designpatterns-08.pdf

7/29/2019 designPatterns-08.pdf

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The Abstract Factory Pattern

and

Singleton Pattern

CSCI 3132 Summer 20111


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SimpleFactoryPa0ern

Client

orderProduct()SimpleFactory

createProduct()

Product

productMethod()

ConcreteProductB

ConcreteProductA

ConcreteProductC

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FactoryMethodStructure

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CreatorClass

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ProductClass

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Advantage

TheCreator(PizzaStore)isnot?ghtlycoupledtoanyconcreteproduct(Pizza).

Instan?a?ngconcreteclassesisanareaoffrequentchange.Byencapsula?ngitusingfactoriesweavoidcodeduplica?on

(whichisacodesmell)andmakeiteasiertoembracechange

duringdevelopmentortoperformmaintenance.

TheFactoryPa0ernalsoillustratetheprincipleofcodingtoabstrac?ons(thePizzaproductandthePizzaStoreclientare

abstracts)

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Thisispossiblyadevelopmentandmaintenancenighmare7


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ThisiswhattheFactoryMethodpa0ernhasachieved:

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TheAbstractFactory

Ourcodecannowworkwithdifferentconcretefactories,throughaFactory

interface(Pizzastore) Whatifweneedtocreateseveraltypesof

products,notjustasingletype

Pizzadough(ThininNY,ThickinChicago)Sauce()

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TheAbstractFactory

Answerseemssimple:justuseFactoryMethodpa0erntwice

pizzadough

ThickCrustDough ThinCrustDough

pizzaSauce

PlumTomatoSauce MarinaraSauce

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TheAbstractFactory

Thislooksfine butdoesitreflectourinten?on Woulditmakesensetohaveapizza,with

ChicagoThickCrust+NYMarinaraSauce Modeldoesnotincludeanybinding

betweenrelatedproducts

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TheAbstractFactory

ADoughandaSaucearenotasseenfromatypepoint-of-viewrelated

Wouldbesomewhatar?ficialorperhapsevenimpossibletointroduceacommonbaseclass

However,wecanenforcethebindingthroughasharedfactoryclass!

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TheAbstractFactory

PizzaIngredientFactorycreateDough()

createSauce()

NYPizzaIngredientFactory ChicagoPizzaIngredientFactory

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MoreIngredience Wewanttolisttheexactlistofingredientsforeachconcrete

pizza.Forexample:

ChicagoCheesePizza:PlumtomatoSauce,Mozzarella,Parmesan,Oregano;

NewYorkCheesePizza:MarinaraSauce,Reggiano,Garlic. Eachstyleusesadifferentsetofingredients, WecouldchangeimplementPizzaasin:

public class Pizza {

Dough dough;

Sauce sauce;

Veggies veggies[];

Cheese cheese;

Pepperoni pepperoni;

Clams clam;

. . .

}

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andtheconstructornaturallybecomessomethinglike:public Pizza(Dough d, Sauce s, Cheese c, Veggies v,

Pepperoni p, Clams c)

{

dough = d;

sauce = s;

veggies = v;

cheese = c;

pepperoni = p;

clam = c;

}

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butthen:public class NYPizzaStore extends PizzaStore {

Pizza createPizza(String item) {

if (item.equals("cheese")) {

return new NYStyleCheesePizza(new ThinCrustDough(),

new Marinara(), new Reggiano(), null, null );

} else if (item.equals("veggie")) {

return new NYStyleVeggiaPizza(new ThinCrustDough(),new Marinara(), new Reggiano(), mew Garlic() ,

null);

}

}

This will cause a lot of maintenanceheadaches! Imagine what happens when wecreate a new pizza!

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WeknowthatwehaveacertainsetofingredientsthatareusedforNewYork..yetwehavetokeeprepea?ngthatsetwitheach

constructor.Canwedefinethisuniquesetjustonce A`erallwearecrea?ngconcreteinstancesofdough,

ingredientsetc.:

letsusethefactoryofingredients!public interface PizzaIngredientFactory {

public Dough createDough();

public Sauce createSauce();

public Cheese createCheese();

public Veggies[] createVeggies();

public Pepperoni createPepperoni();public Clams createClam();

}

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Wethenprogramtotheinterfacebyimplemen?ngdifferentconcreteingredientsfactories.Forexample

herearetheingredientsusedinNewYorkpizzastyle:

public class NYPizzaIngredientFactory : PizzaIngredientFactory {

public Dough createDough() {

return new ThinCrustDough();}

public Sauce createSauce() {

return new MarinaraSauce();

}

public Cheese createCheese() {

return new ReggianoCheese();

}

//...

}

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OurPizzaclasswillremainanabstractclass:public abstract class Pizza {

protected string name;protected Dough dough;

protected Sauce sauce;

protected ArrayList toppings = new ArrayList();

abstract void Prepare(); //now abstract

public virtual string Bake() {Console.WriteLine("Bake for 25 minutes at 350 \n);

}

public virtual string Cut() {

Console.WriteLine("Cutting the pizza into diagonal slices \n);

}

// ...}

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andourconcretePizzasimplybevome:public class CheesePizza : Pizza {PizzaIngredientFactory ingredientFactory;

public CheesePizza(PizzaIngredientFactory ingredientFactory) {

this.ingredientFactory = ingredientFactory;

}

void prepare() {dough = ingredientFactory.createDough();

sauce = ingredientFactory.createSauce();

cheese = ingredientFactory.createCheese();

}

}

thecrea?onoftheingredientsisdelegatedtoafactory.

