Packages and Interfaces

Chapter 9

Packages and Interfaces

This chapter examines two of Java’s most innovative features: packages and interfaces. Packages are containers for classes. They are used to keep the class name space compartmentalized. For example, a package allows you to create a class named List, which you can store in your own package without concern that it will collide with some other class named List stored elsewhere. Packages are stored in a hierarchical manner and are explicitly imported into new class definitions.

In previous chapters, you have seen how methods define the interface to the data in a class. Through the use of the interface keyword, Java allows you to fully abstract an interface from its implementation. Using interface, you can specify a set of methods that can be implemented by one or more classes. In its traditional form, the interface, itself, does not actually define any implementation. Although they are similar to abstract classes, interfaces have an additional capability: A class can implement more than one interface. By contrast, a class can only inherit a single superclass (abstract or otherwise).