Java: Comparison method violates its general contract

Question

I got the exception from above which I know was discussed often here on SO. Anyways, the others don't tell me what's wrong with my code. These StyleAlbum are just Music albums. getAdded() returns the time when their last song was added to Android's MediaStore in milliseconds as int. This happens only on some devices!

Code:

public static List<StyleAlbum> sortAdded() {
    List<StyleAlbum> mAlbums = new ArrayList<>();
    mAlbums.addAll(Library.getAlbums());
    Collections.sort(mAlbums, new Comparator<StyleAlbum>() {
        @Override
        public int compare(StyleAlbum lhs, StyleAlbum rhs) {
            if (lhs.getAdded() > rhs.getAdded()) {
                return -1;
            } else return 1;
        }
    });
    return mAlbums;
}

Could you edit your question and post the full exception, including stack trace? — Stig Perez, Mar 01 '16 at 21:25
Possible duplicate of ["Comparison method violates its general contract!"](https://stackoverflow.com/questions/8327514/comparison-method-violates-its-general-contract) — Julian Os, Apr 07 '18 at 03:54

Raffaele · Answer 1 · 2016-03-01T21:03:30.183

1

Your comparator does not account for two items being equal, ie it never returns zero. You should use something like

return Integer.compare(lhs.getAdded(), rhs.getAdded());

otherwise the result is at least inconsistent, and depending on the algorithm it may even be completely wrong. In this case, Tim sort (the algorithm used in the JDK) is smarter enough to tell you that there's an error.

Note: I compared integers with Integer.compare instead of a subtraction (thanks Louis Wasserman) to avoid overflow errors

edited Mar 01 '16 at 21:03

answered Mar 01 '16 at 20:26

Raffaele

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Generally speaking, you should use `Integer.compare` instead of just subtracting, which can lead to overflow errors. – Louis Wasserman Mar 01 '16 at 20:59

score 0 · Answer 2 · answered Mar 01 '16 at 20:26

0

You should return 0 in the compare method if both values are the same. Especially, a comparison from an object to itself should give 0.

answered Mar 01 '16 at 20:26

J Fabian Meier

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Java: Comparison method violates its general contract

2 Answers2