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

I got the error message in the title whilst using Flyway to run database scripts at work¹.

I find it very satisfying that sorting methods in the JDK can detect if your comparer is not obeying the contract.

There are a lot of errors that cause the contract to be violated according to [2].

Here are the rules in short as described in [3]:

• sign(compare(x,y)) == -sign(compare(y,x))
• (compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0 (transitivity)
• compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z))

Sounds like an excellent starting point for some good JUnit Tests.

References

[1] GitHub - Flyway - Issue 1249

https://github.com/flyway/flyway/issues/1249

[2] StackOverflow - “Comparison method violates its general contract!”

http://stackoverflow.com/questions/8327514/comparison-method-violates-its-general-contract

[3] Oracle Javadoc - Interface Comparator>T>

https://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html