General engineering questions.

Chat with management: typical questions include which IBM products you've used, what you expect from working at IBM, and so on.

Then, lots of JVM internals questions (how finalizers work, what to use instead, how GC works).

These questions felt ad-hoc, so I don't think just preparing for them will do any good.

As I mentioned above, these questions felt much like ad-hoc, and there's no point in mentioning them here. However, if you don't want to fail miserably, please consider:

You should read the JLS and books on JVM internals. They expect you to have a Master's degree in CS or equivalent knowledge, so also please familiarize yourself with these books.

This applies to general design questions. Learn transaction isolation levels, normal forms, and so on. There will be no questions you could see on lists like "100 Spring Interview Questions: Guaranteed to Get You a Job!"