Learn the tools that you'll use to develop software. Becoming proficient in the tooling will help you greatly with the actual development.

Some things you can do for example:

- Using linux? Learn to use the CLI

- Learn to use git/svn/... (whichever you use at work)

- Get comfortable with your IDE (shortcuts, customize to your liking, ...)

- Learn to use a profiler (JProfile, ..)

Those are some pretty general things but they'll help you in any job where you write code.

The tools are (imo) just as important as knowing the language you use.