Sun's Java compiler and libraries aren't in the Debian archives because they aren't
free, even though they can be downloaded at no cost from
Sun's Java website. Sun does provide this software for linux, but if you want to use free software, look for information about "gcj". It should be in the Debian archive. I don't think its libraries provide as much GUI support as Sun's, but I imagine its developers are working on that (quite a monumental task, really).