Advanced Java topics

Hey guys.

My name is David and I have been developing games in Libgdx for six months and I feel like my coding skills are improving every day but because this is a Java Forum, I would like to know, for your opinion, what else can improve my coding skills?

Also, I would like to learn new advanced topics in Java that may help improving my progamming a little bit. What do you think are good subjects to learn?