ClassCastException with Set Set Integer

Question

I'm trying to create a set that holds sets of Integers, but I'm running into a weird ClassCastException. The code works fine so long as I'm adding to an empty set; as soon as I try to add a Set<Integer> to the Set<Set<Integer>>, however, the exception is thrown. Isn't this what generics were supposed to prevent? I'm confused. (Sample code is below in case that explanation wasn't clear...)
Any ideas?
Thanks in advance,
Justin
Set<Set<Integer>> masterSet = new TreeSet<Set<Integer>>();
Set<Integer> s1 = new TreeSet<Integer>();
s1.add(1);               
masterSet.add(s1);
Set<Integer> s2 = new TreeSet<Integer>();
s2.add(2);       
masterSet.add(s2); //throws ClassCastException

Answer 1

Well, you know they say, post in haste, repent in leisure.
Your post would have been more helpful if you'd
pointed out that I have to use a HashSet rather than
a TreeSet because neither Set nor SortedSet
implements Comparable (something I hadn't realized).You can use a TreeSet if you really want to, but if you do, you will have to supply a Comparator. Of course, how exactly you would want to compare two sets escapes me.
The Java compiler doesn't give me the errorNo, it cannot, because it doesn't know that the TreeSet was compiled without a comparator.
Perhaps it would have been better to have designed two otherwise equivalent classes, one a TreeSet that could only accept mutually comparable objects, the other that required a comparator at construction-time.
Thanks so much for your help,Welcome, and sorry if I was brusque.
M.