CompareIdenticalValues.qhelp

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


<overview>
<p>If two identical expressions are compared (that is, checked for equality or
inequality), this is typically an
indication of a mistake, because the Boolean value of the comparison
is always the same. Often, it indicates that the wrong qualifier has been
used on a field access.</p>

<p>An exception applies to inequality (<code>!=</code>) and equality (<code>==</code>) tests
of a floating point variable with itself: the special floating point value <code>NaN</code> ("not-a-number")
is the only value that is not considered to be equal to itself. Thus, the test <code>x != x</code> where
<code>x</code> is a <code>float</code> or <code>double</code> variable is equivalent to checking whether
<code>x</code> is <code>NaN</code>, and similarly for <code>x == x</code>.</p>

</overview>
<recommendation>

<p>It is never good practice to compare a value with itself. If you require constant
behavior, use the Boolean literals <code>true</code> and
<code>false</code>, rather than encoding them obscurely as <code>1 == 1</code>
or similar.</p>

<p>If an inequality test (using <code>!=</code>) of a floating point variable with itself is intentional, it
should be replaced by <code>Double.isNaN(...)</code> or <code>Float.isNaN(...)</code> for readability.
Similarly, if an equality test (using <code>==</code>) of a floating point variable with itself is intentional, it
should be replaced by <code>!Double.isNaN(...)</code> or <code>!Float.isNaN(...)</code>.</p>

</recommendation>
<example>

<p>In the example below, the original version of <code>Customer</code> compares <code>id</code> with
<code>id</code>, which always returns <code>true</code>. The corrected version of <code>Customer</code>
includes the missing qualifier <code>o</code> in the comparison of <code>id</code> with <code>o.id</code>.</p>

<sample src="CompareIdenticalValues.java" />

</example>
<references>


<li>
Help - Eclipse Platform:
<a href="https://help.eclipse.org/2020-12/advanced/content.jsp?topic=/org.eclipse.jdt.doc.user/reference/preferences/java/compiler/ref-preferences-errors-warnings.htm">Java Compiler Errors/Warnings Preferences</a>.
</li>
<li>
Java Language Specification:
<a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-15.html#jls-15.21.1">15.21.1. Numerical Equality Operators</a>.
</li>


</references>
</qhelp>