Java error: Comparison method violates its general contract

You can use the following class to pinpoint transitivity bugs in your Comparators:

/** * @author Gili Tzabari */
public final class Comparators
{ /** * Verify that a comparator is transitive. * * @param <T> the type being compared * @param comparator the comparator to test * @param elements the elements to test against * @throws AssertionError if the comparator is not transitive */ public static <T> void verifyTransitivity(Comparator<T> comparator, Collection<T> elements) { for (T first: elements) { for (T second: elements) { int result1 = comparator.compare(first, second); int result2 = comparator.compare(second, first); if (result1 != -result2) { // Uncomment the following line to step through the failed case //comparator.compare(first, second); throw new AssertionError("compare(" + first + ", " + second + ") == " + result1 + " but swapping the parameters returns " + result2); } } } for (T first: elements) { for (T second: elements) { int firstGreaterThanSecond = comparator.compare(first, second); if (firstGreaterThanSecond <= 0) continue; for (T third: elements) { int secondGreaterThanThird = comparator.compare(second, third); if (secondGreaterThanThird <= 0) continue; int firstGreaterThanThird = comparator.compare(first, third); if (firstGreaterThanThird <= 0) { // Uncomment the following line to step through the failed case //comparator.compare(first, third); throw new AssertionError("compare(" + first + ", " + second + ") > 0, " + "compare(" + second + ", " + third + ") > 0, but compare(" + first + ", " + third + ") == " + firstGreaterThanThird); } } } } } /** * Prevent construction. */ private Comparators() { }
}

Simply invoke Comparators.verifyTransitivity(myComparator, myCollection) in front of the code that fails.