Learning Visual Basic .NET

Chapter 11. Inheritance and Polymorphism

In Chapter 8, you learned how to create new types by declaring classes, and in Chapter 3, you saw a discussion of the principle object relationships of association, aggregation, and specialization. This chapter focuses on specialization, which is implemented in VB.NET through inheritance. This chapter also explains how instances of more specialized classes can be treated as if they were instances of more general classes, a process known as polymorphism. This chapter ends with a consideration of not inheritable classes, which cannot be specialized, and a discussion of the root of all classes, the Object class.

Specialization and Generalization

Classes and their instances (objects) do not exist in a vacuum but rather in a network of interdependencies and relationships, just as we, as social animals, live in a world of relationships and categories. One of the most important relationships among objects in the real world is specialization, which can be described as an is-a relationship. When we say that a Dog is-a mammal, we mean that the dog is a specialized kind of mammal. It has all the characteristics of any mammal (it bears live young, nurses with milk, has hair), but it specializes these characteristics to the familiar characteristics of canine domesticus. A Cat is also a mammal. As such we expect it to share certain characteristics with the dog that are generalized in Mammal, but to differ in those characteristics that are specialized ...