public interface Subject {
  public void registerObserver(Observer o);
  public void removeObserver(Observer o);
  public void notifyAllObservers();
}

public interface Observer {
  public void update(Subject s);
}

import java.util.ArrayList;
 
public class HeadHunter implements Subject{
 
  //define a list of users, such as Mike, Bill, etc.
  private ArrayList<Observer> userList;
  private ArrayList<String> jobs;
 
  public HeadHunter(){
    userList = new ArrayList<Observer>();
    jobs = new ArrayList<String>();
  }
 
  @Override
  public void registerObserver(Observer o) {
    userList.add(o);
  }
 
  @Override
  public void removeObserver(Observer o) {}
 
  @Override
  public void notifyAllObservers() {
    for(Observer o: userList){
      o.update(this);
    }
  }
 
  public void addJob(String job) {
    this.jobs.add(job);
    notifyAllObservers();
  }
 
  public ArrayList<String> getJobs() {
    return jobs;
  }
 
  public String toString(){
    return jobs.toString();
  }
}

public class JobSeeker implements Observer {
 
  private String name;
 
  public JobSeeker(String name){
    this.name = name;
  }
  @Override
  public void update(Subject s) {
    System.out.println(this.name + " got notified!");
    //print job list
    System.out.println(s);
  }
}

public class Main {
  public static void main(String[] args) {
    HeadHunter hh = new HeadHunter();
    hh.registerObserver(new JobSeeker("Mike"));
    hh.registerObserver(new JobSeeker("Chris"));
    hh.registerObserver(new JobSeeker("Jeff"));
 
    //每次添加一个个job，所有找工作人都可以得到通知。
    hh.addJob("Google Job");
    hh.addJob("Yahoo Job");
  }
}