code: public void setSentinel (int sentinel)
      {
          if (_sentinel == sentinel) {
              return;
          }
          if (contains(sentinel)) {
              throw new IllegalArgumentException("Set contains sentinel value " + sentinel);
          }
          // replace every instance of the old sentinel with the new
          for (int ii = 0; ii < _buckets.length; ii++) {
              if (_buckets[ii] == _sentinel) {
                  _buckets[ii] = sentinel;
              }
          }
          _sentinel = sentinel;
      }
docs: Sets the sentinel value, which cannot itself be stored in the set because it is used
  internally to represent an unused location.
  
  @exception IllegalArgumentException if the set currently contains the requested sentinel.