CovariantCompareTo.qhelp

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<qhelp>


<overview>
<p>Classes that implement <code>Comparable&lt;T&gt;</code> and define 
a <code>compareTo</code> method whose parameter type is not <code>T</code>
<i>overload</i> the <code>compareTo</code> method instead of <i>overriding</i> it. This may not be intended. 
</p>

</overview>
<example>

<p>In the following example, the call to <code>compareTo</code> on line 17 calls the method
defined in class <code>Super</code>, instead of the method defined in class <code>Sub</code>, because 
the type of <code>a</code> and <code>b</code> is <code>Super</code>. This may not be the method that
the programmer intended.</p>

<sample src="CovariantCompareTo.java" />

</example>
<recommendation>

<p>To <i>override</i> the <code>Comparable&lt;T&gt;.compareTo</code> method,   
the parameter of <code>compareTo</code> must have type <code>T</code>.
</p>

<p>In the example above, this means that the type of the parameter of <code>Sub.compareTo</code> should be changed
to <code>Super</code>.</p>

</recommendation>
<references>


    <li>J. Bloch, <em>Effective Java (second edition)</em>, Item 12. 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>


</references>
</qhelp>