Notes

// interface Shape {
// void draw();
// }

// class Circle implements Shape{
// @Override
// public void draw(){
// System.out.println("Circle:: draw()");
// }
// }

// class ShapeMaker{
// private Shape circle;

// public ShapeMaker(){
// circle = new Circle();
// }

// public void drawCircle()
// {
// circle.draw();
// }
// }

// public class FacadeTest {

// public static void main(String[] args) {
// ShapeMaker shapeMaker = new ShapeMaker();
// shapeMaker.drawCircle();
// }
// }


//---------------RestaurantOrderSystem---------
// import java.util.ArrayList;
// import java.util.List;

// class Chef{
// public void makeBurger(){
// System.out.println("Chef is making Burger");
// }
// }

// interface Command{
// void execute();
// }

// // Concrete Command
// class BurgerCommand implements Command{
// private Chef chef;

// public BurgerCommand(Chef chef){
// this.chef = chef;
// }

// @Override
// public void execute(){
// chef.makeBurger();
// }
// }

// class Waiter{
// private List<Command> orders = new ArrayList<>();

// public void takeOrder(Command command){
// orders.add(command);
// }

// public void submitOrders(){
// for(Command command : orders){
// command.execute();
// }
// orders.clear();
// }
// }

// public class RestaurantTest {
// public static void main(String[] args) {
// Chef chef = new Chef();
// Command burgerCommand = new BurgerCommand(chef);

// Waiter waiter = new Waiter();
// waiter.takeOrder(burgerCommand);
// waiter.submitOrders();
// }
// }




// -------------------------------------------------
import java.util.ArrayList;
import java.util.List;

// Subject interface
interface Subject {
void registerObserver(Observer observer);

void removeObserver(Observer observer);

void notifyObservers(String post);
}

// Concrete Subject
class FacebookUser implements Subject {
private String username;
private List<Observer> followers;
private String latestPost;

public FacebookUser(String username) {
this.username = username;
this.followers = new ArrayList<>();
}

public void postUpdate(String post) {
latestPost = post;
notifyObservers(post);
}

@Override
public void registerObserver(Observer observer) {
followers.add(observer);
}

@Override
public void removeObserver(Observer observer) {
followers.remove(observer);
}

@Override
public void notifyObservers(String post) {
for (Observer follower : followers) {
follower.update(username, post);
}
}
}

// Observer interface
interface Observer {
void update(String username, String post);
}

// Concrete Observer
class FacebookFollower implements Observer {
private String followerName;

public FacebookFollower(String followerName) {
this.followerName = followerName;
}

@Override
public void update(String username, String post) {
System.out.println(followerName + " received an update from " + username + ":");
System.out.println(post);
System.out.println();
}
}

public class ObserverPatternDemo {
public static void main(String[] args) {
// Create Facebook users
FacebookUser alice = new FacebookUser("Alice");
FacebookUser bob = new FacebookUser("Bob");

// Create followers
FacebookFollower follower1 = new FacebookFollower("Follower 1");
FacebookFollower follower2 = new FacebookFollower("Follower 2");

// Register followers to users
alice.registerObserver(follower1);
alice.registerObserver(follower2);
bob.registerObserver(follower2);

// Users post updates
alice.postUpdate("Hello, everyone! This is my first post.");
bob.postUpdate("Feeling excited about the weekend!");

// Follower 1 decides to unfollow Alice
alice.removeObserver(follower1);

// Alice posts another update
alice.postUpdate("A beautiful sunset today!");

// Follower 1 doesn't receive the new update
}
}