tracking-ad" data-mod="popu_170"><a href="https://code.csdn.net/snippets/132604/fork" target="_blank" title="派生到我的代码片" style="text-indent:0;"><img src="https://code.csdn.net/assets/ico_fork.svg" width="12" height="12" alt="派生到我的代码片" style="position:relative;top:2px;left:2px;"></a></span></div></div><ol start="1" class="dp-j"><li class="alt"><span><span class="comment">// both will cause compliration error</span><span>  </span></span></li><li class=""><span>System.out.println(IFaceImpl.<span class="keyword">class</span><span> </span><span class="keyword">instanceof</span><span> IFace);  </span></span></li><li class="alt"><span>System.out.println(IFaceImplExt.<span class="keyword">class</span><span> </span><span class="keyword">instanceof</span><span> IFace.</span><span class="keyword">class</span><span>);  </span></span></li></ol><div class="save_code tracking-ad" data-mod="popu_249" style="display: none;"><a href="javascript:;" target="_blank"><img src="http://static.blog.csdn.net/images/save_snippets.png"></a></div></div><pre code_snippet_id="132604" snippet_file_name="blog_20131228_6_4346698" name="code" class="java" style="display: none;">// both will cause compliration errorSystem.out.println(IFaceImpl.class instanceof IFace);System.out.println(IFaceImplExt.class instanceof IFace.class);</pre><span style="font-family:Arial; font-size:14px; line-height:26px">通常在向某些参数严格限制的API传递参数时，为了避免产生编译警告，这个方法比较有用！</span><p></p><p><span style="font-family:Arial; font-size:14px; line-height:26px">原文转载自：<a target="_blank" href="http://jasonhan-sh-hotmail-com.iteye.com/blog/1838413">http://jasonhan-sh-hotmail-com.iteye.com/blog/1838413</a></span></p>   </div>