This should work faster:

    private void findUnCommon{

        Set<Integer> a = new HashSet<>(Arrays.asList(1, 2, 3, 4));
        Set<Integer> b = new HashSet<>(Arrays.asList(3, 4, 5, 6));

        Set<Integer> result = new HashSet<>();
        for (Integer el: a) {
          if (!b.contains(el) {
            result.add(el);
        }
        for (Integer el: b) {
          if (!a.contains(el) {
            result.add(el);
        }
        System.out.println("Uncommon elements of set a and set b is : "
            + result);
    }