What is the compare contract?

Question

My java code threw the following exception:

java.lang.IllegalArgumentException: Comparison method violates its general contract!

I researched the message on StackOverflow and found several questions that posted their poorly written compare methods, but I was wondering where the contract is written and what exactly does it state regarding what the compare method must do.

I think you should post some of your code in order to get some real help. — Olle Söderström, Apr 02 '13 at 13:19
Probable duplicate [here](http://stackoverflow.com/q/11441666/823393). — OldCurmudgeon, Apr 02 '13 at 13:19
From looking at other questions, it seems it would raise that exception if your compare method violates transitivity. — Sotirios Delimanolis, Apr 02 '13 at 13:19
You should have researched the Javadoc for `java.util.Compare` first. — user207421, Apr 03 '13 at 01:48

Ajinkya · Accepted Answer · 2013-04-05T05:12:22.763

Looks like Java Doc have it

The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x)) for all x and y. (This implies that compare(x, y) must throw an exception if and only if compare(y, x) throws an exception.)

The implementor must also ensure that the relation is transitive: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.

Finally, the implementor must ensure that compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.

It is generally the case, but not strictly required that (compare(x, y)==0) == (x.equals(y)). Generally speaking, any comparator that violates this condition should clearly indicate this fact. The recommended language is "Note: this comparator imposes orderings that are inconsistent with equals."

WHo would believe it - API docs for `Comparator` hidden away in the API docs for `Comparator`. — Tom Hawtin - tackline, Apr 02 '13 at 13:27
@TomHawtin-tackline: I think only John Skeet could know that. — Ajinkya, Apr 02 '13 at 13:30

What is the compare contract?

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