code: public Object[] nearestNeighbour( STRtreeJGT tree, ItemDistance itemDist ) {
          BoundablePair bp = new BoundablePair(this.getRoot(), tree.getRoot(), itemDist);
          return nearestNeighbour(bp);
      }
docs: Finds the two nearest items from this tree
  and another 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.
  The result value is a pair of items,
  the first from this tree and the second
  from the argument tree.
  
  @param tree another tree
  @param itemDist a distance metric applicable to the items in the trees
  @return the pair of the nearest items, one from each tree