This is a prototype of an Inflatatble Transpose Tree as described in this blog post:

The Inflatable Transpose Tree: a highly compact data structure

To use a Transpose Tree, you extend TransposeTree and add additional field-arrays to hold whatever would have been in fields, had you used ordinary key and value objects. The subclass must implement a compare method that allows TransposeTree to set up the tree references without ever manipulating the keys directly. This way, a key can be any primitive, or any combination of primitives. In other words, compound keys are straightforward. (And hey—the keys can even be objects, if you want.)

For more information, see the blog post.