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构建定制的树型视图

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在<a href="MagnitudeTree-3-1.html">定制模型</a>中，已经介绍了
MagnitudeNodeHelper 接口。该接口定义了对 "helper" 对象的要求，具体如下：<ol>
              <li>返回对象的文本表示法。</li>
              <li>返回对象是否是容器。</li>
              <li>返回层次结构中对象的大小。</li>
              <li>返回对象的子对象。</li>
            </ol>
          </p><p>
在 Java 编程语言中，文件系统中的文件和目录由 <code style="font-family: Courier New, Courier, monospace;
font-size: 12">java.io</code> 包中 File 类的实例表示。File 类的方法可用于提供以下的必要信息：<ol>
              <li>可以用 <code
style="font-family: Courier New, Courier, monospace; font-size:
12">getName()</code> 方法获取的文件或目录的名称。</li>
              <li>File 类表示文件和目录，而 <code style="font-family: Courier New, Courier, monospace; font-size:
12">isDirectory()</code> 方法可以区别它们。</li>
              <li>用 <code
style="font-family: Courier New, Courier, monospace; font-size:
12">length()</code> 方法获取的文件或目录的大小。请注意，对于目录（<code style="font-family: Courier New, Courier,
monospace; font-size: 12">length()</code> 方法通常会返回 0），这个大小不包括任何包含的文件或目录的大小。</li>
              <li>可以使用 <code style="font-family: Courier New, Courier, monospace;
font-size: 12">listFiles()</code> 方法获取的目录中包含的文件。</li>
            </ol>
          </p><p>
这部分教程的下一个定制组件是 <b>FileMagnitudeNodeHelper</b> 类，它实现了 MagnitudeNodeHelper 接口，具体如下：<pre><code style="font-family: Courier New, Courier, monospace; font-size: 12">
<span class="keyword">public</span> <span class="keyword">class</span> <span class="type">FileMagnitudeNodeHelper</span>
       <span class="keyword">implements</span> <span class="type">MagnitudeNodeHelper</span>
{
  <span class="comment">/** Return the magnitude of the supplied File. */</span>
  <span class="keyword">public</span> <span class="type">long</span> <span class="function-name">getSize</span>(<span class="keyword">final</span> <span class="type">Object</span> <span class="variable-name">object</span>)
  {
    <span class="keyword">return</span> ((<span class="type">File</span>)<span class="variable-name">object</span>).<span class="function-name">length</span>();
  }

  <span class="comment">/** Return whether the supplied File represents a directory. */</span>
  <span class="keyword">public</span> <span class="type">boolean</span> <span class="function-name">isContainer</span>(<span class="keyword">final</span> <span class="type">Object</span> <span class="variable-name">object</span>)
  {
    <span class="keyword">return</span> (((<span class="type">File</span>)<span class="variable-name">object</span>).<span class="function-name">isDirectory</span>());
  }

  <span class="comment">/** Return the name of the supplied File. */</span>
  <span class="keyword">public</span> <span class="type">String</span> <span class="function-name">toString</span>(<span class="keyword">final</span> <span class="type">Object</span> <span class="variable-name">object</span>)
  {
    <span class="keyword">return</span> ((<span class="type">File</span>)<span class="variable-name">object</span>).<span class="function-name">getName</span>();
  }

  <span class="comment">/** Return an array of Files that are the children of the supplied
    * File. */</span>
  <span class="keyword">public</span> <span class="type">Object</span>[] <span class="function-name">getChildren</span>(<span class="keyword">final</span> <span class="type">Object</span> <span class="variable-name">object</span>)
  {
    <span class="keyword">final</span> <span class="type">File</span>[] <span class="variable-name">children</span> = ((<span class="type">File</span>)<span class="variable-name">object</span>).<span class="function-name">listFiles</span>();
    <span class="keyword">return</span> (<span class="variable-name">children</span> != <span class="constant">null</span>) ? <span class="variable-name">children</span> : <span class="keyword">new</span> <span class="type">File</span>[<span class="constant">0</span>];
  }
}
            </code></pre>
          </p><p>
请注意，实现 helper 类比较简单：所有艰苦的工作都由定制的节点对象完成。这就易于将数量树型视图类应用于其它层次结构以及显示与其相关的数量信息，但又不必投入很多精力使其生效。
          </p><br x="7"></font></td></tr></table><TABLE border="0" cellpadding="0" cellspacing="0" width="100%"><TR><TD background="../i/sw-gold.gif"><a border="0" href="index.html" onMouseOver="iOver('topmain'); iOver('bottommain'); self.status=mainblurb; return true;" onMouseOut="iOut('topmain'); iOut('bottommain'); self.status=''; return true;"><img alt="主菜单" border="0" src="../i/main.gif" name="bottommain"></a></TD><TD background="../i/sw-gold.gif"><a border="0" onMouseOver="iOver('topsection'); iOver('bottomsection'); self.status=sectionblurb; return true;" onMouseOut="iOut('topsection'); iOut('bottomsection'); self.status=''; return true;" href="index5.html"><img alt="章节菜单" border="0" src="../i/section.gif" name="bottomsection"></a></TD><TD background="../i/sw-gold.gif"><a border="0" onMouseOver="iOver('topfeedback'); iOver('bottomfeedback'); self.status=feedbackblurb; return true;" onMouseOut="iOut('topfeedback'); iOut('bottomfeedback'); self.status=''; return true;" href="MagnitudeTree-7-4.html"><img alt="提交反馈意见" border="0" src="../i/feedback.gif" name="bottomfeedback"></a></TD><TD width="100%" background="../i/sw-gold.gif"><img src="../i/c.gif"></TD><TD background="../i/sw-gold.gif"><a border="0" onMouseOver="iOver('topprevious'); iOver('bottomprevious'); self.status=previousblurb; return true;" onMouseOut="iOut('topprevious'); iOut('bottomprevious'); self.status=''; return true;" href="MagnitudeTree-5-1.html"><img alt="上一屏" border="0" src="../i/previous.gif" name="bottomprevious"></a></TD><TD background="../i/sw-gold.gif"><a border="0" onMouseOver="iOver('topnext'); iOver('bottomnext'); self.status=nextblurb; return true;" onMouseOut="iOut('topnext'); iOut('bottomnext'); self.status=''; return true;" href="MagnitudeTree-5-3.html"><img alt="下一屏" border="0" src="../i/next.gif" name="bottomnext"></a></TD></TR><TR><TD width="150" height="1" bgcolor="#000000" colspan="6"><IMG alt="" height="1" width="150" src="../i/c.gif"></TD></TR></TABLE><TABLE cellpadding="0" cellspacing="0" border="0"><TR><TD width="182"><TABLE cellpadding="0" cellspacing="0" border="0" width="182">
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