I made a Java version of Pong. A paddle, a ball, and somebody trying to keep the ball from getting past them: a pretty good starting point for figuring out an animated game.
The applet uses AWT mouse and keyboard events for input, images for the paddles and ball, and an audio clip for the bounce sound. It keeps separate player and computer scores, includes a computer-paddle routine, and has pause and new-game states.
There is also an offscreen image and graphics context for double buffering. Drawing a whole frame there before copying it to the applet is a useful way to keep the screen from visibly clearing between updates. It is a lot of plumbing for a very old game, but watching all those little pieces turn into something playable is the enjoyable part.