Java Design Pattern: Mediator

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Mediator design pattern is used to collaborate a set of colleagues. Those colleagues do not communicate with each other directly, but through the mediator.

In the example below, Colleague A want to talk, and Colleague B wants to fight. When they do some action(i.e., doSomething()), they invoke mediator to do that.

Mediator Class Diagram

mediator-design-pattern

Mediator Java Code

package designpatterns.mediator;
 
interface IMediator {
	public void fight();
	public void talk();
	public void registerA(ColleagueA a);
	public void registerB(ColleagueB a);
}
 
//concrete mediator
class ConcreteMediator implements IMediator{
 
	ColleagueA talk;
	ColleagueB fight;
 
	public void registerA(ColleagueA a){
		talk = a;
	}
 
	public void registerB(ColleagueB b){
		fight = b;
	}
 
	public void fight(){
		System.out.println("Mediator is fighting");
		//let the fight colleague do some stuff
	}
 
	public void talk(){
		System.out.println("Mediator is talking");
		//let the talk colleague do some stuff
	}
}
 
abstract class Colleague {
	IMediator mediator;
	public abstract void doSomething();
}
 
//concrete colleague
class ColleagueA extends Colleague {
 
	public ColleagueA(IMediator mediator) {
		this.mediator = mediator;
	}
 
	@Override
	public void doSomething() {
		this.mediator.talk();
		this.mediator.registerA(this);
	}
}
 
//concrete colleague
class ColleagueB extends Colleague {
	public ColleagueB(IMediator mediator) {
		this.mediator = mediator;
		this.mediator.registerB(this);
	}
 
	@Override
	public void doSomething() {
		this.mediator.fight();
	}
}
 
public class MediatorTest {
	public static void main(String[] args) {
		IMediator mediator = new ConcreteMediator();
 
		ColleagueA talkColleague = new ColleagueA(mediator);
		ColleagueB fightColleague = new ColleagueB(mediator);
 
		talkColleague.doSomething();
		fightColleague.doSomething();
	}
}

Among other behavioral pattern, Observer is the most similar pattern with Mediator. You can read Observer pattern to compare the difference.

2 thoughts on “Java Design Pattern: Mediator”

  1. You have an error in CollegueA code:
    this.mediator.registerA(this) should be called in constructor, not in doSomething() method.

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