As a person who knows Java and took a step to LibGDX, I can say most of the stuff actually relies on your Java knowledge, not your library knowledge.
To be honest, there are lots of ready-to-use codes for certain tasks. For instance, if you look for “the most efficient way to render in Java”, you’ll find some pages. Same goes for any other concepts like collision detection, path finding, score keeping etc.
Don’t take anyone’s word for it, go look for these codes. You’ll see that the codes are more or less library independent.
There are some libraries that removes the need to learn some concepts, though. For instance, people like me who use Box2D for all the physics won’t know the logic behind how a character’s direction is decided after the collision.
But I imagine you’re just talking about libraries like Libgdx, Slick2D or similar stuff.
I have to tell you, it is inevitable for anyone to look at the source code of the library he’s using. And that’s because of the mistakes he’s going to make and the errors he’s going to get. I don’t know any C/C++ but because of the errors I get, I feel the need to take a look at the source code of Box2D which is written in C/C++.
If you are curious and the library you are using is open source, you still have the chance to learn the deepest stuff.
It is told you many times in this thread that game development skills are independent from library knowledge. I’ll let you hear it from a person who is nothing more than a beginner; These two are different things. Just imagine the pseudo-code and the actual code of the game you’re trying to build. You’ll find out that it’s more logic than rendering the graphics. And, the logic is not handled by any of the libraries. It’s still you who have to move the player, shoot the bullet, make the enemy attack and make the platform move.
But for a while, you’ll have to stop learning game development to learn the library. This is interestingly a short time. Because you have to practice what you’ve learnt, and the only way to do this is to try something.
I repeat, game development is more about logic than the other things, so most of the stuff you learn will stay with you if you switch to another library. To be honest, I think libraries make it easy for you to learn the game development logic.