<p>
According to the Java <code>Comparable.compareTo(T o)</code> documentation:
</p>

<blockquote>
It is strongly recommended, but not strictly required that <code>(x.compareTo(y)==0) == (x.equals(y))</code>.
Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact.
The recommended language is "Note: this class has a natural ordering that is inconsistent with equals."
</blockquote>

<p>
If this rule is violated, weird and unpredictable failures can occur.
For example, in Java 5 the <code>PriorityQueue.remove()</code> method relied on <code>compareTo()</code>, but since Java 6 it relies on <code>equals()</code>.
</p>

<h2>Noncompliant Code Example</h2>

<pre>
public class Foo implements Comparable&lt;Foo&gt; {
  @Override
  public int compareTo(Foo foo) { /* ... */ }      // Noncompliant as the equals(Object obj) method is not overridden
}
</pre>

<h2>Compliant Solution</h2>

<pre>
public class Foo implements Comparable&lt;Foo&gt; {
  @Override
  public int compareTo(Foo foo) { /* ... */ }      // Compliant

  @Override
  public boolean equals(Object obj) { /* ... */ }
}
</pre>