This toy program in Java is a useful teaching example. It's sufficiently 
complex that it will pose problems for students who have never worked with
code written by others. There some inheritance, use of exception classes, 
and some design patterns. Some jUnit 3 test classes are included.

The program is meant to emulate a self-check out device, such as those 
in grocery stories and other retailers. This one in particular was modeled
after the one in an Alberton's store. Ones in Home Depot are similar.

The App package contains the back end for the system. To keep things simple, 
there is no external DBMS. Instead, a set of utilities have been created to
fake a DBMS.

The GUI package is provided for convenience. It is not meant to be a user 
interface for the customers. Instead, it is a means for students to interact
with the system and to provide starting points for reading the code. As 
another simplification, I didn't want students to get caught up in making a
beautiful UI.

The Devices package is meant to be a Facade for APIs for the I/O devices
in a self-check out. Right now, they are just mock ups.