What is the cost of ContainsAll in Java?

Question

I discovered containsAll() (a List interface method) during some coding today, and it looks pretty slick. Does anyone know how much this costs in

Answer 1

Use the source, Luke :)

Edit: As Bozho pointed out, you're asking about List.containsAll() which overrides Collection.containsAll(). The following ramblings are mainly concerned with the latter:

Most Collection classes will use the implementation of containsAll from AbstractCollection, which does it like this:

public boolean containsAll(Collection c) {
    for (Object e : c)
        if (!contains(e))
            return false;
    return true;
}

There's no guarantee that some implementation does it completely differently, though -- which could result in either better or worse runtime behavior.

The above implementation of containsAll will be at least O(n) where n is the number of items in the Collection parameter you pass in, plus whatever time contains takes:

• For a HashSet/HashMap this might be O(1) (best case, no collisions), so the overall runtime of containsAll would still be O(n)
• For an ArrayList, contains will take O(m) where m is the number items in the list (not the parameter), so the overall time for containsAll would be O(n*m)