code: public Object[] nearestNeighbour( ItemDistance itemDist ) {
          BoundablePair bp = new BoundablePair(this.getRoot(), this.getRoot(), itemDist);
          return nearestNeighbour(bp);
      }
docs: Finds the two nearest items in the tree,
  using {@link org.locationtech.jts.index.strtree.ItemDistance} as the distance metric.
  A Branch-and-Bound tree traversal algorithm is used
  to provide an efficient search.
  
  @param itemDist a distance metric applicable to the items in this tree
  @return the pair of the nearest items