Wonko gave you the solution: OpenJDK. Thats the best you are going to get; The only Java Sun has released for ARM, IIRC, won't work with Maemo 5/N900.
OpenJDK is just about as good anyway, and being an OSS app, can/will be compiled -for- the N900's processor, and not just a generic ARM processor(So it can take advantage of all features).

-Rob