$2.(10$ pts$)$ The following is a typical example about head hunter. There are two roles in this example $-$HeadHunter and JobSeeker. Job seekers subscribe to a head hunter, and head hunter notifies job seekers when there is a new job opportunity. Draw the UML diagram $($class diagram$)$ for this design pattern. Explain the importance of this design pattern.

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 userList;

private ArrayList jobs;

public HeadHunter$()\{$

userList $=$ new ArrayList$()$;

jobs $=$ new ArrayList$()$;

$\}$

@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 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$"))$;

hh$.$addJob$("$Google Job"$)$;

hh$.$addJob$("$Yahoo Job"$)$;

$\}$

$\}$