Java Programming

A CLASS REPRESENTS A SET OF OBJECTS which share the same structure and behaviors. The class determines the structure of objects by specifying variables that are contained in each instance of the class, and it determines behavior by providing the instance methods that express the behavior of the objects. This is a powerful idea. However, something like this can be done in most programming languages. The central new idea in object-oriented programming -- the idea that really distinguishes it from traditional programming -- is to allow classes to express the similarities among objects that share some, but not all, of their structure and behavior. Such similarities can be expressed using inheritance and polymorphism.

The topics covered in this section are relatively advanced aspects of object-oriented programming. Any programmer should know what is meant by subclass, inheritance, and polymorphism. However, it will probably be a while before you actually do anything with inheritance except for extending classes that already exist.

The term inheritance refers to the fact that one class can inherit part or all of its structure and behavior from another class. The class that does the inheriting is said to be a subclass of the class from which it inherits. If class B is a subclass of class A, we also say that class A is a superclass of class B. (Sometimes the terms derived class and base class are used instead of subclass and superclass.) A subclass can add to the structure and behavior that it inherits. It can also replace or modify inherited behavior (though not inherited structure). The relationship between subclass and superclass is sometimes shown by a diagram in which the subclass is shown below, and connected to, its superclass.

In Java, when you create a new class, you can declare that it is a subclass of an existing class. If you are defining a class named "B" and you want it to be a subclass of a class named "A", you would write

        class B extends A {
            .
            .  // additions to, and modifications of,
            .  // stuff inherited from class A
            .
        }
      

Several classes can be declared as subclasses of the same superclass. The subclasses, which might be referred to as "sibling classes," share some structures and behaviors -- namely, the ones they inherit from their common superclass. The superclass expresses these shared structures and behaviors. In the diagram to the left, classes B, C, and D are sibling classes. Inheritance can also extend over several "generations" of classes. This is shown in the diagram, where class E is a subclass of class D which is itself a subclass of class A. In this case, class E is considered to be a subclass of class A, even though it is not a direct subclass.