Martin provided a patch on list to make the QuadTreeNode, and Quadrant classes package private. This makes sense to me.
From Martin Desruisseaux:
On a related question about QuadTree, I wonder if the QuadTreeNode class could be considered as internal mechanic? QuadTreeNode is used publicly only be QuadTree.getRoot(), itself used only internally be QuadTreeReader/QuadTreeWriter. Unless they are actually used by other projects outside SIS, could we turn the following into package-private elements?