I don't agree with jfkallens' last comparison between Abstract Classes & Object Interfaces completely.
In an Abstract Class, you can define how some methods work, where as in an Object Interface you can not.
An Object Interface is essentually nothing but a list of function names that a class must define if the class implements that interface.
An Abstract Class is essentually a prototype which hints towards what extending classes should be doing.
An Abstract Class can also be thought of as a Base Class that provides some basic functionality, & also defines a built-in Object Interface that all extending classes will implement.
So, an Object Interface is really a built-in part of an Abstract Class.
