code: public Set<E> keysAbove(int countThreshold) {
      Set<E> keys = new HashSet<E>();
      for (E key : map.keySet()) {
        if (getIntCount(key) >= countThreshold) {
          keys.add(key);
        }
      }
      return keys;
    }
docs: Returns the set of keys whose counts are at or above the given threshold.
  This set may have 0 elements but will not be null.