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public interface ChatMediator {
    public void sendMessage(String msg, User user);
    void addUser(User user);
}

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import java.util.ArrayList;
import java.util.List;

public class ChatMediatorImpl implements ChatMediator {

    private List<User> users;

    public ChatMediatorImpl() {
        this.users = new ArrayList<>();
    }

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

    @Override
    public void sendMessage(String msg, User user) {
        for (User u : this.users) {
            //message should not be received by the user sending it
            if (u != user) {
                u.receive(msg);
            }
        }
    }

}

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public abstract class User {
    protected ChatMediator mediator;
    protected String name;
     
    public User(ChatMediator med, String name){
        this.mediator=med;
        this.name=name;
    }
     
    public abstract void send(String msg);
     
    public abstract void receive(String msg);
}

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public class UserImpl extends User {
  
    public UserImpl(ChatMediator med, String name) {
        super(med, name);
    }
 
    @Override
    public void send(String msg){
        System.out.println(this.name+": Sending Message="+msg);
        mediator.sendMessage(msg, this);
    }
    @Override
    public void receive(String msg) {
        System.out.println(this.name+": Received Message:"+msg);
    }
 
}

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public class ChatClient {
  
    public static void main(String[] args) {
        ChatMediator mediator = new ChatMediatorImpl();
        User user1 = new UserImpl(mediator, "a");
        User user2 = new UserImpl(mediator, "b");
        User user3 = new UserImpl(mediator, "c");
        User user4 = new UserImpl(mediator, "d");
        mediator.addUser(user1);
        mediator.addUser(user2);
        mediator.addUser(user3);
        mediator.addUser(user4);
         
        user1.send("Hi All");
         
    }
 
}