java.lang.IllegalArgumentException: Comparison method violates its general contract [duplicate]

Your compare() method is not transitive. If A == B and B == C, then A must be equal to C.

Now consider this case:

For A, B, and C, suppose the containsKey() method return these results:

• childMap.containsKey(A.getID()) returns true
• childMap.containsKey(B.getID()) returns false
• childMap.containsKey(C.getID()) returns true

Also, consider orders for A.getId() != B.getId().

So,

1. A and B would return 0, as outer if condition will be false => A == B
2. B and C would return 0, as outer if condition will be false => B == C

But, A and C, could return -1, or 1, based on your test inside the if block. So, A != C. This violates the transitivity principle.

I think, you should add some condition inside your else block, that performs check similar to how you do in if block.