In the Observer Design Pattern, the subject class needs to maintain a list of observers that should be notified of any changes in its state. In a Java implementation, this is often facilitated by the addObserver$($Observer o$)$ method. What is the purpose of the addObserver$($Observer o$)$ method in the subject class?

Example code:

import java.util.ArrayList;

import java.util.List;

$//$ Abstract Subject class that manages observers and notifications

abstract class Subject $\{$

private List observers $=$ new ArrayList$<>()$;

public void addObserver$($Observer observer$)\{$

observers.add$($observer$)$;

$\}$

public void removeObserver$($Observer observer$)\{$

observers.remove$($observer$)$;

$\}$

public void notifyObservers$()\{$

for $($Observer observer : observers$)\{$

observer.update$()$;

$\}$

$\}$

$\}$

$//$ Concrete implementation of Subject

class ConcreteSubject extends Subject $\{$

private String state;

public void setState$($String state$)\{$

this.state $=$ state;

notifyObservers$()$;

$\}$

public String getState$()\{$

return this.state;

$\}$

$\}$

$//$ Observer interface

interface Observer $\{$

void update$()$;

$\}$

$//$ Concrete implementation of Observer

class ConcreteObserver implements Observer $\{$

public void update$()\{$

System.out.println$("$Observer notified of change!"$)$;

$\}$

$\}$

$//$ Demo class to show the Observer Pattern in action

public class ObserverPatternDemo $\{$

public static void main$($String$[]$ args$)\{$

ConcreteSubject subject $=$ new ConcreteSubject$()$;

Observer observer$1=$ new ConcreteObserver$()$;

subject.addObserver$($observer$1)$;

subject.setState$("$New State"$)$; $//$ This should trigger observer notification

$\}$

$\}$

It updates the state of the subject based on the observer's state.

It sends an update notification to all registered observers.

It registers a new observer to receive updates from the subject.

It removes an observer from the subject's list of observers.