docs: public String serializeToString() {
      final StringBuilder sb = new StringBuilder();
      //start the string with parameters of the sketch
      final int serVer = SER_VER;                 //0
      final int famID = Family.FREQUENCY.getID(); //1
      final int lgMaxMapSz = lgMaxMapSize;        //2
      final int flags = (hashMap.getNumActive() == 0) ? EMPTY_FLAG_MASK : 0; //3
      final String fmt = "%d,%d,%d,%d,%d,%d,";
      final String s =
          String.format(fmt, serVer, famID, lgMaxMapSz, flags, streamWeight, offset);
      sb.append(s);
      sb.append(hashMap.serializeToString()); //numActive, curMaplen, key[i], value[i], ...
      // maxMapCap, samplesize are deterministic functions of maxMapSize,
      //  so we don't need them in the serialization
      return sb.toString();
    }