Observable is a class and Observer is an interface
Observable class maintain a list of observers
and When an Observable object is updated it invokes the update() method of each of its observers to notify that, it is changed
for example
import java.util.Observable;
import java.util.Observer;
class MessageBoard extends Observable {
private String message;
public String getMessage() {
return message;
}
public void changeMessage(String message) {
this.message = message;
setChanged();
notifyObservers(message);
}
public static void main(String[] args) {
MessageBoard board = new MessageBoard();
Student bob = new Student();
Student joe = new Student();
board.addObserver(bob);
board.addObserver(joe);
board.changeMessage("More Homework!");
}
}
class Student implements Observer {
public void update(Observable o, Object arg) {
System.out.println("Message board changed: " + arg);
}
}
