FastTreeMap is not compatible with TreeMap

FastTreeMap extends TreeMap in a way that doesn't preserve the superclass behavior. For example, the following code prints 'null', but I would expect it to print '1=1', which is what TreeMap does:

TreeMap map = new FastTreeMap();
//TreeMap map = new TreeMap();

map.put(1, "1");
map.put(3, "3");

Entry e = map.floorEntry(2);
System.out.println(e);

This behavior is surprising and can hit you every time a reference of type TreeMap refers to an instance of FastTreeMap. A subclass instance used through a superclass interface shouldn't change the visible behavior of its superclass.

The reason for this problem seems to be that FastTreeMap both extends TreeMap and delegates to a TreeMap via the 'map' field. I.e., there are two map instances for a single FastTreeMap instance.