DefineEqualsWhenAddingFields.qhelp

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


<overview>
<p>If a class overrides the default implementation of equality defined by
the <code>Object.equals</code> method, and a subclass of that class
declares additional fields to the ones that it inherits, the results of <code>equals</code>
may be wrong, unless that subclass also redefines <code>equals</code>.
</p>

</overview>
<recommendation>

<p>See if the subclass should provide its own implementation of <code>equals</code> to take into
account the additional fields that it declares.</p>

<p>If the subclass cannot provide its own implementation of <code>equals</code> because
the inherited <code>equals</code> method is <code>final</code>, consider replacing inheritance
by composition; instead of class <code>B</code> extending class <code>A</code>, class <code>B</code>
could define a field of type <code>A</code>.</p>

</recommendation>
<example>

<p>In the following example, rectangles <code>r1</code> and <code>r2</code> are calculated to be equal,
even though they have different dimensions. This is because the class <code>Rectangle</code> does
not override <code>Square.equals</code>, so it uses a test for equality that is only applicable to squares,
not rectangles. (Note that, in practice, the example should also include an implementation of
<code>hashCode</code>.)</p>

<sample src="DefineEqualsWhenAddingFields.java" />

<p>To get the correct result, you must override <code>Square.equals</code> in class <code>Rectangle</code>.
</p>

</example>
<references>


<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>.
</li>


</references>
</qhelp>