TDDB84 Design Patterns Lecture 06

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1 Lecture 06 Mediator, Adapter, Bridge Peter Bunus Dept of Computer and Information Science Linköping University, Sweden The Mediator Peter Bunus 2 1

2 Peter Bunus 3 The Mediator Non Software Example Control Tower Mediator Flight 34 Flight 456 Air Force One Flight 111 The Mediator defines an object that controls how a set of objects interact. The pilots of the planes approaching or departing the terminal area communicate with the tower, rather than explicitly communicating with one another. The constraints on who can take off or land are enforced by the tower. the tower does not control the whole flight. It exists only to enforce constraints in the terminal area. Peter Bunus 4 2

3 The Mediator Another Example Bob lives in the HouseOfFuture where everthing is automated: When Bob hits the snooze button of the alarm the coffee maker starts brewing coffee No coffee in weekends... onevent(){ checkcalendar(); checksprinkler(); startcoffee(); //do more stuff onevent(){ checkcalendar(); checkalarm(); //do more stuff onevent(){ checkdayoftheweek(); doshower(); docoffee(); doalarm(); //do more stuff onevent(){ checkcalendar(); checkshower(); checktemperature //do more stuff Peter Bunus 5 The Mediator in Action With a Mediator added to the system all the appliance objects can be greatly simplified They tell the mediator when their state changes They respond to requests from the Mediator It s such a relief, not having to figure out that Alarm clock picky rules if(alarmevent)(){ checkcalendar(); checkshower(); checktemp(); //do more stuff if(weekend){ checkweather(); if(trashday){ resetalarm(); Peter Bunus 6 3

4 Mediator and MFC (Microsoft Foundation Classes) The Client creates afontdialog and invokes it. The list box tells the FontDialog ( it's mediator ) that it has changed The FontDialog (the mediator object) gets the selection from the list box The FontDialog (the mediator object) passes the selection to the entry field edit box Peter Bunus 7 Actors in the Mediator Pattern Mediator defines an interface for communicating with Colleague objects ConcreteMediator implements cooperative behavior by coordinating Colleague objects knows and maintains its colleagues Colleague classes (Participant) each Colleague class knows its Mediator object (has an instance of the mediator) each colleague communicates with its mediator whenever it would have otherwise communicated with another colleague Peter Bunus 8 4

5 Yet Another Example Robbery in progress. I need backup Officer down, officer down!!! We have casualties Peter Bunus 9 Mediator advantages and disadvantages Changing the system behavior means just sub classing the mediator. Other objects can be used as is. Since the mediator and its colleagues are only tied together by a loose coupling, both the mediator and colleague classes can be varied and reused independent of each other. Since the mediator promotes a One-to-Many relationship with its colleagues, the whole system is easier to understand (as opposed to a many-to-many relationship where everyone calls everyone else). It helps in getting a better understanding of how the objects in that system interact, since all the object interaction is bundled into just one class - the mediator class. Since all the interaction between the colleagues are bundled into the mediator, it has the potential of making the mediator class very complex and monolithically hard to maintain. Peter Bunus 10 5

6 Issues When an event occurs, colleagues must communicate that event with the mediator. This is somewhat reminiscent of a subject communicating a change in state with an observer. One approach to implementing a mediator, therefore, is to implement it as an observer following the observer pattern. Peter Bunus 11 Time For Course Evaluation What I have liked about this course What I didn t liked about this course What should be improved Peter Bunus 12 6

7 The Adapter Peter Bunus 13 Adapter Non Software Example Peter Bunus 14 7

8 Time to be Adaptive Remember my Ducks? public interface Duck { public void quack(); public void fly(); public class MallardDuck implements Duck { public void quack() { System.out.println("Quack"); I m flying Quack public void fly() { System.out.println("I'm flying"); Peter Bunus 15 The New Bird Now it is time for the newest fowl on the block public interface Turkey { public void gobble(); public void fly(); public class WildTurkey implements Turkey { public void gobble() { System.out.println("Gobble gobble"); public void fly() { System.out.println("I'm flying a short distance"); Peter Bunus 16 8

