Learning Java

Chapter 6. Relationships Among Classes

So far, we know how to create a Java class and to create objects, which are instances of a class. But an object by itself isn’t very interesting—no more interesting than, say, a table knife. You can marvel at a table knife’s perfection, but you can’t really do anything with it until you have some other pieces of cutlery and food to use the cutlery on. The same is true of objects and classes in Java: they’re interesting by themselves, but what’s really important comes from relationships that you establish among them.

That’s what we’ll cover in this chapter. In particular, we’ll be looking 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 supports 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 of another class definition

Subclassing and Inheritance

Classes in Java exist in a class 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 uses them as if they were declared within the subclass itself:

class Animal {  
    float weight;  
    ...  
    void eat( ) {  
        ...  
    }  
    ...  
}  
  
class Mammal extends Animal {  
    int heartRate;  
    // inherits weight  
    ...  
    void breathe( ) {  
        ...  
    }  
    // inherits eat( )  
}

In this example, ...