What Is Inheritance?

A brief discussion of an object-oriented concept known as inheritance may help you to understand one of the concepts in the first lesson. Inheritance is a relationship between classes in the system that allows for one class to use another class as a basis for specialization of behavior.

In our real-world example, doors are a class of things that you can shut and open and use as entry/exit points. Beyond that commonality, you can specialize doors: There are automatic doors, elevator doors, bank vault doors, and so on. These are all still doors that open and close, but they each provide a bit of additional specialized behavior.

In your security system, you have the common Door class that specifies secure and release behavior and allows each Door object to store a location. However, you must also support specialty doors that provide additional behaviors and attributes. An AlarmDoor provides the ability to trigger an audible alarm when activated and is also able to operate as a secure door. Since the secure and release behaviors of the AlarmDoor are the same as those in the Door class, the AlarmDoor can designate that it is inheriting from the Door class and need only provide specification for alarm activation.

Inheritance provides the AlarmDoor class with the ability to reuse the secure and release behaviors without having to specify them. For an AlarmDoor object, these behaviors appear as if they were defined in the AlarmDoor class.

Figure 5. Inheritance

Note that the relationship between AlarmDoor and Door is also a navigable association. AlarmDoor is dependent upon Doorit cannot exist without Door, since AlarmDoor inheXrits its code behavior and attributes from Door. UML shows the inheritance relationship as a directional arrow with the arrowhead closed instead of opened.

In the first lesson, you'll use inheritance to take advantage of a test framework. In a later lesson, you'll learn about inheritance in greater depth.