In Chapter 7, you learned
how to create new types by declaring classes, and in Chapter 6, you saw a discussion of the
principle object relationships of association, aggregation, and
specialization . This chapter focuses on
specialization, which is implemented in C# 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
sealed classes, which cannot be specialized, and a
discussion of the root of all classes, the
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 the 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 canis 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 in ...