Java Reference
In-Depth Information
Sidebar 8.1 Observer
Java provides the java.util.Observable class and the java.util.Observer interface for implementing the
event notification mechanism. The Observable is the broadcaster object that provides information
to a set of listener objects ( Observers ). The underlying architectural model is known as
Model - View - Controller and has been documented as pattern Observer .
Class Observable offers method notifyObservers() to inform observer objects that some data have
changed. When this occurs, observers update their state to reflect the new data. State transitions
are defined in the update() method that every subclass of interface Observer must implement.
The call to notifyObservers() results in a call to the update() method of each Observer . This means
that the Observable object remains blocked until all of the Observer objects have completed the exe-
cution of their update() methods.
The code below implements an example, where the Equipment class plays the role of the
observable that changes state at random time instants. Class Controller plays the role of the observer
that is notified of the observable's state changes. Class Monitor initializes the communication
between Equipmet and Controller by adding Controller in the Equipment 's list of observer objects.
import java.util.*;
public class Equipment extends Observable implements Runnable {
int counter # 0;
public void run() {
while ( true ) {
try {
System.out.println("Equipment: state change :
" ! ( !! counter));
this .setChanged();
this .notifyObservers( new Integer(counter));
Thread.sleep(Math.round(Math.random()*2000.0 ! 500.0));
catch (InterruptedException i.e.) {}
public class Controller implements Observer{
public void update(Observable observable, Object object) {
Integer value # (Integer) object;
if (Math.IEEEremainder(value.intValue(), 2.0) ## 0.0)
System.out.println("Even value " ! value.intValue());
System.out.println("Odd value " ! value.intValue());
public class Monitor {
public static void main(String[] args) {
Equipment equipment # new Equipment();
Controller controller # new Controller();
Thread thread # new Thread(equipment);
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