Often books on Java teach the following topics in one chapter:

• deriving classes from base classes
• polymorphism (dynamic binding)
• abstract classes
• interfaces

These topics are taught in this order because it seems to be an obvious order: first introduce the advantages of deriving classes from base classes; then explain polymorphism (dynamic binding); the explain that an abstract class is a special form of base class; and finally introduce an interface as a special form of an abstract class. However, a chapter that has these topics introduces a lot of new ideas and explaining them all at once will overload the reader.

Although there is some logic to teaching interfaces in this way, it is better to make a clearer separation between implementation inheritance and interfaces. I think it is important to teach interfaces early, and to leave implementation inheritance until later.

Although the lectures on implementation inheritance appears late in this course, some of the basic ideas are introduced in earlier lectures:

• When toString, equals and hashCode are introduced, you can also introduce the idea that, by default, classes are derived from Object; that you can use methods of the class Object on other objects; and that methods of a superclass can be overriden.
• When discussing the classes of the Swing API you can demonstrate the use of implementation inheritance and explain the usefulness of organising classes in a class hierarchy;
• When using java.util.Observable, you need to produce a class that is derived from a class of the Core APIs and explain why this is being done.

It is only towards the end of this course that the students learn how to derive a class from one of their other classes. These lectures also introduce the idea of abstract classes.