Mediator Pattern

// ─── EXAMPLE 1 ──────────────────────────────────────────────────────────────
// WHAT WE ARE IMPLEMENTING:
// An aviation control tower regulating flight paths and runway clearances for
// multiple landing airplanes.
//
// WHERE THE MEDIATOR FITS IN:
// AirportControlTower acts as the Mediator. Flight acts as the Colleague
// interface. Planes communicate solely through the tower to prevent
// collisions.
// ────────────────────────────────────────────────────────────────────────────
import java.util.ArrayList;
import java.util.List;

// --- Mediator ---
interface ChatMediator {
    void sendMessage(String message, User sender);
    void addUser(User user);
}

// --- Concrete mediator ---
class ChatRoom implements ChatMediator {
    private final List<User> users = new ArrayList<>();

    public void addUser(User user) {
        users.add(user);
        System.out.println("  [Room] " + user.getName() + " joined");
    }

    public void sendMessage(String message, User sender) {
        for (User user : users) {
            // Don't send back to sender
            if (user != sender) {
                user.receive(message, sender.getName());
            }
        }
    }
}

// --- Colleague ---
class User {
    private final String name;
    private final ChatMediator mediator;

    public User(String name, ChatMediator mediator) {
        this.name = name;
        this.mediator = mediator;
    }

    public String getName() { return name; }

    public void send(String message) {
        System.out.println("  [" + name + "] Sending: " + message);
        mediator.sendMessage(message, this);
    }

    public void receive(String message, String from) {
        System.out.println("  [" + name + "] Received from " + from + ": " + message);
    }
}

public class Main {
    public static void main(String[] args) {
        ChatMediator chatRoom = new ChatRoom();

        User alice = new User("Alice", chatRoom);
        User bob = new User("Bob", chatRoom);
        User charlie = new User("Charlie", chatRoom);

        chatRoom.addUser(alice);
        chatRoom.addUser(bob);
        chatRoom.addUser(charlie);

        System.out.println("\n--- Alice sends a message ---");
        alice.send("Hello everyone!");
    }
}