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.
The is-a relationship is one of specialization. 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 in Cat.
The specialization and generalization relationships are both reciprocal and hierarchical. They are reciprocal because specialization is the obverse side of the coin from generalization. Thus, Dog and Cat specialize Mammal, and Mammal generalizes from Dog and Cat.
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 and 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).
Similarly, when you say that ListBox and Button are Windows, you indicate that there are characteristics and behaviors of Windows that you expect to find in both of these types. In other words, Window generalizes the shared characteristics of both ListBox and Button, while each specializes its own particular characteristics and behaviors.
About the Unified Modeling Language
The Unified Modeling Language (UML) is a standardized "language" for describing a system or business. The part of the UML that is useful for the purposes of this chapter is the set of diagrams used to document the relationships among classes.
In the UML, 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. In the UML, you model these relationships as shown in Figure 5-1. Note that the arrow points from the more specialized class up to the more general class.
It is common to note that two classes share functionality, and then to factor out these commonalities into a shared base class. This provides you with easier-to-maintain code and greater reuse of common code. For example, suppose you started out creating a series of objects as illustrated in Figure 5-2.
After working with RadioButtons, CheckBoxes, and Command buttons for a while, you realize that they share certain characteristics and behaviors that are more specialized than Window but more general than any of the three. You might factor these common traits and behaviors into a common base class, Button, and rearrange your inheritance hierarchy as shown in Figure 5-3. This is an example of how generalization is used in object-oriented development.
This UML diagram depicts the relationship between the factored classes and shows that both ListBox and Button derive from Window, and that Button is in turn specialized into CheckBox and Command. Finally, RadioButton derives from CheckBox. You can thus say that RadioButton is a CheckBox, which in turn is a Button, and that Buttons are Windows.
This is not the only, or even necessarily the best, organization for these objects, but it is a reasonable starting point for understanding how these types (classes) relate to one another.