Collections.sort() Comparison method violates its general contract in Java [duplicate]

Answer 1

You need to find where sgn(compare(x, y)) == -sgn(compare(y, x)) doesn't hold. I suggest you use brute force to find examples.

Comparator comp = ...
for (MiniUser a : users) {
    for (MiniUser b: users) {
         if (a == b) continue;
         if (comp.compare(a, b) != -comp.compare(b, a)) {
            // print an error message with these two.
         }
    }
}