Mediator Pattern

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Mediator Pattern - Object Behavioral Pattern

Intent

Define an object that encapsulates how a set of objects interact. The Mediator pattern promotes loose coupling by keeping objects from referring to each other explicitly, allowing you to vary their interaction independently.

Structure

Mediator Pattern

Participants:

Mediator: Defines an interface for communicating with Colleague objects.
ConcreteMediator: Implements cooperative behavior by coordinating Colleague objects; it knows and maintains its colleagues.
Colleague class: Each colleague knows its Mediator object; colleagues communicate with the mediator whenever they would have otherwise communicated with another colleague.

Applicability

Use the Mediator pattern when:

A set of objects communicate in well-defined but complex ways, resulting in tangled and hard-to-understand dependencies.
An object references many other objects and communicates with them directly, making the object difficult to reuse.
A behavior distributed among several classes should be customizable without creating many subclasses.

Example 1

Online payment is a crucial part of e-commerce, and different platforms provide different payment interfaces. We need to integrate these various interfaces so that customers don’t have to worry about specific payment details when shopping. Here is a Mediator pattern implementation for this requirement:

Mediator Pattern Example

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interface WebServiceMediator {
    public void buy(double money, WebService service);
    public void SetAmazon(WebService amazon);
    public void SetEbay(WebService ebay);
}

abstract class WebService {
    protected WebServiceMediator mediator;
    public abstract void SetMediator(WebServiceMediator mediator);
    public abstract void buyService(double money);
    public abstract void search(double money);
}

class ConcreteServiceMediator implements WebServiceMediator {
    private WebService amazon;
    private WebService ebay;
    
    public ConcreteServiceMediator() {
        amazon = null;
        ebay = null;
    }
    
    public void SetAmazon(WebService amazon) {
        this.amazon = amazon;
    }
    
    public void SetEbay(WebService ebay) {
        this.ebay = ebay;
    }
    
    public void buy(double money, WebService service) {
        if (service == amazon)
            amazon.search(money);
        else
            ebay.search(money);
    }
}

class Amazon extends WebService {
    public void SetMediator(WebServiceMediator mediator) {
        this.mediator = mediator;
    }
    
    public void buyService(double money) {
        mediator.buy(money, this);
    }
    
    public void search(double money) {
        System.out.println("Amazon received: " + money);
    }
}

class Ebay extends WebService {
    public void SetMediator(WebServiceMediator mediator) {
        this.mediator = mediator;
    }
    
    public void buyService(double money) {
        mediator.buy(money, this);
    }
    
    public void search(double money) {
        System.out.println("Ebay received: " + money);
    }
}

Example 2

Simple communication between two colleagues.

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public class MediatorPattern {
    public static void main(String[] args) {
        ConcreteMediator m = new ConcreteMediator();
        Colleague1 c1 = new Colleague1(m);
        Colleague2 c2 = new Colleague2(m);

        m.setC1(c1);
        m.setC2(c2);

        c1.sendMessage("hello");
        c2.sendMessage("hi");
    }
}

abstract class Colleague{
    protected Mediator mediator;
}

class Colleague1 extends Colleague{
    public Colleague1(Mediator mediator){
        this.mediator = mediator;
    }

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

    public void Notify(String message){
        System.out.println("Colleague1 received: " + message);
    }
}

class Colleague2 extends Colleague{
    public Colleague2(Mediator mediator){
        this.mediator = mediator;
    }

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

    public void Notify(String message){
        System.out.println("Colleague2 received: " + message);
    }
}

abstract class Mediator{
    public abstract void sendMessage(String message, Colleague c);
}

class ConcreteMediator extends Mediator{
    // Defined directly since there are few objects in this example
    private Colleague1 c1;
    private Colleague2 c2;
    
    // If there were many colleagues:
    // List<Colleague> list = new ArrayList<>();
    // public void Add(Colleague c){
    //     list.add(c);
    // }

    public void setC1(Colleague1 c1){
        this.c1 = c1;
    }
    public void setC2(Colleague2 c2){
        this.c2 = c2;
    }

    @Override
    public void sendMessage(String message, Colleague c){
        if(c == c1){
            // Pass message to colleague 2
            c2.Notify(message);
        }else{
            // Pass message to colleague 1
            c1.Notify(message);
        }
    }
}