I might suggest a few things:
1: In the README, you shouldn't be worried about if it runs on mac. This is java! It will run on pretty much everything (Even microwaves use java (no, microwaves will not use your game)). Unless you are working with a native library, but... baby steps.
2: The control seems a bit lagged.
3: Organize and clean up your code a bit. 'Keying' does not make much sense as it is your character and your key event listener. How about a 'Player' and a 'World' and a 'Keying.' It is just more organized. Trust me. Organization is key for coding. It will pay off in debugging.
Hope this helps!
Good luck on your coding endeavors!
My formula for finding your amount of java learned: your geuss * 0.1You can never stop learning! (unless your guess is >= 1000%)