"Comparison method violates its general contract!"

Your comparator is not transitive.

Let A be the parent of B, and B be the parent of C. Since A > B and B > C, then it must be the case that A > C. However, if your comparator is invoked on A and C, it would return zero, meaning A == C. This violates the contract and hence throws the exception.

It's rather nice of the library to detect this and let you know, rather than behave erratically.

One way to satisfy the transitivity requirement in compareParents() is to traverse the getParent() chain instead of only looking at the immediate ancestor.

The violation of the contract often means that the comparator is not providing the correct or consistent value when comparing objects. For example, you might want to perform a string compare and force empty strings to sort to the end with:

if ( one.length() == 0 ) {
    return 1;                   // empty string sorts last
}
if ( two.length() == 0 ) {
    return -1;                  // empty string sorts last                  
}
return one.compareToIgnoreCase( two );

But this overlooks the case where BOTH one and two are empty - and in that case, the wrong value is returned (1 instead of 0 to show a match), and the comparator reports that as a violation. It should have been written as:

if ( one.length() == 0 ) {
    if ( two.length() == 0 ) {
        return 0;               // BOth empty - so indicate
    }
    return 1;                   // empty string sorts last
}
if ( two.length() == 0 ) {
    return -1;                  // empty string sorts last                  
}
return one.compareToIgnoreCase( two );

Just because this is what I got when I Googled this error, my problem was that I had

if (value < other.value)
  return -1;
else if (value >= other.value)
  return 1;
else
  return 0;

the value >= other.value should (obviously) actually be value > other.value so that you can actually return 0 with equal objects.