Watch What is Going On with the Observer Patterns : Observable and Observer « Design Pattern « Java Tutorial

1. Java Tutorial
2. Design Pattern
3. Observable and Observer

import java.util.Vector;

public class TestObserver {
  public static void main(String args[]) {
    Database database = new Database();
    Archiver archiver = new Archiver();
    Client client = new Client();
    Boss boss = new Boss();

    database.registerObserver(archiver);
    database.registerObserver(client);
    database.registerObserver(boss);
    database.editRecord("delete", "record 1");
  }
}

interface Observer {
  public void update(String operation, String record);
}

interface Subject {
  public void registerObserver(Observer o);

  public void removeObserver(Observer o);

  public void notifyObservers();
}

class Database implements Subject {
  private Vector<Observer> observers;

  private String operation;

  private String record;

  public Database() {
    observers = new Vector<Observer>();
  }

  public void registerObserver(Observer o) {
    observers.add(o);
  }

  public void removeObserver(Observer o) {
    observers.remove(o);
  }

  public void notifyObservers() {
    for (int loopIndex = 0; loopIndex < observers.size(); loopIndex++) {
      Observer observer = (Observer) observers.get(loopIndex);
      observer.update(operation, record);
    }
  }

  public void editRecord(String operation, String record) {
    this.operation = operation;
    this.record = record;
    notifyObservers();
  }
}

class Client implements Observer {
  public Client() {
  }

  public void update(String operation, String record) {
    System.out.println("The client says a " + operation + " operation was performed on " + record);
  }
}

class Boss implements Observer {
  public Boss() {
  }

  public void update(String operation, String record) {
    System.out.println("The boss says a " + operation + " operation was performed on " + record);
  }
}

class Archiver implements Observer {
  public Archiver() {
  }

  public void update(String operation, String record) {
    System.out
        .println("The archiver says a " + operation + " operation was performed on " + record);
  }
}