11.2 Composition and generalization > 11.2.1 The generalization and IS-A relationship (inheritance)

11.2.1 The generalization and IS-A relationship (inheritance)

Generalization simply means identification of the commonalities among classes. The commonality may be in state, behavior, or both. For example, a problem description such as "All automobiles have a engine" expresses a common property (state) “engine” among all automobiles. Similarly, the statement, "All automobiles can be driven." expresses a common behavior among all automobiles.

You can express this commonality in a hierarchical structure. At the root of hierarchy, you need to put the root class that has a state and behavior common to all of its descendents. The descendent classes typically extend the root class to add a specialized state and/or behavior. Figure 11.2 shows a simple class hierarchy with generalization.

Figure 11.2 Animal hierarchy with generalized Animal class at root and specialized classes at bottom

The root class represents common (generalized) state and behavior shared among all its descendents. The subclasses, on the other hand, represent special behavior. For instance, the class Animal represents state and behavior common to all aircrafts. But its subclasses represent special behavior. For instance, Dog is a special kind of Animal and so is the Cat. But each of them is-an Animal.  Thus, whenever a class represents “common to all” behavior, it represents generalization. Similarly, if the class IS-A special kind of some class, it represents specialization. Therefore, it is often easy to spot generalization when you look for any of the following phrases in problem definition.

         "is a"

         "is a type of"

         "is a kind of"

Note that when you read the class hierarchy from bottom up, the IS-A relationship is revealed. For instance, when you read the animal hierarchy in figure 11.2, you can conclude that  Dog IS-AN Animal. ShowDog IS-A Dog.