code: int getActiveMaps() {
      int levels = 0;
      final int iMapsLen = maps_.length;
      for (int i = 0; i < iMapsLen; i++) {
        if (maps_[i] != null) { levels++; }
      }
      return levels;
    }
docs: Returns the number of active internal maps so far.
  Only the base map is initialized in the constructor, so this method would return 1.
  As more keys are promoted up to higher level maps, the return value would grow until the
  last level HLL map is allocated.
  @return the number of active levels so far