How can I solve 'Comparison method violates its general contract!' problem in Kotlin?

Question

I got this error to compare two contourAreas in openCV project.

java.lang.IllegalArgumentException: Comparison method violates its general contract!

And I've tried:

        Collections.sort(contours) { o1, o2 ->
            val area1 = Imgproc.contourArea(o1)
            val area2 = Imgproc.contourArea(o2)
            (area2 - area1).toInt()
        }

        contours.sortWith(Comparator { o1, o2 ->
            val area1 = Imgproc.contourArea(o1)
            val area2 = Imgproc.contourArea(o2)
            (area2 - area1).toInt()
        })

And I thought those above are dubious so I tried:

        contours.sortWith(Comparator { o1, o2 ->
            val area1 = Imgproc.contourArea(o1)
            val area2 = Imgproc.contourArea(o2)
            val res = (area2 - area1)

            if (res > 0) {
                return@Comparator 1
            } else if(res < 0){
                return@Comparator -1
            }
            return@Comparator 0
        })

But none of them worked properly and my app crushed. How can I solve this problem?

Does this answer your question? ["Comparison method violates its general contract!"](https://stackoverflow.com/questions/8327514/comparison-method-violates-its-general-contract) — Pawel, Mar 30 '20 at 01:20
From my zero knowledge of OpenCV, that comparator _looks_ transitive...does `Imgproc.contourArea` not always return the same value for the same object? — Ryan M, Mar 30 '20 at 04:05
[mre] is required. -- you misunderstand. `contourArea` always returns the same value for the same contour... however, it will return **negative** values if the contour is a **hole** (reversed order of points) — Christoph Rackwitz, Sep 15 '22 at 21:00

How can I solve 'Comparison method violates its general contract!' problem in Kotlin?

0 Answers0