I currently use Textmate for most of my editing, but am taking a Java course and am wondering if there's a better editor out there, especially one that might have some form of intellisense or active debugging.
Most, if not all of the usual suspects when it comes to Java IDEs work on OS X. I've personally used Eclipse and NetBeans on OS X - in fact I still have NetBeans installed as it's also a good environment for writing Ruby code.
Of course if you're just looking for an editor and not an IDE, you might want to try out Aquamacs...
Eclipse works okay. It has awesome features, but is somewhat buggy / crashy. It has very good "look and feel" for a Java app on OSX. Feature parity (as best as I can tell) with Windows/Linux.
I use Eclipse quite heavily on OS X. Lots of plugins, artifact downloading/caching, building, etc. Just make sure you've got the latest Java installed on OS X, and maybe verify that your system will launch Eclipse using Java 6, 64-bit, and make sure you're running Eclipse 3.5, and you should be good to go.
I leave mine running for days at a time, but your mileage may vary depending upon plugins.
If you want code completion, you are not looking for an editor but an IDE.
Eclipse, Netbeans, IntelliJ IDEA all work well on OS X. JDeveloper I have not tried but the generic version runs under OS X.
Choose the one your instructor recommends (because then he can help you getting started).
I like NetBeans and Eclipse, but Xcode comes with Mac OS X and TextWrangler is a great stand-alone programming editor.