Fatal Exception: java.lang.IllegalArgumentException: Comparison method violates its general contract

Question

I know there many similar questions and I have received big help by reading answers to those questions, however I am not able to see how is my client facing this problem. And there is only one client who is facing this problem.

I have a List, and I am sorting that list using Comparator interface. Does any of you see problem with the following code?

    private static class BiologySamplesComparator implements Comparator<BiologySample>, Serializable {
        @Override
        public int compare(BiologySample left, BiologySample right) {
            if (left == right || (left != null && right != null && left.getSampleDateTime() == right.getSampleDateTime())) {
                return 0;
            }

            if (left == null || left.getSampleDateTime() == null) {
                return 1;
            }

            if (right == null || right.getSampleDateTime() == null) {
                return -1;
            }

            return right.getSampleDateTime().compareTo(left.getSampleDateTime());
        }
    }

And this how I am calling this function

Collections.sort(biologySamples, new BiologySamplesComparator());

I know that the main problem in this kind of scenario is Transitivity. However I couldn't figure what is violating that rule.

This how getSampleDateTime() is returning date Fri Apr 09 17:00:00 PDT 2021

---

Update This is how I was able to fix my problem. I hope this helps, I was stuck for so long on this problem.

    private static class BiologySamplesComparator implements Comparator<BiologySample>, Serializable {
        @Override
        public int compare(BiologySample left, BiologySample right) {
            if (left == null) {
                if (right == null) {
                    return 0;
                } else {
                    return 1;
                }
            } else if (right == null) {
                return -1;
            } else if (left == right) {
                return 0;
            }
            if (left.getSampleDateTime() == null) {
                if (right.getSampleDateTime() == null) {
                    return 0;
                } else {
                    return 1;
                }
            } else if (right.getSampleDateTime() == null) {
                return -1;
            } else if (left.getSampleDateTime() == right.getSampleDateTime()) {
                return 0;
            }

            return right.getSampleDateTime().compareTo(left.getSampleDateTime());
        }
    }

Answer 1

I was missing some conditional for some cases and this is how I was able to solve my problem.

    private static class BiologySamplesComparator implements Comparator<BiologySample>, Serializable {
        @Override
        public int compare(BiologySample left, BiologySample right) {
            if (left == null) {
                if (right == null) {
                    return 0;
                } else {
                    return 1;
                }
            } else if (right == null) {
                return -1;
            } else if (left == right) {
                return 0;
            }
            if (left.getSampleDateTime() == null) {
                if (right.getSampleDateTime() == null) {
                    return 0;
                } else {
                    return 1;
                }
            } else if (right.getSampleDateTime() == null) {
                return -1;
            } else if (left.getSampleDateTime() == right.getSampleDateTime()) {
                return 0;
            }

            return right.getSampleDateTime().compareTo(left.getSampleDateTime());
        }
    }

courtesy of Why does my compare methd throw IllegalArgumentException sometimes?

Answer 2

You have a possible inconsistency when comparing a null "sample" to a non-null sample with a null timestamp.

Sample a = null;
Sample b = new Sample(null);

bsc.compare(a, b); // -> 1, a > b
bsc.compare(b, a); // -> 1, b > a

First, you should replace the Date in your sample class with Instant if at all possible, and then make your life simpler by saying this:

public static final Comparator<Sample> ORDER_BY_TIMESTAMP =
  Comparator.nullsLast(Comparator.comparing(
      Sample::getDateTime,
      Comparator.nullsLast(Comparator.naturalOrder())
  );

If you can rule out null values, even simpler:

Comparator.comparing(Sample::getDateTime);