Thursday, 28 April 2016

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.


[1] GitHub - Flyway - Issue 1249
[2] StackOverflow - “Comparison method violates its general contract!”
[3] Oracle Javadoc - Interface Comparator>T>

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