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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>除去對象實例</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 9 頁（共11 頁）</nobr></font></td>
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<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
Collection employees = architectureDepartment.getEmployees();
pm.retrieveAll(employees);
// remove the first element
PersonImpl removedEmployee = null;
if(!employees.isEmpty()) {
         Iterator employIt = employees.iterator();
         removedEmployee = (PersonImpl)employIt.next();
         employees.remove(removedEmployee);
         architectureDepartment.setEmployees(employees);
         pm.deletePersistent(removedEmployee);
}
</code>
</pre>
<p>在上述代碼中，我們首先從特殊體系結構部門上檢索雇員集合。接著，我們調用持久性管理器上的 <code>retrieveAll</code> 方法。這樣將整個集合裝入到內存中（如果表過大，那么這個操作會產生問題）。</p>
<p>要成功除去一名雇員，我們必須：</p>
<ul>
<li>從部門內的集合上除去這名雇員</li>
<li>從數據庫上刪除這名雇員的持久性表示</li>
</ul>
<p>如果我們只是刪除了表上的持久性表示，當部門的雇員集合試圖檢索它時，會收到一個空指針異常。另一方面，如果我們只是從部門中的雇員集合上除去這名雇員，最終將產生一名不屬于任何部門的雇員，而且在不斷膨脹的數據庫記錄集合中該雇員可能被當作&ldquo;無用信息&rdquo;。</p>
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