# Mediator

Paul Tomoiu

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Examples,

Design patterns

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Published 2022-12-03

This tutorial explains to you the design pattern named Mediator (which is a Structural Pattern).

# Mediator Pattern - theory

Mediator Design Pattern in Java is used when we want more objects communicate one with each other in a simple manner. It helps to promote a looser coupling and centralizes the control. This pattern is used in message-based systems.

Take a look at the following UML diagram representing the Mediator design pattern (for my example):

In this example you can see how a list of users are defined into a mediator class. When a user send a message the others receive it.

# Mediator Pattern - example

Here is an example of using the Mediator Design Pattern in Java:

package mediator.java.pattern.example;
 
public class User {
       private String name;
 
       public User(String name){
          this.name  = name;
       }
 
       public void sendMessage(String message){
           
           System.out.println("The user "+name+" SEND the message '"+ message +"' to ALL "); 
           MediatorClass.sendToAllMessage(this,message);
           
       }
        
       public void receiveMessage(String message){
           System.out.println("The user "+name+" receive the message: "+message); 
       }
        
    }

package mediator.java.pattern.example;
 
import java.util.ArrayList;
import java.util.List;
 
public class MediatorClass {
     
    private static List<User> users;
 
    public MediatorClass(){
        MediatorClass.users=new ArrayList<>();
    }
 
     
    public static void sendToAllMessage(User user, String message){
        for (User u : users){   
            //message should not be received by the user sending it
            if(u != user){
                 u.receiveMessage(message);
            }
        }
    }   
 
    public static void addUser(User user){
        users.add(user);
    }
 
}

package mediator.java.pattern.example;
 
public class MediatorJavaPatternExample {
 
     
    public static void main(String[] args) {
          User robert = new User("Robert");
          User john = new User("John");
          User dana = new User("Dana");
 
          MediatorClass mediator = new MediatorClass();
           
          MediatorClass.addUser(robert);
          MediatorClass.addUser(john);
          MediatorClass.addUser(dana);      
           
          robert.sendMessage("Hi!");
          System.out.println("\n");
          john.sendMessage("Hello!");
       }
}

When you run this example you will receive the following result: