Java Cookbook

Providing Callbacks via Interfaces

Problem

You want to provide callbacks ; that is, have unrelated classes call back into your code.

Solution

One way is to use a Java interface.

Discussion

An interface is a class-like object that can contain only abstract methods and final fields. As we’ve seen, interfaces are used a lot in Java! In the standard API, the following are a few of the commonly used interfaces:

• Runnable, Comparable, and Cloneable (in java.lang)

• List, Set, Map, and Enumeration/Iterator (in the Collections API; see Chapter 7)

• ActionListener, WindowListener, and others (in the AWT GUI; see Section 13.5)

• Driver, Connection, Statement, and ResultSet (in JDBC; see Section 20.4)

• The "remote interface” -- the contact between the client and the server -- is specified as an Interface (in RMI, CORBA, and EJB)

Subclass, Abstract Class, or Interface?

There is usually more than one way to skin a cat. Some problems can be solved by subclassing, by use of abstract classes, or by interfaces. The following general guidelines may help:

• Use an abstract class when you want to provide a template for a series of subclasses, all of which may inherit some of their functionality from the parent class but are required to implement some of it themselves. (Any subclass of a geometric Shapes class might have to provide a computeArea( ) method; since the top-level Shapes class cannot do this, it would be abstract. This is implemented in Section 8.9.)

• Subclass whenever you want to extend a class and add ...