Getting the difference between two sets
Try this
test2.removeAll(test1);Removes from this set all of its elements that are contained in the specified collection (optional operation). If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.
If you use Guava (former Google Collections) library there is a solution:
SetView<Number> difference = com.google.common.collect.Sets.difference(test2, test1);The returned SetView is a Set, it is a live representation you can either make immutable or copy to another set. test1 and test2 are left intact.