The Java API provides a class called java.util.Observable which can be extended by any Java class. The child class can then be an “observable” class and other objects can “observe” changes in state of this object.

The class that performs the “observing” must implement the java.util.Observer interface. There is a single method:

 public void update(Observable obj, Object arg)

This method is called whenever the observed object is changed. An application calls an Observable object’s notifyObservers method to have all the object’s observers notified of the change.

In the following example, a Manager observes an Employee. Suppose a change in salary occurs in Employee. It then notifies the observers using setChanged() and notifyObservers(). Manager’s update() method is called to inform him that Employee has changed.

public class Employee extends Observable{  float salary;  public setSalary(float newSalary){    salary=newSalary;  // salary has changed    setChanged();      // mark that this object has changed, MANDATORY    notifyObservers(this, new Float(salary));  // notify all observers, MANDATORY  }}public class Manager implements Observer{   public void update(Observable obj, Object arg){     System.out.println("A change has happened and new salary is " + arg);   }}public class Demo(){  public static void main(String[] args){    Employee e = new Employee();    Manager m = new Manager();    e.addObserver(m); // register for observing  }}