I was checking one of your tutorials here One of the last paragraphs says:
Then you would export a .jar file that contains your code. Make sure your classes are in packages!
I think you could add a bit more why packages are important, specially if you are creating a jar. Another suggestion on this explanation is related to the last example you provided between the sketch and Ball class. The latter could be stored in the Ball.java file at the same level as the sketch.java file so for sketch.java to be able to see the Ball class.
Two small details to improve this docs.Other than that, this tutorials are very useful.