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Choosing to implement the Comparable interface represents a commitment that the implementation of the compareTo() method adheres to the general contract for that method regarding how the method is to be called. Library classes such as TreeSet and TreeMap accept Comparable objects and use the associated compareTo() methods to sort the objects. However, a class that implements the compareTo() method in an unexpected way can cause undesirable results.

Wiki MarkupThe general contract for {{compareTo()}} from Java SE 6 API \[ [API 2006|AA. References#API 06]\] states that

1. The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y. (This implies that x.compareTo(y) must throw an exception if y.compareTo(x) throws an exception.)
2. The implementor must also ensure that the relation is transitive: (x.compareTo(y) > 0 && y.compareTo(z) > 0) implies x.compareTo(z) > 0.
3. Finally, the implementor must ensure that x.compareTo(y) == 0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)) for all z.
4. It is strongly recommended, but not strictly required, that (x.compareTo(y) == 0) == x.equals(y). 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.

In the foregoing description, the notation sgn(expression) designates the mathematical signum function, which is defined to return either -1, 0, or 1 depending on whether the value of the expression is negative, zero or positive.

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Bibliography

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[ [API 2006AA. References#API 06]]	[Method compareTo()	http://java.sun.com/javase/6/docs/api/java/lang/Comparable.html#compareTo(java.lang.Object)]	]]></ac:plain-text-body></ac:structured-macro>
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MET09-J. Classes that define an equals() method must also define a hashCode() method      05. Methods (MET)