That really depends on the school you are attending.
There are really 2 kinds Computer Science schools, those that focus on windows (boo! hiss!) and those that focus on UNIX. Will you find one that focuses on the mac? nah, but yours might treat Win and UNIX equally.
If your school does push unix in its CS programs, you'll be extremely
happy using Mac OS X
. I wouldn't even consider Mac OS 9 and previous an option for doing CompSci work.
In short, you get the mac interface on top of an unholy marriage between the 3 major BSDs (FreeBSD, NetBSD, and OpenBSD), combined with a bunch of (very cool) apple additions.
The development tools (which are provided as a free download from apple) are pretty much the standard open source unix tools (gcc, gdb, and other good stuff), plus a bunch of optional GUI tools (Project Builder and Interface Builder, most notably) to make your life easier.
In your specific case, Mac OS X has Java support that puts Windows to shame (WinXP doesn't even include Java by default. Mac OS X contains full support for Java 1.3, and there are some major enhancments coming with OS X 10.1, due late september). You have your choice between using Apple's tools for java development, or paying a bit to use Borland's jBuilder, which is (as far as I know) the standard software package for java development. I don't do Java myself, but I intend to try soon.
If your school is primarily windows, then perhaps a windows machine would suit you best, although OS X is a dream for CompSci students, because there are just so many options, and most of them are free. If you need to do some development on Windows, but not a lot, then you should look into buying VirtualPC, a PC emulator. It's not fast enough to play games in, but it'll run MS VisualC++ just fine.
Your best bet is not to buy until you actually go to school, and ask the instructors if the CompSci department is unix or windows based. If it's unix, go with Mac OS X, and if it's windows, buy a windows machine. I personally plan on buying an iBook to run Mac OS X on at the end of September.
If you have any more specific questions, you can mail me at firstname.lastname@example.org