difference method

Set<E> difference (Set<Object> other)

Returns a new set with the elements of this that are not in other.

That is, the returned set contains all the elements of this Set that are not elements of other according to other.contains.

Implementation 

Set<E> difference(Set<Object> other) {
  Set<E> result = toSet();
  for (E element in this) {
    if (other.contains(element)) result.remove(element);
  }
  return result;
}