Use these simple classes to get an idea what abstract class
and abstract method mean.

For example, load the three classes into DrJava and do the following:

1. Compile. In the interactions pane, try doing this:
   new Shape(3, 4)

   Now delete the word "abstract" in the first line of class Shape and
   try this again:
   new Shape(3 4)

2. Make sure class Shape is abstract.
   Comment out abstract method area in class Shape and try to compile.
