designpatterns-06.pdf

7/29/2019 designPatterns-06.pdf

1/19

The Decorator Pattern

CSCI 3132 Summer 20111


2/19

StarbuzzCoffee

Wanttoofferavarietyofcombina5onsofcoffeeandcondiments

Costofacupdependsonthecombina5onthatwasordered

2


3/19

FirstDesign

Makeabeverageclassandasubclassforeachlegalcombina5on

3

HouseBlend

cost()

DarkRoast

cost()

Decaf

cost()

Espressocost

()

HouseBlendWithSteamedMilkcost()

DarkRoastWithSteamedMilkcost()

DecafWithSteamedMilkcost()

EspressoWiSteamedMicost()HouseBlend

Mochacost()

DarkRoastMochacost()

DecafMochacost()

EspressoMochacost()HouseBlend

WithWhip

cost

DarkRoastWithWhip

DecafWithWhip

cost

EspressoWithWhipcost()

BeverageString description

getDescription() cost()

3


4/19

ProblemswithInheritance

Toomanysubclassesmaybeneeded. Needtodefinetheimplementa5onofeach

subclass.

Allclassesinheritasta5cbehaviourwhichmayhavetobeoverridden.

Theinheritedbehaviourcannotbechangedatrun5me,i.e.behaviourissta5catrun5me.

Thus,thedesignisnotflexibleormaintainable.

4


5/19

SecondDesign

Makethesuperclasscontainbooleanstospecifywhichcondimentsareincludedandsubclassesforeachtypeofcoffee

Howdowecomputecost? Whatdoesittaketoaddanewcondiment?

5


6/19

BeverageString descriptionmilk

soymochawhipgetDescription()cost

()

hasMilk()setMilk()hasSoy()setSoy()hasMocha()setMocha()hasWhip()setWhip()

HouseBlend

cost()

DarkRoastcost()

Decafcost()

Espressocost()

6


7/19

DesignPrinciple

Classesshouldbeopenforextension,butclosedformodifica5on

extensionisNOTsubclassing

Itmeanstheaddi5onofnewbehavior(withoutmodifyingthecode!)

7


8/19

DecoratorPaern

Startwithaninstanceofthebasicclassesandthendecorateitwithnewcapabili5es

Dark Roast

Mocha

cost()cost()

Whip

cost() 0.990.99+0.20

0.99+0.20+0.10

8


9/19

KeyPoints

Decoratorshavethesamesupertypesastheobjectstheydecorate

Thisletsuspassaroundthedecoratedobjectinsteadoftheoriginal(unwrapped)object

Decoratoraddbehaviorbydelega5ngtotheobjectitdecoratesandthenaddingitsownbehavior

Canadddecora5onsatany5me

9


10/19

ClassDiagramfortheDecoratorPaern

10


11/19

StarbuzzCoffeeExample

11


12/19

public abstract class Beverage{!String description = Unknown;!public String getDescription() {!return description;!}!public abstract double cost();!

}!public abstract class CondimentDecorator extends Beverage {!public abstract String getDescription();!

public abstract double cost();!}!public class Espresso extends Beverage {!

public Espresso() {!description = Espresso;

!}!public double cost() {!

return 1.99;!}!

}! 12


13/19

public class Mocha extends CondimentDecorator {!Beverage bev;!public Mocha(Beverage bev) {!

this.bev = bev;!}!public String getDescription {!

return bev.getDescription() + , Mocha;!}!public double cost() {!

return .20 + bev.cost();!}!

}!

13


14/19

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

Beverage bev = new Espresso); !bev = new Mocha(bev);!bev = new Mocha(bev);!bev = new Whip(bev);!bev = new Soy(bev);!System.out.println(bev.getDescription() + $+!

bev.getCost()); !}!

14


15/19

DecoratorsinJava

FileI/OExample

FileInputStream

BufferedInputStream

LineNumberInputStream

Component

Concrete decorators

15


16/19

DesignPrinciplefortheDecoratorPaern

Classesshouldbeopenforextensionbutclosedformodifica5on.

Systemscanbeextendedwithoutchangingexis5ngcode. Tradeoffs:

Takes5meandeffort. Introducesnewlevelsofabstrac5onwhichmakes

designsmorecomplicatedandcodehardtounderstand.

Duetothetradeoffsthedecoratorpaernshouldnotbeusedthroughoutthedesignbutinareasthataremostlikelytochange.

Experienceandlookingatotherexampleshelpsonedeterminewhichareasarelikelytochange.

16


17/19

DecoratorPaern

Providesaflexiblealterna5vetousinginheritancetoextendfunc5onality.

Thisachievedbyintroducingdecoratorsthatdecorateobjects. Decoratorshavethesametypeastheobjects

theydecorate.

Anobjectcanhaveoneormoredecorators.17


18/19

TheDecoratorPaern

Thedecoratedobjectandtheoriginalobjecthavethesametype.Thus,thedecoratedobjectcanbepassedinsteadoftheoriginal

object. Thedecoratordelegatespartofitsbehaviour

totheobjectitdecorates.

ItaddsitsownbehaviourbeforeoraZerthedelega5on.

Objectscanbedelegateddynamicallyatrun5me.

Theobjectsarewrappedwithdecorators.18


19/19

DecoratorPaernSummary

Maindesignprinciple:Classesshouldbeopenforextensionbutclosedformodifica5on.

Achievesflexibility-behaviourcanbeextendedwithoutchangingexis5ngcode.

Composi5onanddelega5onisusedtoaddnewbehavioursatrun5me.

Asetofdecoratorclassesareusedtowrapconcretecomponents. Overuseofdecoratorscanbecomplexastheycanintroduceanumberofsmallobjectsintoadesign.

19

designpatterns-06.pdf

Documents