Make sure that, everything you plan to do in your game, you can already do. You'll probably plan to make an amazing GUI, make sure you know how to use Swing. You'll probably plan to use inheritance and interfaces, so know how to use them in isolation. (If you don't plan to use them, you should probably keep learning Java. Sorry if that was blunt, but it is very true.) Everything you want to do in your game, you should be able to do already. Learning things as you go along will only make the game 10 times more painful to code and take you 10 times longer to create. If you want to know how much Java you should know, read the Oracle tutorials, http://docs.oracle.com/javase/tutorial/java/index.html . If you know EVERYTHING in there, and you have used it in ISOLATION, you are set to start making your game. Please don't try to learn Java as you go along. I've tried it, and it ended in heart break when I was told "Judging by the quality of questions you've submitted in the past hour, I'd recommend the book, Baby Steps In Java." After that, I read another 3 Java books, then started making a game. I havent had to ask any questions since.
Also, add core elements of your game. If it's a FPS, make sure that you create the shooting and moving before you create the main menu. After all, you dont want the ability to shoot be be limited by the code main menu.