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finallywemusthaveourconcretePizzastoree.g.public class NYPizzaStore : PizzaStore {protected Pizza createPizza(String item) {

Pizza pizza = null;

PizzaIngredientFactory ingredientFactory =

new NYPizzaIngredientFactory();

if (item.equals("cheese")) {

pizza = new CheesePizza(ingredientFactory);

} else if (item.equals("veggie")) {

pizza = new VeggiePizza(ingredientFactory);

} else if (item.equals("clam")) {

pizza = new ClamPizza(ingredientFactory);

} else if (item.equals("pepperoni")) {

pizza = new PepperoniPizza(ingredientFactory);

}

return pizza;

}

}

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AbstractFactoryPa0ern

AbstractFactorycreateProductA()

createProductB()

AbstractProductA

AbstractProductB

Client

ConcreteFactory1

createProductA()createProductB()

ConcreteFactory2

createProductA()createProductB()

ConcreteProductA1

ConcreteProductA2

ConcreteProductB1

ConcreteProductB2

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AbstractFactorySummary TheAbstractFactoryPa0ernprovidesaninterfaceforcrea?ngfamiliesofrelatedordependentobjectswithoutspecifying

theirconcreteclasses.

FactoryMethod: UsesinheritancetocreateaConcreteProduct SubclassesdecidewhichConcreteProducttouse

AbstractFactory: Usescomposi?ontocreateobjects Theobjectscreatedwereapartofafamilyofobjects.For

example,NYregionhadaspecificsetofingredients.

AnabstractfactoryactuallycontainsoneormoreFactoryMethods!

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TheAbstractFactory

Bymakingacreatorclasswithseveralcreatemethods,werestricttheproductcombina?onsthe

clientcancreate

ThemethodsintheAbstractFactoryareproduct-typedependent,soifweaddanotherproduct,we

needtochangetheinterfaceofthebaseclass

Thisisapricewemustpayforbinding(formally)non-relatedtypestogether

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Crea?ngaSingleInstanceofaClass

Insomecasesitmaybenecessarytocreatejustoneinstanceofaclass.

Thismaybenecessarybecause:MorethanoneinstancewillresultinincorrectprogrambehaviorMorethanoneinstancewillresultintheoveruseof

resources

Morethanoneinstancewillresultininconsistentresults

Thereisaneedforaglobalpointofaccess Howwouldyouensurethatjustoneinstance

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SingletonPa0ernOverview

Insomecasesthereshouldbeatmostoneinstanceofaclass.

Thisoneinstancemustbeaccessiblebyallclients,e.g.aprinterspooler.

Thisusuallyoccurswhenaglobalresourcehastobeshared.

Thesingletonpa0ernensuresthataclasshasonlyoneinstanceandprovidesonlyonepointofentry.

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Implemen?ngtheSingletonPa0ern

Implementaprivateconstructortopreventotherclassesfromdeclaringmorethanone

instance. Implementamethodtocreateasingle

instance.Makethismethodsta?c.

Createalazyinstanceoftheclassintheclass. Makethedataelementsta?c.

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ThreadExample

A A dual processor machine, with two threads calling thegetInstance() method for the chocolate boiler

Thread1 Thread2

public stat ChocolateBoiler!! getInstance()!public stat ChocolateBoiler!! getInstance()!

if (uniqueInstance == null)!if (uniqueInstance == null)!

uniqueInstance = !new ChocolateBoiler()!

uniqueInstance = !new ChocolateBoiler()!

return uniqueInstance;!return uniqueInstance;! 28


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ProblemswithMul?threading

Inthecaseofmul?threadingwithmorethanoneprocessorthegetInstance()methodcouldbecalledatmoreorlessthesame?meresul?ngintomorethanoneinstancebeingcreated.

Possiblesolu?ons: SynchronizethegetInstance()method DonothingifthegetInstance()methodisnotcri?caltothe

applica?on.

Movetoaneagerlycreatedinstanceratherthanalazilycreatedone.

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SynchronizingthegetInstance()

Method Codepublic static synchronized Singleton getInstance(){

}

Disadvantagesynchronizingcandecreasesystemperformancebyafactorof1.

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UseanEagerlyCreatedInstance

RatherthanaLazyOne Code://Data elements

public static Singleton uniqueInstance = newSingleton()

private Singleton() {}

public static Singleton getInstance() {

return uniqueInstance

}

DisadvantageMemorymaybeallocated andnotused.

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SingletonPa0ernSummary

Thesingletonpa0ernensuresthatthereisjustoneinstanceofaclass.

Thesingletonpa0ernprovidesaglobalaccesspoint.

Thepa0ernisimplementedbyusingaprivateconstructorandasta?cmethodcombinedwithasta?cvariable.

Possibleproblemswithmul?threading. VersionsofJavaearlierthat1.2automa?cally

clearsingletonsthatarenotbeingaccessedaspartofgarbagecollec?on.

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