Chat system

import java.util.HashSet;
import java.util.Set;

// Interface Definitions

interface User {
    String getUsername();
    void sendMessage(String message, ChatSession session);
    void receiveMessage(String message);
}

interface Message {
    String getContent();
    User getSender();
    User getReceiver();
}

interface ChatSession {
    void addUser(User user);
    void removeUser(User user);
    void sendMessage(User sender, String message);
}

// Implementation Classes

class ChatUser implements User {
    private String username;

    public ChatUser(String username) {
        this.username = username;
    }

    @Override
    public String getUsername() {
        return username;
    }

    @Override
    public void sendMessage(String message, ChatSession session) {
        session.sendMessage(this, message);
    }

    @Override
    public void receiveMessage(String message) {
        System.out.println(username + " received message: " + message);
    }
}

class ChatMessage implements Message {
    private String content;
    private User sender;
    private User receiver;

    public ChatMessage(String content, User sender, User receiver) {
        this.content = content;
        this.sender = sender;
        this.receiver = receiver;
    }

    @Override
    public String getContent() {
        return content;
    }

    @Override
    public User getSender() {
        return sender;
    }

    @Override
    public User getReceiver() {
        return receiver;
    }
}

class SimpleChatSession implements ChatSession {
    private Set<User> users = new HashSet<>();

    @Override
    public void addUser(User user) {
        users.add(user);
    }

    @Override
    public void removeUser(User user) {
        users.remove(user);
    }

    @Override
    public void sendMessage(User sender, String message) {
        for (User user : users) {
            if (!user.equals(sender)) {
                user.receiveMessage(sender.getUsername() + ": " + message);
            }
        }
    }
}

// Main Application

public class ChatApplication {
    public static void main(String[] args) {
        // Create users
        User alice = new ChatUser("Alice");
        User bob = new ChatUser("Bob");

        // Create a chat session
        ChatSession session = new SimpleChatSession();
        session.addUser(alice);
        session.addUser(bob);

        // Users send messages
        alice.sendMessage("Hi Bob!", session);
        bob.sendMessage("Hello Alice!", session);
    }
}