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Superclasses and Subclasses The superclass, or parent class, contains all the attributes and behaviors that are common to classes that inherit from it. All mammals have these attributes and behaviors, so it is not necessary to duplicate them down the inheritance tree for each type of mammal. The Mammal class is considered the superclass of the Dog and the Cat subclasses, or child classes. When you’re designing a Cat class, the Mammal class provides much of the functionality needed. To make it more specifically a cat type of mammal, the Cat class must include any attributes or behaviors that pertain solely to a cat.

Class reuse means that someone has already written a class. Thus, a programmer who uses a class must know how to get the class to work properly. This programmer will combine many classes to create a system. The programmer is the one who needs to understand the interfaces of a class. Therefore, when we talk about users in this chapter, we primarily mean designers and developers—not necessarily end users. Thus, when we talk about interfaces in this context, we are talking about class interfaces, not GUIs.

This said, the obvious question is who are the users? Although the customers are certainly users, the cabbie must be able to successfully provide the service to the customers. In other words, providing an interface that would, no doubt, please the customer, like “Take me to the airport for free,” is not going to go over well with the cabbie. Thus, in reality, to build a realistic and usable interface, both the customer and the cabbie must be considered users. For a software analogy, consider that users might want a programmer to provide a certain function.