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<a name="navskip"></a><td height="25" colspan="4"><img alt="4.垃圾收集 " src="imagemaster/titlebar4.jpg" border="0" height="25" width="562"></td>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>垃圾收集的符合條件 </b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr> 第 2 頁（共5 頁）</nobr></font></td>
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</p>
<font size="2" face="Verdana, Arial, Helvetica">
<p>當一個對象沒有活動線程可訪問它時，它就符合垃圾收集的條件。 </p>
<p>一個對象可以有不同的方式符合垃圾收集的條件： </p>
<ul>
<li>如果指向這個對象的引用變量設置為 null，那么只要沒有其他引用指向它，這個對象就符合垃圾收集的條件。 <br>
<br>
</li>
<li>如果指向這個對象的引用變量改為指向別的對象，那么只要沒有其他引用指向它，這個對象就符合垃圾收集的條件。 <br>
<br>
</li>
<li>在方法本地創建的對象當方法返回時就符合垃圾收集的條件，除非它被導出到方法以外(即它被返回或者作為異常拋出)。 <br>
<br>
</li>
<li>彼此引用的對象，如果沒有活動線程訪問它們中的任意一個，那么它們就符合垃圾收集的條件。 </li>
</ul>
<p>考慮以下例子： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public class TestGC
{
public static void main(String [] args)
{
Object o1 = new Integer(3); // Line 1
Object o2 = new String("Tutorial"); // Line 2
o1 = o2; // Line 3
o2 = null; // Line 4
// Rest of the code here
}
}
</code>
</pre>
<p>在這個例子中，一開始被引用 <code>o1</code> 引用的 <code>Integer</code> 對象在第 3 行后變得符合垃圾收集的條件。這是因為現在 <code>o1</code> 引用 <code>String</code> 對象。即使 <code>o2</code> 現在引用 <code>String</code> 對象也不符合垃圾收集的條件，因為 <code>o1</code> 引用了它。 </p>
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