Is there a way to check if two Collections contain the same elements, independent of order?

Question

I've been looking for a method that operates like Arrays.equals(a1, a2), but ignoring the element order. I haven't been able to find it in either Google Collections (something like Iterables.elementsEqual(), but that does account for ordering) and JUnit (assertEquals() obviously just calls equals() on the Collection, which depends on the Collection implementation, and that's not what I want) It would be best if such a method would take Iterables, but I'm also fine with simply taking Collections Such a method would of course take into account any duplicate elements in the collection (so it can't simply test for containsAll()).

Note that I'm not asking how to implement such a thing, I was just wondering if any of the standard Collections libraries have it.

Answer 1

If you want to ignore order, then how about testing sets for equality?

new HashSet(c1).equals(new HashSet(c2))

Answer 2

This is three method calls and uses Google Collections, but is possibly as simple as it gets:

HashMultiset.create(c1).equals(HashMultiset.create(c2));

Creating the temporary Multisets may appear wasteful, but to compare the collections efficiently you need to index them somehow.

Answer 3

Note that I'm not asking how to implement such a thing, I was just wondering if any of the standard Collections libraries have it.

Well, the answer is no, AFAIK.

The most likely reason is that it is difficult to define an intuitive and consistent semantic for the 3 methods equals(Collection, Collection), equals(Set, Set) and equals(List, List). Another reason is that most applications/algorithms use either List or Set rather than Collection, and hence there is not much call for comparing Collections in the way you suggest.

Answer 4

Apache commons-collections has CollectionUtils#isEqualCollection:

Returns true iff the given Collections contain exactly the same elements with exactly the same cardinality.

That is, iff the cardinality of e in a is equal to the cardinality of e in b, for each element e in a or b.

Which is, I think, exactly what you're after.