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10: Arrays
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Inheritance and Polymorphism
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In 6, we explained how classes derive from one another and described how classes can inherit properties and methods from their parent classes. In 7, you learned how to create your own classes and use objects of those classes, but you didn t see how the inheritance aspect works in practice. That s about to change. We mentioned in 6 that the three key principles of object-oriented programming are encapsulation (discussed in 7), specialization, and polymorphism. This chapter focuses on specialization, which is implemented in C# through inheritance. You ll see how to create your own class hierarchy, and how to enforce that child classes implement the methods of their parent classes. You ll even see how to create completely abstract classes, and why you d want to. You can t create an instance of an abstract class; you can only inherit from it. This chapter also explains how instances of a child class can be treated as though they were instances of one of the child class s ancestor 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 Object class.
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Before we can start to show you the syntax of inheritance, we first have to give you a little more object-oriented background, so you can see why inheritance works the way it does. 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
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that are generalized in Mammal, but to differ in those characteristics that are specialized in cats. The specialization and generalization relationships are both reciprocal and hierarchical. Specialization is just generalization in the opposite direction: Mammal generalizes what is common among dogs and cats, and dogs and cats specialize mammals to their own specific subtypes. These relationships are hierarchical because they create a relationship tree, with specialized types branching off from more generalized types. As you move up the hierarchy, you achieve greater generalization. You move up toward Mammal to generalize that dogs, cats, and horses all bear live young. As you move down the hierarchy, you specialize. Thus, the cat specializes Mammal in having claws (a characteristic) and purring (a behavior). To use a more programming-specific example, every widget that you see in a standard Windows interface is called a control. So, when you say that ListBox and Button are Controls, you indicate that there are characteristics and behaviors of Controls that you expect to find in both of these types. In other words, Control generalizes the shared characteristics of both ListBox and Button, while each specializes its own particular characteristics and behaviors. To put it another way, all Controls, which includes ListBoxes and Buttons, have certain common behaviors they re all drawn on the screen, for one thing. But a Button can be clicked, which a ListBox can t. A ListBox has contents, which can be sorted. A Button can t do that. The Unified Modeling Language (UML) is a standardized language for describing an object-oriented system. UML has many different visual representations, but in this case, all you need to know is that classes are represented as boxes. The name of the class appears at the top of the box, and (optionally) methods and members can be listed in the sections within the box. You can use UML to model specialization relationships, as shown in Figure 11-1. Note that the arrow points from the more specialized class up to the more general class. In the figure, the more specialized Button and ListBox classes point up to the more general Control class. When you start out designing classes from scratch, you ll often find that you have several classes that do the same thing. When this occurs, you can factor out these commonalities into a shared base class, which is more general than the specialized classes. This factoring is beneficial to you, because it allows you to reuse common code, and anytime you can reuse code instead of copying it to a new class is a good thing. That gives you code that is easier to maintain, because the changes are located in a single class rather than scattered among numerous classes. For example, suppose you started out creating a series of objects, as illustrated in Figure 11-2. After working with RadioButtons, CheckBoxes, and Command buttons for a
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