Well first I'd like to warn you that I don't think anyone here does much more than hobby game development. That said...
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Do you want to learn C++ first, or 3d graphics?
If you want to learn 3d graphics, you need to pick either OpenGL or DirectX. It really doesn't matter which one you pick, and you can always go and learn the other later, but trying to learn both at the same time isn't going to fly well.
If you want to learn C++, try rewriting your Java programs in it.
I would not recommend trying to learn a new language AND a new API simultaneously.
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I'm not suggesting that you don't take classes on the subject, but if you like reading and programming in your free time and are willing to dedicate a couple of hours everyday to doing just that then I'm willing to bet you could teach yourself most of what you need to know.
Just speculation but if you had some impressive games to show and $(GAMESTUDIO) asks you what language they're written in and you answer with Java, I'm guessing that's probably not going to somehow diminish the quality of your work.
The advice I've gotten about trying to get a job at a game studio is that they're more looking for someone who is experienced than someone who is technically qualified. If you've written a bunch of games in Java and they see that, they'll probably be willing to give you a little time on the job to pickup C++. Honestly once you learn one programming language really well picking up another one
isn't that hard. The only thing that really changes is the paradigm and the syntax, and Java and C++ both are mainly OO programming languages with pretty similar "curly brace" syntax so personally I can't say that should be much of a jump.
At the end of the day it's your decision though.