MultiplyRemainder.qhelp

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


<overview>
<p>
Using the remainder operator <code>%</code> with the multiplication operator may not give you the result that
you expect unless you use parentheses. This is because the
remainder operator has the same precedence as the multiplication operator, and the operators are
left-associative.</p>

</overview>
<recommendation>

<p>
When you use the remainder operator with the multiplication operator, ensure that the expression is 
evaluated as you expect. If necessary, add parentheses.</p>

</recommendation>
<example>

<p>Consider a time in milliseconds,
represented by <code>t</code>. To calculate the number of milliseconds remaining after the time has been converted to whole minutes,
you might write <code>t % 60 * 1000</code>. However, this is equal to <code>(t % 60) * 1000</code>, which gives the wrong result.
Instead, the expression should be <code>t % (60 * 1000)</code>.
</p>

</example>
<references>


<li>
  J. Bloch and N. Gafter, <em>Java Puzzlers: Traps, Pitfalls, and Corner Cases</em>, Puzzle 35. Addison-Wesley, 2005.
</li>
<li>
  The Java Tutorials: <a href="https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html">Operators</a>.
</li>


</references>
</qhelp>