Introduction
This chapter continues our discussion of object-oriented programming (OOP) by introducing one of its primary featuresinheritance, which is a form of software reuse in which a new class is created by absorbing an existing class's members and embellishing them with new or modified capabilities. With inheritance, programmers save time during program development by reusing proven and debugged high-quality software. This also increases the likelihood that a system will be implemented effectively.
When creating a class, rather than declaring completely new members, the programmer can designate that the new class should inherit the members of an existing class. The existing class is called the superclass, and the new class is the subclass. (The C++ programming language refers to the superclass as the base class and the subclass as the derived class.) Each subclass can become the superclass for future subclasses.
A subclass normally adds its own fields and methods. Therefore, a subclass is more specific than its superclass and represents a more specialized group of objects. Typically, the subclass exhibits the behaviors of its superclass and additional behaviors that are specific to the subclass.
The direct superclass is the superclass from which the subclass explicitly inherits. An indirect superclass is any class above the direct superclass in the class hierarchy, which defines the inheritance relationships between classes. In Java, the class hierarchy begins with class Object (in package java.lang), which every class in Java directly or indirectly extends (or "inherits from"). Section 9.7 lists the methods of class Object, which every other class inherits. In the case of single inheritance, a class is derived from one direct superclass. Java, unlike C++, does not support multiple inheritance (which occurs when a class is derived from more than one direct superclass). In Chapter 10, Object-Oriented Programming: Polymorphism, we explain how Java programmers can use interfaces to realize many of the benefits of multiple inheritance while avoiding the associated problems.
Experience in building software systems indicates that significant amounts of code deal with closely-related special cases. When programmers are preoccupied with special cases, the details can obscure the big picture. With object-oriented programming, programmers focus on the commonalities among objects in the system rather than on the special cases.
We distinguish between the "is-a" relationship and the "has-a" relationship. "Is-a" represents inheritance. In an "is-a" relationship, an object of a subclass can also be treated as an object of its superclass. For example, a car is a vehicle. By contrast, "has-a" represents composition (see Chapter 8). In a "has-a" relationship, an object contains one or more object references as members. For example, a car has a steering wheel (and a car object has a reference to a steering wheel object).
New classes can inherit from classes in class libraries. Organizations develop their own class libraries and can take advantage of others available worldwide. Some day, most new software likely will be constructed from standardized reusable components, just as automobiles and most computer hardware are constructed today. This will facilitate the development of more powerful, abundant and economical software.