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href="/developerWorks/java/">Java</a> : <a
href="/developerWorks/education/java/">教学 - 在线课程</a></font><br x="7"><img src="imagemaster/masthead.jpg" alt="构建定制的树型视图"></td><td width="*" valign="bottom" align="right"><a border="0" href="../MagnitudeTree.zip"><img alt="下载教程 zip 文件" border="0" src="../i/icon-zip.gif"></a><a href=../MagnitudeTree_eng/index.html target=_blank><img alt="英文原文" border="0" src="../i/icon-source.gif"></a></td></tr><tr><td height="2" colspan="2"><img height="2" width="1" src="../i/c.gif"></td></tr></table><TABLE border="0" cellpadding="0" cellspacing="0" width="100%"><TR><TD width="150" height="1" bgcolor="#000000" colspan="6"><IMG alt="" height="1" width="150" src="../i/c.gif"></TD></TR><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="topmain"></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="index3.html"><img alt="章节菜单" border="0" src="../i/section.gif" name="topsection"></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="topfeedback"></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-3-7.html"><img alt="上一屏" border="0" src="../i/previous.gif" name="topprevious"></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-3-9.html"><img alt="下一屏" border="0" src="../i/next.gif" name="topnext"></a></TD></TR></TABLE><table bgcolor="ffffff" cellspacing="0" cellpadding="2" border="0" height="400" width="100%"><tr valign="bottom"><a name="navskip"></a><td height="25" colspan="4"><img border="0" height="25" width="562" src="imagemaster/titlebar3.jpg" alt="3. 定制模型"></td></tr><tr><td bgcolor="ffffff" width="15">&nbsp;</td><td bgcolor="ffffff" width="12">&nbsp;</td><td valign="top" align="left" bgcolor="ffffff" width="*"><p><br x="7"><table cellspacing="0" cellpadding="0" border="0"><tr><td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>构造层次结构</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>    第 8 页（共 9 页）</nobr></font></td></tr></table><br x="7"><br x="7"></p><font size="2" face="Verdana, Arial, Helvetica"><p>
本章节中还有最后一个改进要添加到 DefaultMagnitudeNode 类中。目前，有一个应用程序正在等待显示树型视图以及数量信息，它需要：<ol>
              <li>创建合适的 "helper" 对象（实现 MagnitudeNodeHelper）</li>
              <li>逐步通过整个层次结构，以便创建每个节点的 DefaultMagnitudeNode 的实例，并想每个构造器提供 helper 对象。</li>
              <li>对根节点调用 <code style="font-family: Courier New, Courier, monospace;
font-size: 12">aggregate()</code> 方法</li>
              <li>创建 JTree，提供根节点并建立适当的定制绘制（将在下一章节中讨论）</li>
            </ol>
          </p><p>
但是，在许多情况下可以自动执行以上序列中的第 2 步：如果层次结构已经存在，并且只需要探测该层次结构，那么一般过程就可以用来完成该步骤。文件系统就是这种层次结构的一个示例，因为它提供了枚举任何目录的“子代”的机制：从根目录开始，可以构造整个层次结构。
          </p><p>
因此向 MagnitudeNodeHelper 接口添加如下方法：<pre><code style="font-family: Courier New, Courier, monospace; font-size: 12">
<span class="comment">/** Return the set of user objects for the child nodes of the node
  * represented by the supplied user object.
  * @param object the information-store-specific object for a node.
  * @return Object[] the set of user objects, or a zero length
  * array if there are no child nodes. */</span>
<span class="keyword">public</span> <span class="type">Object</span>[] <span class="function-name">getChildren</span>(<span class="type">Object</span> <span class="variable-name">object</span>);
            </code></pre>
          </p><p>
还向 DefaultMagnitudeNode 类添加了一个新方法，它是 <code style="font-family: Courier New,
Courier, monospace; font-size: 12">explore()</code>。该方法将自动创建必需的子节点，并向它们的构造器提供相同的 helper 对象。应注意 <code style="font-family:
Courier New, Courier, monospace; font-size: 12">explore()</code> 只能添加一次子节点，因此使用一个布尔数据成员以记录是否已创建了子节点。
            <pre><code style="font-family: Courier New, Courier, monospace; font-size: 12">
<span class="comment">// Indicates whether the children of this node have been constructed.</span>
<span class="keyword">private</span> <span class="type">boolean</span> <span class="variable-name">fExplored</span> = <span class="constant">false</span>;

<span class="keyword">public</span> <span class="type">void</span> <span class="function-name">explore</span>()
{
  <span class="comment">// If we haven't already done this...</span>
  <span class="keyword">if</span> (!<span class="variable-name">fExplored</span>)
  {
    <span class="comment">// Get the set of child nodes (if any).</span>
    <span class="keyword">final</span> <span class="type">Object</span>[] <span class="variable-name">children</span> = <span class="variable-name">fHelper</span>.<span class="function-name">getChildren</span>(<span class="function-name">getUserObject</span>());

    <span class="comment">// For each one...</span>
    <span class="keyword">for</span> (<span class="type">int</span> <span class="variable-name">child</span> = <span class="constant">0</span>; <span class="variable-name">child</span> &lt; <span class="variable-name">children</span>.<span class="variable-name">length</span>; <span class="variable-name">child</span>++)
    {
      <span class="comment">// Construct a new magnitude node, using the same helper
      // instance, and add it to our self.</span>
      <span class="function-name">add</span>(<span class="keyword">new</span> <span class="type">DefaultMagnitudeNode</span>(<span class="variable-name">children</span>[<span class="variable-name">child</span>], <span class="variable-name">fHelper</span>));
    }

    <span class="comment">// Remember that we've done this.</span>
    <span class="variable-name">fExplored</span> = <span class="constant">true</span>;
  }
}
            </code></pre>
          </p><p>
通过将 <code style="font-family: Courier New,
Courier, monospace; font-size: 12">explore()</code> 调用插到 DefaultMagnitudeNode
上 <code style="font-family: Courier New, Courier,
monospace; font-size: 12">aggregate()</code> 方法的第一行，可以确保及时创建了子节点，因而可以正确计算合计大小。只有在用到子节点时才将子节点填充到层次结构中，这种做法有时称作“懒惰”填充，因为它确保了不会做不必要的工作：如果永远不需要子节点，那么就永远不会创建它们。但是，请注意，<code style="font-family: Courier New, Courier,
monospace; font-size: 12">aggregate()</code> 的递归性质意味着对根节点调用一次 <code style="font-family: Courier New, Courier,
monospace; font-size: 12">aggregate()</code> 就会创建节点的整个层次结构并计算所有合计大小。
          </p><p>
目前，有一个应用程序正在等待显示树型视图以及数量信息，它需要：<ol>
              <li>创建合适的 "helper" 对象（实现 MagnitudeNodeHelper）</li>
              <li>创建 DefaultMagnitudeNode 的单个实例作为根节点，将 helper 对象提供给构造器</li>
              <li>对根节点调用 <code style="font-family: Courier New, Courier, monospace;
font-size: 12">aggregate()</code> 方法</li>
              <li>创建 JTree，提供根节点并建立适当的定制绘制（将在下一章节中讨论）</li>
            </ol>
          </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="index3.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-3-7.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-3-9.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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