InformationLoss.qhelp

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<qhelp>


<overview>
<p>Compound assignment statements of the form <code>x += y</code> or
<code>x *= y</code> perform an implicit narrowing conversion
if the type of <code>x</code> is narrower than the type of <code>y</code>.
For example, <code>x += y</code> is equivalent to <code>x = (T)(x + y)</code>,
where <code>T</code> is the type of <code>x</code>.
This can result in information loss and numeric errors such as overflows.
</p>

</overview>
<recommendation>

<p>Ensure that the type of the left-hand side of the compound assignment statement
is at least as wide as the type of the right-hand side.</p>

</recommendation>
<example>

<p>If <code>x</code> is of type <code>short</code> and <code>y</code> is of type <code>int</code>,
the expression <code>x + y</code> is of type <code>int</code>. However, the expression <code>x += y</code>
 is equivalent to <code>x = (short) (x + y)</code>. The expression <code>x + y</code> is cast to the type of
the left-hand side of the assignment: <code>short</code>, possibly leading to information loss.</p>

<p>To avoid implicitly narrowing the type of <code>x + y</code>, change the type of <code>x</code> to
<code>int</code>. Then the types of <code>x</code> and <code>x + y</code> are both <code>int</code> and there is no
 need for an implicit cast.</p>

</example>
<references>


<li>
  J. Bloch and N. Gafter, <em>Java Puzzlers: Traps, Pitfalls, and Corner Cases</em>, Puzzle 9. Addison-Wesley, 2005.
</li>
<li>
  The Java Language Specification:
  <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.26.2">Compound Assignment Operators</a>,
  <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1.3">Narrowing Primitive Conversion</a>.
</li>
<li>The CERT Oracle Secure Coding Standard for Java:
  <a href="https://www.securecoding.cert.org/confluence/display/java/NUM00-J.+Detect+or+prevent+integer+overflow">NUM00-J. Detect or prevent integer overflow</a>.</li>


</references>
</qhelp>