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<a name="navskip"></a><td height="25" colspan="4"><img alt="3.流程控制、斷言和異常處理 " src="imagemaster/titlebar3.jpg" border="0" height="25" width="562"></td>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>異常處理 </b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr> 第 3 頁（共5 頁）</nobr></font></td>
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<p>異常是 Java 對象，異常類是從 <code>java.lang.Throwable</code>派生出來的。 </p>
<p>拋出異常用于表示出現了異常情況，如訪問超出數組大小的索引。異?？梢允?checked 或者是 unchecked。 <code>RuntimeException</code> 和 <code>Error</code> 的子類稱為 <i>unchecked 異常 </i>。所有從 <code>Exception</code> 繼承的其他類都是 <i>checked 異常 </i>。所有可能拋出 checked 異常的方法都必須用 <code>throws</code> 關鍵詞(有關關鍵詞的更多內容請參閱 <a href="j-scjp-5-3.html">Java 關鍵詞和標識符 </a>) 來聲明異常，或者用 <code>try/catch</code> 塊來處理異常。 </p>
<p>
<b>
<code>try</code>/<code>catch</code> 塊 </b>
<br>
可能拋出異常的代碼被包圍在 <code>try</code> 塊中?？梢蕴峁┮粋€或者多個 <code>catch</code> 子句來處理不同的異常類型： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
try
{
// code that might throw exceptions
}
catch(Exception e)
{
//code to handle the exception
}
</code>
</pre>
<p>更特定的異常類型應該首先處理。例如，我們必須從特定到最一般的順序排列 <code>catch</code> 塊，否則就會出現編譯時錯誤。 </p>
<p>例如，假設 <code>MyException</code> 繼承自 <code>Exception</code>:</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
try
{
throw new MyException();
}
catch(Exception e)
{
System.out.println("Exception");
}
catch(MyException e)
{
System.out.println("MyException");
}
</code>
</pre>
<p>這里編譯器會報怨無法達到 <code>MyException</code> 的 <code>catch</code> 子句，因為所有拋出的異常都被第一個一般性的 <code>catch</code> 子句捕獲了。 </p>
<p>
<b>
<code>finally</code> 塊 </b>
<br>
還可以提供 <code>finally</code> 塊來執行任何清除操作。如果拋出一個異常，那么相匹配的 <code>catch</code> 子句就會執行，然后控制就會進入 <code>finally</code> 塊（如果有的話）。 <code>finally</code> 塊的語法如下： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
try
{
// code that throws exceptions
}
catch(Exception e)
{
// handle exception
}
finally
{
// cleanup code
}
</code>
</pre>
<p>
<code>finally</code> 塊即使在沒有異常拋出的情況下也會執行。惟一的例外是當 <code>try</code> 或者 <code>catch</code> 塊調用 <code>System.exit()</code> 時。一個 <code>try</code> 塊應當有一個 <code>catch</code> 塊或者有一個 <code>finally</code> 塊，但是不一定要同時有這兩者。 </p>
<p>
<b>聲明和拋出異常 </b>
<br>
要在代碼中顯式地拋出異常，可使用 <code>throw</code> 關鍵詞。 </p>
<p>方法可以拋出的 checked 異常必須用 <code>throws</code> 關鍵詞(有關關鍵詞的更多內容請參閱 <a href="j-scjp-5-3.html">Java 關鍵詞和標識符 </a>)聲明。例如： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
void f() throws MyException
{
if(x > y) throw new MyException(); // MyException is a subclass of Exception
}
</code>
</pre>
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