java.lang.ClassCastException：com.MyComp.model.Image无法转换为java.lang.Comparable

Question

I try to sort a List with the following code:

        List<Image> sortedImages = new LinkedList<Image>(inputImages);
        Collections.sort(sortedImages, imageScoreComparator);

and

class ImageScoreComparator implements Comparator<Image> {

    ImageScoreCalculator imageScoreCalculator;

    ImageScoreComparator() {
        this(new ImageScoreCalculator(Calendar.getInstance()));
    }

    @VisibleForTesting
    ImageScoreComparator(ImageScoreCalculator imageScoreCalculator) {
        this.imageScoreCalculator = imageScoreCalculator;
    }

    @Override
    public int compare(Image image1, Image image2) {
        if (image2.isPromoted() && !image1.isPromoted()) {
            return 1;
        } else if (image1.isPromoted() && !image2.isPromoted()) {
            return -1;
        } else {
            return Double.compare(imageScoreCalculator.getScoreForImage(image1), imageScoreCalculator.getScoreForImage(image2));
        }
    }
}

But I get this error:

java.lang.ClassCastException: com.MyComp.model.Image cannot be cast to java.lang.Comparable

This is how I init imageScoreComparator

imageScoreComparator = mock(ImageScoreComparator.class);
        when(imageScoreComparator.compare(image1, image2)).thenReturn(0);

Answer 1

It sounds like your imageScoreComparator variable is null. If the Comparator passed as the second parameter to Collections.sort is null, it does not treat that as an error; rather it will try to cast each item in the list to Comparable , sorting them according to their natural sort order, which won't work for these objects.

Make sure you're initializing the imageScoreComparator variable to something non-null.