public Set<Set<Integer>> calculate(Set<Integer> originalSet) {
    validateInputSet(originalSet);

    Set<Set<Integer>> sets = new HashSet<Set<Integer>>();
    //Base class returns the empty set.
    if (originalSet.isEmpty()) {
      sets.add(new HashSet<Integer>());
      return sets;
    }
    //Take the first element in head.
    List<Integer> list = new ArrayList<Integer>(originalSet);
    Integer head = list.get(0);
    //Generate a hash set without the first element
    Set<Integer> rest = new HashSet<Integer>(list.subList(1, list.size()));
    //Recursive call to iterate over every combination generated with the rest list.
    for (Set<Integer> set : calculate(rest)) {
      //Add every element and the head, the old set and the new set.
      Set<Integer> newSet = new HashSet<Integer>();
      newSet.add(head);
      newSet.addAll(set);
      sets.add(newSet);
      sets.add(set);
    }
    return sets;
  }
Combination of recursive and iterative approach to resolve this problem.
src/main/java/com/github/pedrovgs/problem43/CombinationOfIntegers.java