code: public long getMemoryUsageBytes() {
      long total = 0;
      for (int i = 0; i < maps_.length; i++) {
        if (maps_[i] != null) {
          total += maps_[i].getMemoryUsageBytes();
        }
      }
      return total;
    }
docs: Returns total bytes used by all internal maps
  @return total bytes used by all internal maps