Farbrausch, is one of the best - if not *the* best demo group (ever?). Their demos are well thought out, technically superior, and deliver in 64k what others use 5+ MB to do. Having done several 64k demos, and doing articles on the sound engine (kb of farbrausch -> http://kebby.org/
-> .articles -> 'The Workings of fr-08's Sound System') and programs to package textures and executable, has led me to believe that though the code is *extremely* complex it is also very well documented and maintainable.
That said, they have access to asm and c which is made to interact with hardware, where Java is encapsulated in a VM and made for larger abstract programs, and there's not a lot to do about it, untill the support libraries provide access to more stuff (like dx does).
Ofcourse, on j2me, we're doing the same at work. We have at best 64 kB file size and 210 kB memory available - and yet we have a multiplayer (including lounge, chat, finding each other etc.) platform and games on top of that.
For more Farbrausch:http://www.farb-rausch.com/
try fr08, which started it all, fr11, fr19 (still my favorite) and of course fr30 - candytron. There are many more 64kB demos - but these are some of the best IMO.