Mediator is a behavioral design pattern which is used to reduce communication complexity between multiple objects or classes. This pattern provides a mediator class which normally handles all the communications between different classes and supports easy maintenance of the code by loose coupling.

Demonstrational example of Mediator Pattern is s chat room where multiple users can send message to chat room and it is the responsibility of chat room to show the messages to all users. There are two classes – User and ChatRoom, ChatRoom acts as a Mediator: User objects use ChatRoom to share their messages.

Step 1 : Create mediator class

public class ChatRoom {
    public static void showMessage(User user, String message) {
        System.out.println(new Date() + " [" + user.getName() + "] : " + message);
    }
}

Step 2 : Create User class

public class User {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

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

    public void sendMessage(String message) {
        ChatRoom.showMessage(this, message);
    }
}

Step 3 : Use the User object to show communication between users

public class MediatorPatternDemo {
    public static void main(String[] args) {
        User john = new User("John");
        User robert = new User("Robert");

        john.sendMessage("Hi, Robert");
        robert.sendMessage("Hello, John");
    }
}

The output will be :

Thu Aug 20 15:37:36 EDT 2020 [John] : Hi, Robert
Thu Aug 20 15:37:36 EDT 2020 [Robert] : Hello, John