Java Collections.Sort Comparison method violates its general contract

Question

I'm getting an error in my code since yesterday and I'm not sure why! I googled a lot and found this Stack-Post.

I use this funtion to sort users by name and create date. This function has worked since 2 years and now I'm getting an error for one user and I don't know what has changed. I tried to check my data if there is anything corrupt but couldn't find anything wrong.

After reading zhe Stack-Post I still didn't understand fully what's the problem or what has changed in my project.

public void sortUsers(List<Users> sortList) {
    Collections.sort(sortList, new Comparator<Users>() {
        public int compare(Users user1, Users user2) {
            Integer comp = 0;
            comp = user1.getUsername().compareTo(user2.getUsername());
            if (comp == 0) {
                comp = user1.getCreateDate().before(user2.getCreateDate()) ? -1 : 1;
            }

            return comp;
        }
    });
}

`sortList.sort(Comparator.comparing(Users::getUsername).thenComparing(Users::getCreateDate));` This one liner is enough. — Mushif Ali Nawaz, Oct 14 '19 at 08:00

Andy Turner · Accepted Answer · 2019-10-14T15:59:45.920

8

comp = user1.getCreateDate().before(user2.getCreateDate()) ? -1 : 1;

This line can never return zero, in the case that the dates are equal. This means that it's not anti-symmetric (i.e. sgn(a.compareTo(b)) = -sgn(b.compareTo(a))).

The exact fix depends on what class getCreateDate() returns, but it could be something like:

comp = user1.getCreateDate().compareTo(user2.getCreateDate());

However, an easier way to do construct a compliant comparator would be:

Comparator.comparing(Users::getUsername).thenComparing(Users::getCreateDate)

or similar.

edited Oct 14 '19 at 15:59

answered Oct 14 '19 at 07:56

Andy Turner

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@Andy Thank you for your answer I already tried the first solutions which works perfetly. I just don't understand why it just happend since yesterday. If I check my data there has nothing changed in the past few days and there are multiple users with same name and date but they don't produce this error. – Mihawk Oct 14 '19 at 08:05
1

@Mihawk bear in mind that "it didn't used to throw an error" isn't the same as "it used to be correct": errors like this (and fail-fast iterators etc) are provided on a best-effort basis. There is no /requirement/ for implementations of `sort` to throw for particular inputs (it's [optional](https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#sort-java.util.List-)). – Andy Turner Oct 14 '19 at 08:09

Java Collections.Sort Comparison method violates its general contract

1 Answers1