code: public void setTracking (boolean track)
      {
          if (track != _tracking) {
              _tracking = track;
              if (track) {
                  _seenKeys = new HashSet<K>();
                  _misses = _hits = 0;
  
                  // oh boy, but to properly track we need to clear the hash
                  clear();
              } else {
                  _seenKeys = null;
              }
          }
      }
docs: Turn performance tracking on/off.