The link doesn't work?
A single class seems a bit odd...
I have programmed in java for several years now. Having started several times to create games but something always comes up that makes me stop (usually i get fustrated or system corrupts). I have use Swing and AWT for a while but it was always bare basic stuff I want to dive deeper. I have done file handling but not very good with it (dont fully understand the reader/writer stuff beyond text file usage)
As for the rest I know of them but have very little understandung of them.
As for photo editting they are not my friends... I know MSPaint after that the others confuse me.
I strongly advise not to read the code for the Java4K entries. A lot of it is heavily optimised and hard to understand. If you didn't know, Java4K is where developers try to make games that are <4 kilobytes, which usually requires a lot of crude optimization.
If you're serious about game development then using just the Java API isn't a good choice as it lacks the performance that most games need. However, for beginners it's a great place to start. Try searching on Youtube for some simple Java game development tutorials, then once you've set up a basic window and set up a basic game loop I suggest searching through Google for articles/tutorials/forum posts regarding the topics I listed above (game loops, aabb collision, etc). Don't just watch/read articles and tutorials though; maybe try creating a pong or snake game. Once you're at the stage of feeling comfortable with those principles of game development, I challenge you to create a very simple platform game using coloured rectangles for tiles (load the map from a text file) and a coloured rectangle for the player.
After that is where it gets more difficult. If you don't care too much about performance for your game, you could continue to use swing & AWT for it, however if you want to create faster, better games then you should consider looking into some game engines (I recommend LibGDX for Java). Game engines can be hard to learn; they're much more advanced and use OpenGL for rendering. You could also consider learning how to use OpenGL (for Java learn LWJGL) which can be confusing and is a vast, vast topic. Learning OpenGL will give you a much better understanding of how things work and you'll even be able to create your own game engine if you want.
As a whole, game development is a huge and confusing topic, but the number one thing is that when learning, don't give up
. Good luck.