code: @Override
    public void setHopByHopIdentifier(long hopByHopId) {
      if (hopByHopId < 0) {
        this.hopByHopId = -hopByHopId;
        this.notMutableHopByHop = true;
      }
      else {
        if (!this.notMutableHopByHop) {
          this.hopByHopId = hopByHopId;
        }
      }
    }
docs: Inner API