Well, your post is kind of unclear, however I'm guessing you want to understand Java in terms of game development.
Java is great. C++ fanboys love to talk about how it's slower because it's an interpreted language and how crappy it is, but in reality, it's great. There is a abundance of game development libraries, and developing in it is really easy. With Java you can get a good game together relatively quickly (Look at Prelude Of The Chambered, it was developed by the creator of Minecraft and made in forty-eight hours.). Also, why do you want to know if it'll help you learn C++. The answer is yes, however why does it matter to you? If you know Java, C++ will be easier however chances are you won't learn it in some time. Short Answer:
1) Java is fine performance wise
2) Java will make transitioning to C++ easier, however Dynamic Memory will probably confuse you.
3) It can produce great games. It's not the language, it's the Programmer that makes the game.
4) Everyone is different. Learning Java takes a long time. To actually learn a language and understand how it actually works you need to work with it for a long time and go through an insane amount of trial and error.

See my signature, depending on the type (2D or 3D) you're interested in, there's a good Java lib. Although, I'd still highly recommend you first get your core Java skills & general programming experience before doing anything serious with those, particularly 3D.