BadCheckOdd.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>


<overview>
<p>Avoid using <code>x % 2 == 1</code> or <code>x % 2 > 0</code>
to check whether a number <code>x</code> is odd, or 
<code>x % 2 != 1</code> to check whether it is even.
Such code does not work for negative numbers. 
For example, <code>-5 % 2</code> equals <code>-1</code>, not <code>1</code>.
</p>

</overview>
<recommendation>

<p>
Consider using <code>x % 2 != 0</code> to check for odd and <code>x % 2 == 0</code> to check for even.
</p>

</recommendation>
<example>
<p>-9 is an odd number but this example does not detect it as one. This is because <code>-9 % 2
</code> is -1, not 1.</p>
<sample src="BadCheckOdd.java" />

<p>It would be better to check if the number is even and then invert that check.</p>
<sample src="BadCheckOddGood.java" />

</example>
<references>


<li>
  J. Bloch and N. Gafter, <em>Java Puzzlers: Traps, Pitfalls, and Corner Cases</em>, Puzzle 1. Addison-Wesley, 2005.
</li>
<li>
  Java Language Specification:
  <a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.17.3">Remainder Operator %</a>.
</li>


</references>
</qhelp>