CovariantEquals.qhelp

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


<overview>
<p>Classes that define an <code>equals</code> method whose parameter type is not <code>Object</code>
<i>overload</i> the <code>Object.equals</code> method instead of <i>overriding</i> it. This may not be intended.</p> 

</overview>
<recommendation>

<p>To <i>override</i> the <code>Object.equals</code> method,   
the parameter of the <code>equals</code> method must have type <code>Object</code>.</p>

</recommendation>
<example>

<p>In the following example, the definition of class <code>BadPoint</code> does not override the <code>Object.equals</code> method.
This means that <code>p.equals(q)</code> resolves to the default definition of <code>Object.equals</code> and
returns <code>false</code>. Class <code>GoodPoint</code> correctly overrides <code>Object.equals</code>,
so that <code>r.equals(s)</code> returns <code>true</code>.</p>

<sample src="CovariantEquals.java" />

</example>
<references>


    <li>J. Bloch, <em>Effective Java (second edition)</em>, Item 8. Addison-Wesley, 2008.</li>
    <li>Java Language Specification:
  <a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-8.html#jls-8.4.8.1">Overriding (by Instance Methods)</a>,
  <a href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-8.html#jls-8.4.9">Overloading</a>.</li>
    <li>The Java Tutorials: <a href="https://docs.oracle.com/javase/tutorial/java/IandI/override.html">Overriding and Hiding Methods</a>.</li>


<!-- A related issue is examined by the query <a href="MissingInstanceOfInEquals.html">Equals Method Does Not Inspect Argument Type</a>. -->

</references>
</qhelp>