It depends, what is your background? Java is a run-the-mill object oriented language. If you've used any OO language with dynamic dispatch, you're immediately

It depends, what is your background? Java is a run-the-mill object oriented language. If you've used any OO language with dynamic dispatch, you're immediately set 90% of the way to fluency. The tricky part is the generic part of the type system, the part with the `? extends` and `? super` annotations.