9 Adapting Turkeys Now let s say you are short on Ducks and you would like to use some Turkey objects in their place. So let s write an adapter. public class TurkeyAdapter implements Duck { Turkey turkey; public TurkeyAdapter(Turkey turkey) { this.turkey = turkey; public void quack() { turkey.gobble(); First you need to adapt the interface of the type you are adapting to. This is the interface your client expect to see Next we need to get a refence to the object that we are adapting Now we need to implement all the methods in the interface public void fly() { for(int i=0; i < 5; i++) { turkey.fly(); Even though both interface have a fly() method, Turkeys fly a short spurts they can do long distance flying like ducks Peter Bunus 17 Duck Test Drive public class DuckTestDrive { public static void main(string[] args) { MallardDuck duck = new MallardDuck(); WildTurkey turkey = new WildTurkey(); Duck turkeyadapter = new TurkeyAdapter(turkey); System.out.println("The Turkey says..."); turkey.gobble(); turkey.fly(); System.out.println("\nThe Duck says..."); testduck(duck); System.out.println("\nThe TurkeyAdapter says..."); testduck(turkeyadapter); static void testduck(duck duck) { duck.quack(); duck.fly(); Peter Bunus 18 9

10 Adapting Ducks public class DuckAdapter implements Turkey { Duck duck; Random rand; public DuckAdapter(Duck duck) { this.duck = duck; rand = new Random(); public void gobble() { duck.quack(); public void fly() { if (rand.nextint(5) == 0) { duck.fly(); Peter Bunus 19 Turkey Test Drive public class TurkeyTestDrive { public static void main(string[] args) { MallardDuck duck = new MallardDuck(); Turkey duckadapter = new DuckAdapter(duck); for(int i=0;i<10;i++) { System.out.println("The DuckAdapter says..."); duckadapter.gobble(); duckadapter.fly(); Peter Bunus 20 10

11 Adapter Example Peter Bunus 21 Class Adapter A class adapter uses multiple inheritance to adapt one interface to another Peter Bunus 22 11

12 Object Adapter An object adapter relies on object composition Peter Bunus 23 The Bridge Peter Bunus 24 12

13 Bridge Non Software Example Peter Bunus 25 Remote Control Imagine that you are going to revolutionize extreme lounging by writting the code for a new remote control for TVs There will be a lots of implementations one for each model of a TV Peter Bunus 26 13

14 The Remote Control Example This an abstraction. It could be an iterface or an abstract class RemoteControl +on() +off() +setchannel() Every Remote has the same abstraction LGControl SonyControl GrundigControl Lots of implementations for each TV +on() +off() +setchannel() +on() +off() +setchannel() { tunechannel(chanel) +on() +off() +setchannel() Peter Bunus 27 Dilema A product usually is refined many times. So the remote are going to change We have already abstracted the user interface so we can vary the implementation over many TVs We need to vary the abstraction as well since remotes are changing (eventually improving) Peter Bunus 28 14

15 The Bridge Pattern Remote Control Example The Bridge allows you to vary the implementation and the abstraction by placing the two in a separate class hierarchy Abstraction class hierarchy The relantionship between the two is refered to as the bridge Implementation class hierarchy All methods in the abstraction are implemented in terms of the abstraction Concrete subclasses are implemented in terms of the abstraction, not the implementation Peter Bunus 29 Advantages/Disadvantages Decouples an implementation so that is not bound permanently to an interface Abstraction and implementation can be extended independently Changes to concrete abstraction classes don t affect the client Useful in graphic and windowing systems that need to run over multiple platforms Useful any time you need to vary an interface and an implementation in different ways Increases complexity Peter Bunus 30 15

16 The Bridge Pattern Structure Peter Bunus 31 The Bridge Example Peter Bunus 32 16

17 Bridge An Example Peter Bunus 33 17

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