PowerSetOptimized.java

private static Set<List<Item>> generateAllSubLists(List<Item> items) {
  Set<List<Item>> subLists = new HashSet<>();
  int allBitMasks = (int) Math.pow(2, Math.min(items.size(), 12)); 
  for (long i = 1; i < allBitMasks; i++) {
    List<Item> sublist = new ArrayList<>();
    for (int j = 0; j < items.size(); j++) {
      if ((i & (1 << j)) > 0) {
        sublist.add(items.get(j));
      }
    }
    subLists.add(sublist);
  }
  return subLists;
}