When I started out programming in Java (and programming altogether) I tried making a mod for Minecraft, despite being partially succesful in making throwing knives it made me realise I had a lot to learn about Java. So I read "Head First Java" from O'Reilly, it almost felt like putting on glasses for the first time
Really nice, non boring-wall-of-text book, quite engaging. Plus it made me feel ashamed of the code I had written for my MC mod.
Moral of the story; Read a Java programming book. Most don't require you to use an IDE or even tell you to do everything in a console, but you're free to ignore that. Actually do the exercises, they help a lot in understanding and retaining that understanding. Do it beside making your game and you'll see that discovering new possibilities in programming helps you think of stuff for your game. Plus, actively thinking about putting fresh learned knowledge to use makes reading & learning easier. And when you're done you have a handy book to fall back on when you need info about (almost) anything java.
Side notes about Head First Java:
Starts out with pure basics, how to write a barebones java program, compile it and run it. Without IDE. When you've done that you can apply that knowledge to any IDE because you know the terminology.
It also covers a bunch of Java's built in libraries.
Already know about a subject (like operators) just skip it!