Rules for compareTo()

With all objects, compareTo() works the way number comparisons work in ordinary arithmetic. Here are a few rules. Most of these are fairly clear if you think about numbers. Say that A, B, and C are Integers.

If A.compareTo(B) > 0 then B.compareTo(A) < 0.

If A.compareTo(B) > 0 and B.compareTo(C) > 0 then A.compareTo(C) > 0.

If A.compareTo(B) == 0 then A.compareTo(Z) and B.compareTo(Z) should give the same result, no matter what Z is.

The classes that come with Java follow these rules. If you write a class that implements Comparable, you need to follow these rules. This is not hard to do because most sensible compareTo() methods will do this naturally.