Chapter 9 Object-Oriented Programming 449 libraries and can
Chapter 9 Object-Oriented Programming 449 libraries and can take advantage of other libraries available worldwide. Someday, most software might be constructed from standardized reusable components, just as hardware is often constructed today. This will help meet the challenges of developing the ever more powerful software we will need in the future. 9.2 Superclasses and Subclasses Often an object of one class is an object of another class as well. A rectangle certainly is a quadrilateral (as are squares, parallelograms and trapezoids). Thus, 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 a quadrilateral is a rectangle (the quadrilateral could be a parallelogram). Figure 9.1 shows several simple inheritance examples of superclasses and potential subclasses. Inheritance normally produces subclasses with more features than their superclasses, so the terms superclass and subclass can be confusing. There is another way, however, to view these terms that makes perfectly good sense. Because every subclass object is an object of its superclass, and because one superclass can have many subclasses, the set of objects represented by a superclass is normally larger than the set of objects represented by any of that superclass s subclasses. For example, the superclass Vehicle represents in a generic manner all vehicles, such as cars, trucks, boats, bicycles and so on. However, subclass Carrepresents only a small subset of all the Vehicles in the world. Inheritance relationships form tree-like hierarchical structures. A superclass exists in a hierarchical relationship with its subclasses. A class can certainly exist by itself, but it is when a class is used with the mechanism of inheritance that the class becomes either a superclass that supplies attributes and behaviors to other classes or a subclass that inherits those attributes and behaviors. Frequently, one class is both a subclass and a superclass. Superclass Subclasses Student GraduateStudent UndergraduateStudent Shape Circle Triangle Rectangle Loan CarLoan HomeImprovementLoan MortgageLoan Employee FacultyMember StaffMember Account CheckingAccount SavingsAccount Fig. 9.1Some simple inheritance examples in which the subclass is a Fig. superclass. Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/7/01