ConstantExpAppearsNonConstant.qhelp

<!DOCTYPE qhelp PUBLIC
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  "qhelp.dtd">
<qhelp>


<overview>
<p>
Some expressions always evaluate to the same result, no matter what their subexpressions are:
</p>

<ul>
<li><code>x * 0</code> always evaluates to <code>0</code>.
</li>
<li><code>x % 1</code> always evaluates to <code>0</code>.
</li>
<li><code>x &lt;&lt; 64</code> (when <code>x</code> is of type <code>long</code>) always evaluates to <code>0</code>.
</li>
<li><code>x &lt;&lt; 32</code> (when <code>x</code> is not of type <code>long</code>) always evaluates to <code>0</code>.
</li>
<li><code>x &amp; 0</code> always evaluates to <code>0</code>.
</li>
<li><code>x || true</code> always evaluates to <code>true</code>.
</li>
<li><code>x &amp;&amp; false</code> always evaluates to <code>false</code>.
</li>
</ul>

<p>
Whenever <code>x</code> is not constant, such an expression is often a mistake.
</p>

</overview>
<recommendation>

<p>
If the expression is supposed to evaluate to the same result every time it is executed, consider replacing the entire expression with its result.
</p>

</recommendation>
<example>
<p>
The following method tries to determine whether <code>x</code> is even by checking whether <code>x % 1 == 0</code>.
</p>

<sample src="ConstantExpAppearsNonConstant.java" />

<p>
However, <code>x % 1 == 0</code> is always true when <code>x</code> is
an integer. The correct check is <code>x % 2 == 0</code>.
</p>

<sample src="ConstantExpAppearsNonConstantGood.java" />

</example>
<references>


<li>
  The Java Language Specification:
  <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.17.1">Multiplication operator *</a>, <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.17.3">Remainder operator %</a>, <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.19">Left shift operator &lt;&lt;</a>, <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.22.1">Bitwise AND operator &amp;</a>, <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.23">Conditional-and operator &amp;&amp;</a> and <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.24">Conditional-or operator ||</a>.
</li>


</references>
</qhelp>