I recommend finding out what the class is using, or if there's a tool recommendation made by the instructor. Then if that tool runs on the Mac, use it. Only after determining the class tool (or its absence) should you look at Mac-specific tools.
Are there platform specific Java IDEs? That would seem pretty silly, given that all of the core things an IDE would have (javac, jdb) are themselves written in Java, which means your IDE would at least need to be capable of interfacing with Java, which generally means some of your code would be written in Java, which would probably lead to you writing the entire thing in Java (and, as far as I'm aware, every Java IDE is written entirely in Java.)
Anyways, it is true that Eclipse is the industry standard. The reason why is because when people ask this question, they are told Eclipse is the industry standard. Most people say that because it's the only thing they've ever used (which means they aren't very good people to ask in the first place, as they have nothing to compare it against.) Other people say it because most of their coworkers use it.
Basically, Eclipse is to Java IDEs as Windows is to computer OSs. God knows how it became standard in the first place, and it remains the standard because it is the standard.
You may have noticed that I didn't say what the "other people" that I mentioned previously actually themselves use. They (and I) use IntelliJ, because it's actually a good Java IDE. Whereas Eclipse is a flakey piece of crap known for stalling and crashing for no apparent reason, or for making retarded code completion suggestions, or for failing to remove error markers after problems are resolved,
IntelliJ works. Flawlessly. All the time. It's quick, too. And smarter. And it has Sublime style multiselection. It's so much better that they actually have a really expensive pro version of it. But you don't need that for learning Java. The standard free version is just fine.
Here you go:
http://www.jetbrains.com/idea/
You're welcome.
