A series of videos talking you through creating this simulation is now here: https://www.youtube.com/playlist?list=PLIWb-FtdAhJjNgdIljwkMEQMhwhVIiV63

Mik, I am running a summer school session and we are using your Contagion videos. Excellent series. Thank you. I have added a few features that focus on moving toward libraries and object composition as a means of increasing reliability and code reuse. It seems to be an area where students struggle in their growth. The goal was to 1) introduce these ideas and integrate them into existing work, and 2) demonstrate how the code in the original solution becomes easier to read and maintain by removing sections of code into libraries and other classes. Code/solutions from of the examples posted on websites and books grow large organically and this "refactoring" of code seems to get missed in the evolution of a developer's education. I am trying to provide an easy to follow path to see beyond helper methods to helper classes. You provide an excellent discussion and examples from Objects First with Java, but most students don't make it all the way through to those chapters and many greenfoot scenarios miss out on the application of this topic. I would be happy to share what I have with you and would suggest that object composition is a more important topic for the later video lessons in the series. It also would flow better with Inheritance (as in your book). Cheers