Which class that looks like MapWritable can be used as the Key in a Hadoop MapReduce program?

Question

I'm writing a program in Java using Hadoop 0.18. Now I would like to use a Map (HashMap/TreeMap/...) kind of datastructure as the Key in my Map Reduce processing. I haven't yet been able to find an official Hadoop class that is essentially an MapWritableComparable (i.e. implements Map, Writable and Comparable).

So for my first tests I currently have a simple class that only does what I need right now.

Have I overlooked it or does such a class really not (yet) exist?