Mediator design pattern

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. 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 providing a mediator between objects for communication and help in implementing loose-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.

UML Diagram Mediator design pattern

Design components

• Mediator :It defines the interface for communication between colleague objects.
• ConcreteMediator : It implements the mediator interface and coordinates communication between colleague objects.
• Colleague : It defines the interface for communication with other colleagues
• ConcreteColleague : It implements the colleague interface and communicates with other colleagues through its mediator

Let’s see an example of Mediator design pattern.

Java

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;
    }
}

interface Command
{
    void land();
}

interface IATCMediator
{

    public void registerRunway(Runway runway);

    public void registerFlight(Flight flight);

    public boolean isLandingOk();

    public void setLandingStatus(boolean status);
}

class Flight implements Command
{
    private IATCMediator atcMediator;

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

    public void land()
    {
        if (atcMediator.isLandingOk())
        {
            System.out.println("Successfully Landed.");
            atcMediator.setLandingStatus(true);
        }
        else
            System.out.println("Waiting for landing.");
    }

    public void getReady()
    {
        System.out.println("Ready for landing.");
    }

}

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);
    }

}

class MediatorDesignPattern
{
    public static void main(String args[])
    {

        IATCMediator atcMediator = new ATCMediator();
        Flight sparrow101 = new Flight(atcMediator);
        Runway mainRunway = new Runway(atcMediator);
        atcMediator.registerFlight(sparrow101);
        atcMediator.registerRunway(mainRunway);
        sparrow101.getReady();
        mainRunway.land();
        sparrow101.land();

    }
}

Output:

Ready for landing.
Landing permission granted.
Successfully Landed.

Advantage

• It limits subclassing. A mediator localizes behavior that otherwise would be distributed among several objects. Changing this behaviour requires subclassing Mediator only, Colleague classes can be reused as is.

Disadvantage

• It centralizes control. The mediator pattern trades complexity of interaction for complexity in the mediator. Because a mediator encapsulates protocols, it can become more complex than any individual colleague. This can make the mediator itself a monolith that’s hard to maintain

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