Mediator Design Pattern

Last modified on August 1st, 2014 by Joe.

Behavioral design pattern can change the way programs run. Mediator design pattern is one of the important and widely used behavioral design pattern. Mediator enables decoupling of objects by introducing a layer in between so that the interaction between objects happen via the layer.

In an enterprise application where we have large number of classes the complexity also increases proportionately. Business logic might be spread across those multiple classes. There will be interaction between classes. If we draw line between classes where we have interaction, then it will end up like Idiyappam (Indian rice noodle).

Before Mediator Design Pattern

So what is the problem we are trying to solve here? This complex interaction between objects creates dependency and tighter coupling. We stand for loose coupling, we want to reduce dependency as much as possible, we want more reuse.

After Mediator Design Pattern

Define an object that acts as a mediator between communicating objects thus removing the direct dependency between those communicating objects. This mediator object encapsulates the interaction information and uses it to enable communication between the objects.

The intermediate mediator object is not only to communicate between two objects, but it helps in interaction for a set of peer objects. This mediator object is like a router in networks, which is routes / controls or coordinates communication between systems.

Real-time Example for Mediator Design Pattern

Air traffic controller (ATC) is a mediator between flights. It helps in communication between flights and co-oridates/controls landing, take-off. Two flights need not interact directly and there is no dependency between them. This dependency is solved by the mediator ATC. If ATC is not there all the flights have to interact with one another and managing the show will be very difficult and things may go wrong.

Mediator Pattern Implementation

In a mediator design pattern implementation we will have

mediator interface – an interface that defines the communication rules between objects
concrete mediator – a mediator object which will enables communication between participating objects
colleague – objects communicating with each other through mediator object

Java Example for Mediator Desing Pattern

package com.javapapers.designpattern.mediator;

public class ATCMediator implements IATCMediator {
	private Flight flight;
	private Runway runway;
	public boolean land;

	public void registerRunway(Runway runway) {
		this.runway = runway;
	}

	public void registerFlight(Flight flight) {
		this.flight = flight;
	}

	public boolean isLandingOk() {
		return land;
	}

	@Override
	public void setLandingStatus(boolean status) {
		land = status;

	}
}

package com.javapapers.designpattern.mediator;

public interface Command {
	void land();
}

package com.javapapers.designpattern.mediator;

public interface IATCMediator {

	public void registerRunway(Runway runway);

	public void registerFlight(Flight flight);

	public boolean isLandingOk();

	public void setLandingStatus(boolean status);
}

package com.javapapers.designpattern.mediator;

public class Flight implements Command {
	private IATCMediator atcMediator;

	public Flight(IATCMediator atcMediator) {
		this.atcMediator = atcMediator;
	}

	public void land() {
		if (atcMediator.isLandingOk()) {
			System.out.println("Landing done....");
			atcMediator.setLandingStatus(true);
		} else
			System.out.println("Will wait to land....");
	}

	public void getReady() {
		System.out.println("Getting ready...");
	}

}

package com.javapapers.designpattern.mediator;

public class Runway implements Command {
	private IATCMediator atcMediator;

	public Runway(IATCMediator atcMediator) {
		this.atcMediator = atcMediator;
		atcMediator.setLandingStatus(true);
	}

	@Override
	public void land() {
		System.out.println("Landing permission granted...");
		atcMediator.setLandingStatus(true);
	}

}