// Tutorial //

Mediator Design Pattern in Java

Published on August 3, 2022
Default avatar
By Pankaj
Developer and author at DigitalOcean.
Mediator Design Pattern in Java

While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.

Mediator design pattern is one of the behavioral design pattern, so it deals with the behaviors of objects. Mediator design pattern is used to provide a centralized communication medium between different objects in a system.

Mediator Design Pattern

mediator pattern, mediator design pattern, mediator pattern java According to GoF, mediator pattern intent is:

Allows loose coupling by encapsulating the way disparate sets of objects interact and communicate with each other. Allows for the actions of each object set to vary independently of one another.

Mediator design pattern is very helpful in an enterprise application where multiple objects are interacting with each other. If the objects interact with each other directly, the system components are tightly-coupled with each other that makes higher maintainability cost and not hard to extend. Mediator pattern focuses on provide a mediator between objects for communication and help in implementing lose-coupling between objects. Air traffic controller is a great example of mediator pattern where the airport control room works as a mediator for communication between different flights. Mediator works as a router between objects and it can have it’s own logic to provide way of communication. The system objects that communicate each other are called Colleagues. Usually we have an interface or abstract class that provides the contract for communication and then we have concrete implementation of mediators. For our example, we will try to implement a chat application where users can do group chat. Every user will be identified by it’s name and they can send and receive messages. The message sent by any user should be received by all the other users in the group.

Mediator Pattern Interface

First of all we will create Mediator interface that will define the contract for concrete mediators. ChatMediator.java

package com.journaldev.design.mediator;

public interface ChatMediator {

	public void sendMessage(String msg, User user);

	void addUser(User user);
}

Mediator Pattern Colleague Interface

Users can send and receive messages, so we can have User interface or abstract class. I am creating User as abstract class like below. User.java

package com.journaldev.design.mediator;

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);
}

Notice that User has a reference to the mediator object, it’s required for the communication between different users.

Concrete Mediator

Now we will create concrete mediator class, it will have a list of users in the group and provide logic for the communication between the users. ChatMediatorImpl.java

package com.journaldev.design.mediator;

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);
			}
		}
	}

}

Mediator Design Pattern Concrete Colleague

Now we can create concrete User classes to be used by client system. UserImpl.java

package com.journaldev.design.mediator;

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);
	}

}

Notice that send() method is using mediator to send the message to the users and it has no idea how it will be handled by the mediator.

Mediator Pattern Example Client Program Code

Let’s test this our chat application with a simple program where we will create mediator and add users to the group and one of the user will send a message. ChatClient.java

package com.journaldev.design.mediator;

public class ChatClient {

	public static void main(String[] args) {
		ChatMediator mediator = new ChatMediatorImpl();
		User user1 = new UserImpl(mediator, "Pankaj");
		User user2 = new UserImpl(mediator, "Lisa");
		User user3 = new UserImpl(mediator, "Saurabh");
		User user4 = new UserImpl(mediator, "David");
		mediator.addUser(user1);
		mediator.addUser(user2);
		mediator.addUser(user3);
		mediator.addUser(user4);
		
		user1.send("Hi All");
		
	}

}

Notice that client program is very simple and it has no idea how the message is getting handled and if mediator is getting user or not. Output of the mediator pattern example program is:

Pankaj: Sending Message=Hi All
Lisa: Received Message:Hi All
Saurabh: Received Message:Hi All
David: Received Message:Hi All

Mediator Pattern Class Diagram

mediator pattern class diagram, mediator design pattern java

Mediator Pattern Example in JDK

Mediator Design Pattern Important Points

  • Mediator pattern is useful when the communication logic between objects is complex, we can have a central point of communication that takes care of communication logic.
  • Java Message Service (JMS) uses Mediator pattern along with Observer pattern to allow applications to subscribe and publish data to other applications.
  • We should not use mediator pattern just to achieve lose-coupling because if the number of mediators will grow, then it will become hard to maintain them.

That’s all for mediator design pattern and it’s implementation in java.

Thanks for learning with the DigitalOcean Community. Check out our offerings for compute, storage, networking, and managed databases.

Learn more about us


About the authors
Default avatar
Pankaj

author

Developer and author at DigitalOcean.

Still looking for an answer?

Ask a questionSearch for more help

Was this helpful?
 
JournalDev
DigitalOcean Employee
DigitalOcean Employee badge
June 4, 2021

This is something mix of mediator and visitor

- Fatih

    JournalDev
    DigitalOcean Employee
    DigitalOcean Employee badge
    December 3, 2020

    nice explanation, but i can’t quite see the difference between mediator pattern and facade pattern, what is the difference between these two ?

    - mohamed

      JournalDev
      DigitalOcean Employee
      DigitalOcean Employee badge
      October 26, 2017

      hello,please help me i want to know, how use jms from mediator pattern? thankful

      - fatemeh

        JournalDev
        DigitalOcean Employee
        DigitalOcean Employee badge
        November 23, 2016

        Nice example.

        - robothy

          JournalDev
          DigitalOcean Employee
          DigitalOcean Employee badge
          November 1, 2016

          I knew at this and I can´t see the differences between mediator pattern and observe pattern

          - David

            JournalDev
            DigitalOcean Employee
            DigitalOcean Employee badge
            March 25, 2016

            Nice explanation.

            - Annappa

              JournalDev
              DigitalOcean Employee
              DigitalOcean Employee badge
              September 27, 2015

              Really crisp and good writeups

              - Ashwin

                JournalDev
                DigitalOcean Employee
                DigitalOcean Employee badge
                September 14, 2015

                how mediator pattern is implemented in ExecutorService API.Kindly explain

                - Aditya

                  JournalDev
                  DigitalOcean Employee
                  DigitalOcean Employee badge
                  July 31, 2015

                  hi. tanck you for your nice tutorial.

                  - mohammad

                    JournalDev
                    DigitalOcean Employee
                    DigitalOcean Employee badge
                    July 31, 2015

                    Awesome explaination thanks :)

                    - Mohit Gupta

                      Try DigitalOcean for free

                      Click here to Sign up and get $200 of credit to try our products over 60 days!