Superclasses and Subclasses

Often, an object of one class "is an" object of another class as well. For example, in geometry, a rectangle is a quadrilateral (as are squares, parallelograms and trapezoids). Thus, in Java, class Rectangle can be said to inherit from class Quadrilateral. In this context, class Quadrilateral is a superclass and class Rectangle is a subclass. A rectangle is a specific type of quadrilateral, but it is incorrect to claim that every quadrilateral is a rectanglethe quadrilateral could be a parallelogram or some other shape. Figure 9.1 lists several simple examples of superclasses and subclassesnote that superclasses tend to be "more general" and subclasses tend to be "more specific."

Because every subclass object "is an" object of its superclass, and one superclass can have many subclasses, the set of objects represented by a superclass is typically larger than the set of objects represented by any of its subclasses. For example, the superclass Vehicle represents all vehicles, including cars, trucks, boats, bicycles and so on. By contrast, subclass Car represents a smaller, more specific subset of vehicles.

Inheritance relationships form tree-like hierarchical structures. A superclass exists in a hierarchical relationship with its subclasses. When classes participate in inheritance relationships, they become "affiliated" with other classes. A class becomes either a superclass, supplying members to other classes, or a subclass, inheriting its members from other classes. In some cases, a class is both a superclass and a subclass.

Let us develop a sample class hierarchy (Fig. 9.2), also called an inheritance hierarchy. A university community has thousands of members, including employees, students and alumni. Employees are either faculty members or staff members. Faculty members are either administrators (such as deans and department chairpersons) or teachers. Note that the hierarchy could contain many other classes. For example, students can be graduate or undergraduate students. Undergraduate students can be freshmen, sophomores, juniors or seniors.

Figure 9.2. Inheritance hierarchy for university CommunityMembers.

Each arrow in the hierarchy represents an "is-a" relationship. As we follow the arrows in this class hierarchy, we can state, for instance, that "an Employee is a CommunityMember" and "a Teacher is a Faculty member." CommunityMember is the direct superclass of Employee, Student and Alumnus, and is an indirect superclass of all the other classes in the diagram. Starting from the bottom of the diagram, the reader can follow the arrows and apply the "is-a" relationship up to the topmost superclass. For example, an Administrator is a Faculty member, is an Employee and is a CommunityMember.

Now consider the Shape inheritance hierarchy in Fig. 9.3. This hierarchy begins with superclass Shape, which is extended by subclasses TwoDimensionalShape and ThreeDimensionalShapeShapes are either TwoDimensionalShapes or ThreeDimensionalShapes. The third level of this hierarchy contains some more specific types of TwoDimensionalShapes and THReeDimensionalShapes. As in Fig. 9.2, we can follow the arrows from the bottom of the diagram to the topmost superclass in this class hierarchy to identify several "is-a" relationships. For instance, a triangle is a TwoDimensionalShape and is a Shape, while a Sphere is a ThreeDimensionalShape and is a Shape. Note that this hierarchy could contain many other classes. For example, ellipses and trapezoids are TwoDimensionalShapes.

Figure 9.3. Inheritance hierarchy for Shapes.

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Not every class relationship is an inheritance relationship. In Chapter 8, we discussed the "has-a" relationship, in which classes have members that are references to objects of other classes. Such relationships create classes by composition of existing classes. For example, given the classes Employee, BirthDate and TelephoneNumber, it is improper to say that an Employee is a BirthDate or that an Employee is a TelephoneNumber. However, an Employee has a BirthDate, and an Employee has a TelephoneNumber.

It is possible to treat superclass objects and subclass objects similarlytheir commonalities are expressed in the members of the superclass. Objects of all classes that extend a common superclass can be treated as objects of that superclass (i.e., such objects have an "is-a" relationship with the superclass). However, superclass objects cannot be treated as objects of their subclasses. For example, all cars are vehicles, but not all vehicles are cars (the other vehicles could be trucks, planes or bicycles, for example). Later in this chapter and in Chapter 10, Object-Oriented Programming: Polymorphism, we consider many examples that take advantage of the "is-a" relationship.

One problem with inheritance is that a subclass can inherit methods that it does not need or should not have. Even when a superclass method is appropriate for a subclass, that subclass often needs a customized version of the method. In such cases, the subclass can override (redefine) the superclass method with an appropriate implementation, as we will see often in the chapter's code examples.