Learning Java, 3rd Edition

Chapter 6. Relationships Among Classes

So far in our exploration of Java, we have seen how to create Java classes and objects, which are instances of those classes. By themselves objects would be little more than a convention for organizing code. It is in the relationships between objects—their connections and privileges with respect to one another—that the power of an object-oriented language is really expressed.

That’s what we’ll cover in this chapter. In particular, we’ll look at several kinds of relationships:

Inheritance relationships

How a class inherits methods and variables from its parent class

Interfaces

How to declare that a class implements certain behavior and define a type to refer to that behavior

Packaging

How to organize objects into logical groups

Inner classes

A generalization of classes that lets you nest a class definition inside another class definition

Subclassing and Inheritance

Classes in Java exist in a hierarchy. A class in Java can be declared as a subclass of another class using the extends keyword. A subclass inherits variables and methods from its superclass and can use them as if they were declared within the subclass itself:

    class Animal {
        float weight;
        ...
        void eat() {
            ...
        }
        ...
    }

    class Mammal extends Animal {
        // inherits weight
        int heartRate;
        ...

        // inherits eat()
        void breathe() {
            ...
        }
    }

In this example, an object of type Mammal has both the instance variable weight and the method eat(). They are inherited from Animal.

A class can extend only one ...