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<div align="center" class="title">JAVA程序員必讀：基礎篇（3）語言基礎</div>
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<div align="center"><font color="#666666">編譯：ZSC/太平洋網絡學院</font></div>
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<div align="right" class="gray"><a href="mailto:tntst@pconline.com.cn">［責編:爆破手］</a></div>
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<p class="article"> <b>語言基礎</b> </p>
<p></p>
<p></p>
<p class="article"><b>3.4.3 if-else語句</b></p>
<p class="article"> if語句可以使程序根據一些條件有選擇地執行語句。舉個例子，假如你的程序根據boolean變量DEBUG的值來打印了調試信息。如果DEBUG是true，你的程序就打印出調試信息，比如變量x的數值；或者就不打印。具體的程序如下：
</p>
<p class="article">if (DEBUG) {</p>
<p class="article"> System.out.println("DEBUG: x = " + x);</p>
<p class="article">}</p>
<p class="article"> 上面是一個簡單的if語句的例子。總得說來，if語句的語法格式為： </p>
<p class="article"> if (表達式) {</p>
<p class="article"> 語句（參數）</p>
<p class="article">}</p>
<p class="article"> 如果你想在if判斷表達式為false的時候執行不同的語句，你可以使用else語句。舉個另外的例子吧，假如你的程序需要執行不同的工作，主要是根據用戶是點擊OK鍵還是擊警告窗口中的其它按鈕。你可以使用if和else語句來實現這個目的：
</p>
<p class="article"> . . .</p>
<p class="article"> // response is either OK or CANCEL depending</p>
<p class="article"> // on the button that the user pressed</p>
<p class="article"> . . .</p>
<p class="article">if (response == OK) {</p>
<p class="article"> // 在這里添加執行OK動作的代碼</p>
<p class="article"> } else {</p>
<p class="article"> // 在這里添加執行Cancel動作的代碼</p>
<p class="article"></p>
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