Object-Oriented Programming: Inheritance

Object-Oriented Programming: Inheritance

This lecture continues our discussion of object-oriented programming (OOP) by introducing one of its primary capabilities—inheritance, 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, you can save time during program development by basing new classes on existing proven and debugged high-quality software. This also increases the likelihood that a system will be implemented and maintained effectively.

When creating a class, rather than declaring completely new members, you can designate that the new class should inherit the members of an existing class. The existing class is called the superclass, and the newclass 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 a superclass for future subclasses.

A subclass can add its own fields and methods. Therefore, a subclass is more specific than its superclass and represents a more specialized group of objects. The subclass exhibits the behaviors of its superclass and can modify those behaviors so that they operate appropriately for the subclass. This is why inheritance is sometimes referred to as specialization.

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 that are inherited by all other Java classes. Java supports only single inheritance, in which each class is derived from
exactly one direct superclass. Unlike C++, Java does not support multiple inheritance (which occurs when a class is derived from more than one direct superclass). Chapter 10, Object- Oriented Programming: Polymorphism, explains how to use Java interfaces to realize many of the benefits of multiple inheritance while avoiding the associated problems.