Learning Java, 4th Edition

Observers and Observables

The java.util.Observer interface and java.util.Observable class are relatively small utilities, but they provide a glimpse of a fundamental design pattern in Java. Observers and observables are part of the MVC (Model-View-Controller) framework. It is an abstraction that lets a number of client objects (the observers) be notified whenever a certain object or resource (the observable) changes in some way. We will see this pattern used extensively in Java’s event mechanism, which is covered in Chapters 16 through 19. Although these classes are not often used directly, it’s worth looking at them in order to understand the pattern.

The Observable object has a method that an Observer calls to register its interest. When a change happens, the Observable sends a notification by calling a method in each of the Observers. The observers implement the Observer interface, which specifies that notification causes an Observer object’s update() method to be called.

In the following example, we create a MessageBoard object that holds a String message. MessageBoard extends Observable, from which it inherits the mechanism for registering observers (addObserver()) and notifying observers (notifyObservers()). To observe the MessageBoard, we have Student objects that implement the Observer interface so that they can be notified when the message changes:

    //file: MessageBoard.java
    import java.util.*;
      
    public class MessageBoard extends Observable {
        private String message;
      
        public ...