To run you must keep the source files in the bouncing_balls directory.
There are two source code files, Animator.java and Model.java.
Animator sets up the window and handles the drawing. It does not need to be modified.
Model handles the movement of the balls. So this is where you write your own code to extend the model.
To compile and then run you need to be in the parent directory of bouncing_balls and write:
javac bouncing_balls/*.java
java bouncing_balls.Animator