Observable and observer : Observer Pattern « Design Pattern « Java

Observable and observer

import java.util.Observable;
import java.util.Observer;

public class MessageBoard extends Observable {
  private String message;

  public String getMessage() {
    return message;

  public void changeMessage(String message) {
    this.message = message;

  public static void main(String[] args) {
    MessageBoard board = new MessageBoard();
    Student bob = new Student();
    Student joe = new Student();
    board.changeMessage("More Homework!");

class Student implements Observer {
  public void update(Observable o, Object arg) {
    System.out.println("Message board changed: " + arg);


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