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 objectstheir connections and privileges with respect to one anotherthat 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