'compareTo()' is incompatible with 'equals()'

If a class implements Comparable, compareTo() method must return zero if and only if equals() returns true for the same non-null argument (and vice versa). Violating this rule may cause unexpected behavior of the objects.

class Product implements Comparable
    ...
    public int compareTo(Object o) {
        if (o == null)
            throw new NullPointerException();
        if (this.equals(o))
            return 0;
        ...    
    }
}

See also:

• ‘equals()’ is overriden while ‘hashCode()’ is not

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