code: public static final Set fromMap(Map map, Object... keys) {
          if (keys != null && map != null) {
              Set answer = new HashSet();
              for (Object key : keys) {
                  if (map.containsKey(key)) {
                      answer.add(map.get(key));
                  }
              }
              return Collections.unmodifiableSet(answer);
          }
          return Collections.EMPTY_SET;
      }
docs: Given a map and a set of keys, return a set containing the values referred-to by the keys.
  If the map is null or the keys are null, the EMPTY_SET is returned.
  If a passed key does not exist in the map, nothing is added to the set.
  However, if the key is present and maps to null, null is added to the set.