This is a simple implementation of a 2-3-4 tree in Java as a Set, which is a Be-Tree of order 4.
The structure implements the Simple Set interface (SSet.java) from Dr. Pat Morin's wonderful Open Data Structures book.
DefaultComparator.java and TwoFourNodeException.java are only helpers. The bulk of the datastructure is found in TwoFourTree.java.
I've tried to document the code as best as possible and be clear in every choice I made.
TODO:
rewrite the recursive delete method as an iterative one.