BadAbsOfRandom.qhelp

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


<overview>
<p>
Using <code>Math.abs</code> on the result of a call to <code>Random.nextInt()</code> (or <code>Random.nextLong()</code>) is not guaranteed to
return a non-negative number. <code>Random.nextInt()</code> can return <code>Integer.MIN_VALUE</code>, which when passed to <code>Math.abs</code>
results in the same value, <code>Integer.MIN_VALUE</code>. (Because of the two's-complement representation of integers in Java, the positive equivalent of <code>Integer.MIN_VALUE</code>
cannot be represented in the same number of bits.) The case for <code>Random.nextLong()</code> is similar.
</p>

</overview>
<recommendation>
<p>
If a non-negative random integer is required, use <code>Random.nextInt(int)</code> instead, and use <code>Integer.MAX_VALUE</code> as its parameter.
The values that might be returned do not include <code>Integer.MAX_VALUE</code> itself, but this solution is likely to be sufficient for most purposes.
</p>

<p>Another solution is to increment the value of <code>Random.nextInt()</code> by one, if it is negative, before passing the result to
<code>Math.abs</code>. This solution has the advantage that <code>0</code> has the same probability as other numbers.</p>

</recommendation>
<example>

<p>In the following example, <code>mayBeNegativeInt</code>
is negative if <code>nextInt</code> returns <code>Integer.MIN_VALUE</code>.
The example shows how using the two solutions described above means that <code>positiveInt</code> is always assigned a positive number.</p>

<sample src="BadAbsOfRandom.java" />

</example>
<references>


<li>
Java API Specification:
<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Math.html#abs(int)">Math.abs(int)</a>,
<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Math.html#abs(long)">Math.abs(long)</a>,
<a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Random.html">Random</a>.
</li>
<li>
Java Language Specification:
<a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-4.html#jls-4.2.1">4.2.1 Integral Types and Values</a>.
</li>


</references>
</qhelp>