InconsistentEqualsHashCode.qhelp

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


<overview>
<p>A class that overrides only one of <code>equals</code> and <code>hashCode</code>
is likely to violate the contract of the <code>hashCode</code> method. The contract
requires that <code>hashCode</code> gives the same integer result for any two equal objects.
Not enforcing this property may cause unexpected results when storing and
retrieving objects of such a class in a hashing data structure.
</p>

</overview>
<recommendation>

<p>Usually, both methods should be overridden to ensure that they are consistent.
</p>

</recommendation>
<example>

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

<sample src="InconsistentEqualsHashCode.java" />

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

<sample src="InconsistentEqualsHashCodeGood.java" />

</example>
<references>


<li>
  J. Bloch, <em>Effective Java (second edition)</em>, Item 9. Addison-Wesley, 2008.
</li>
<li>
  Java API Specification:
  <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>,
  <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#hashCode()">Object.hashCode</a>.
</li>
<li>
  IBM developerWorks: <a href="https://www.ibm.com/developerworks/java/library/j-jtp05273/index.html">Java theory and practice: Hashing it out</a>.
</li>
<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>


</references>
</qhelp>