Learning Java, 4th 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, classes 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