InconsistentCompareTo.qhelp

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


<overview>
<p>A class that overrides <code>compareTo</code> but not <code>equals</code>
may not implement a natural ordering that is consistent with <code>equals</code>.
</p>

</overview>
<recommendation>

<p>Although this consistency is not strictly required by the <code>compareTo</code> contract, 
usually both methods should be overridden to ensure that they are consistent, that is, that
<code>x.compareTo(y)==0</code> is <code>true</code> if and only if <code>x.equals(y)</code>
is <code>true</code>, for any non-null <code>x</code> and <code>y</code>.</p>

</recommendation>
<example>

<p>In the following example, the class <code>InconsistentCompareTo</code> overrides 
<code>compareTo</code> but not <code>equals</code>.</p>

<sample src="InconsistentCompareTo.java" />

<p>In the following example, the class <code>InconsistentCompareToFix</code> overrides both
<code>compareTo</code> and <code>equals</code>.</p>

<sample src="InconsistentCompareToGood.java" />

<p>
If you require a natural ordering that is inconsistent with <code>equals</code>, you should document it clearly.
</p>

</example>
<references>


<li>
  J. Bloch, <em>Effective Java (second edition)</em>, Item 12. Addison-Wesley, 2008.
</li>
<li>
  Java API Specification: 
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Comparable.html#compareTo(T)">Comparable.compareTo</a>,
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)">Object.equals</a>.
</li>


</references>
</qhelp>