Programming Visual Basic .NET, Second Edition

Chapter 6. Inheritance and Polymorphism

Chapter 5 demonstrates how to create new types by declaring classes. The current chapter explores the relationship among objects in the real world and how to model these relationships in your code. 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, as well as a brief overview of nested classes.

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, etc.), 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 ...