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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>断言 </b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 2 页（共5 页）</nobr></font></td>
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<font size="2" face="Verdana, Arial, Helvetica">
<p>断言是一个包含布尔表达式的语句，在执行这个语句时假定该表达式为 true。如果表达式计算为 false，那么系统会报告一个 <code>AssertionError</code>。它用于调试目的： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
  assert(a &gt; 0); // throws an AssertionError if a &lt;= 0
</code>
</pre>
<p>断言可以有两种形式： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">    
  assert Expression1 ; 
  assert Expression1 : Expression2 ; 
</code>
</pre>
<p>
						
<code>Expression1</code> 应该总是产生一个布尔值。 </p>
<p>
						
<code>Expression2</code> 可以是得出一个值的任意表达式。这个值用于生成显示更多调试信息的 <code>String</code> 消息。例如： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
  class AssertionTest
  {
    int a ;
    public void func() 
    {
      assert (a &lt; 0): "a is positive" ;
      // Remaining code
    }
  }  
</code>
</pre>
<p>在上面的代码中，如果 <code>a</code> 为负，则断言表达式为 true。如果 <code>a</code> 为正，则抛出一个 <code>AssertionError</code> 和一条消息&ldquo;a is positive&rdquo;。 </p>
<p>断言在默认情况下是禁用的。要在编译时启用断言，需要使用 <code>source 1.4</code> 标记： </p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">	
  javac -source 1.4 Test.java
</code>
</pre>
<p>要在运行时启用断言，可使用 <code>-enableassertions</code> 或者 <code>-ea</code> 标记。 </p>
<p>要在运行时选择禁用断言，可使用 <code>-da</code> 或者 <code>-disableassertions</code> 标记。 </p>
<p>要系统类中启用断言，可使用 <code>-esa</code> 或者 <code>-dsa</code> 标记。还可以在包的基础上启用或者禁用断言。 </p>
<p>
						
<b>断言的恰当和不恰当使用 </b>
						
<br>
可以在预计正常情况下不会到达的任何位置上放置断言。断言可以用于验证传递给私有方法的参数。不过，断言不应该用于验证传递给公有方法的参数，因为不管是否启用了断言，公有方法都必须检查其参数。不过，既可以在公有方法中，也可以在非公有方法中利用断言测试后置条件。另外，断言不应该以任何方式改变程序的状态。 </p>
<p>有关断言的更多内容请参阅 <a href="j-scjp-11-2.html">参考资料 </a>
					
</p>
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