designPatterns-11.pdf

7/29/2019 designPatterns-11.pdf

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Structure Patters:

Bridgeand

Flyweight

CSCI 3132 Summer 2011 1


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IntroducingtheBridgePa1ern

IntentTodecoupleanabstrac7onfromitsimplementa7onsothatthetwocanvary

independently.

Whatdoesitmean? Decouple:havethingsbehaveindependentlyfromeachother.

Abstrac7on:howthingsarerelatedtoeachotherconceptually.

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AnExample Asimpleproblem

Drawingshapes Rectangles

Twodifferentdrawingprograms

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Example(Contd) Properuseofinheritance

Wedontwantthecodethatdrawstherectanglestoworryaboutwhattypeofdrawingprogramitshoulduse.

VRectangleusesDP V2RectangleusesDP2

class V1Rectangle : public Rectangle!{!public:!

V1Rectangle(double x1, double y1, double x2, double y2) : !Rectangle(x1, y1, x2, y2){} !

void drawLine( double x1, double y1, double x2, double y2){!DP1::draw_a_line( x1, y1, x2, y2);!

}!public:!

~V1Rectangle(void);!};!

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ADesignusingInheritance

Changing requirements: add a new shape - circle

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SequenceDiagram

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AnyProblem? Combinatorialexplosion

Anothernewdrawingprogram SixdifferentkindsofShapes

2Shapeconcepts7mesthreedrawingprogramsAnothertypeofShape

NinedifferentkindsofShapes3Shapeconcepts7mesthreedrawingprograms

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LearningtheBridgePa1ern:

AnExample Reason

Tightlycoupled Theabstrac7on(thekindsofShapes)andtheimplementa7on(thedrawingprograms)

TheBridgepa1ern Todecoupleanabstrac7onfromitsimplementa7onso

thatthetwocanvaryindependently

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AnObserva7on Thewaytolookatdesignpa1erns

Focusonthecontextofthepa1erntheproblemitistryingtosolve

Notonthesolu7onsthepa1ernsoffer Thebridgepa1ern

Anabstrac7onthathasdifferentimplementa7ons Allowtheabstrac7onandtheimplementa7ontovary

independentlyofeachother

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LearningtheBridgePa1ern:DerivingIt Step:findoutwhatvariesandencapsulateit.

Step2:Representthevaria7ons.

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LearningtheBridgePa1ern:DerivingIt Step3.Tietheclassestogether

Whatuseswhat?

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LearningtheBridgePa1ern:DerivingIt Expandingthedesign

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Separa7onof

abstrac7onandimplementa7on

Separate the abstraction class hierarchy from theimplementation class hierarchy and use delegation as abridge between the two.

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ClassDiagram GenericstructureoftheBridgepa1ern

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ThePar7cipants

Abstraction defines the abstractions interface

RefinedAbstraction extends the interface defined byAbstraction

Implementor defines the interface for implementationclasses; can be different from Abstraction interface.Typically, Implementor provides only primitive operations,and Abstraction defines higher-level operations based onthese primitives.

ConcreteImplementor implements the Implementorinterface and defines its concrete implementation.

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UsingtheBridgePa1ern TheBridgepa1ernoZenincorporatestheAdapterpa1ern

Compounddesignpa1erns Instan7a7ngtheobjectsoftheBridgepa1ern

Theobjectsrepresen7ngtheabstrac7onweregiventheirimplementa7onwhilebeinginstan7ated.

Basicstrategiesfordealingwithvaria7on: Findwhatvariesandencapsulateit. Favoraggrega7onoverinheritance.

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Applicability

Usethebridgewhen: Youwanttoavoidapermanentbindingbetweenan

abstrac7onanditsimplementa7on.

Boththeabstrac7onsandtheirimplementa7onsshouldbeextensiblebysubclassing. Changesintheimplementa7onofanabstrac7on

shouldhavenoimpactonclients(i.e.codeshouldnot

havetoberecompiled).

Youwanttoshareanimplementa7onamongmul7pleobjects,andwanttohidethisfromtheclient.

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BenefitsinusingBridgePa1ern

Decouplingabstrac7onfromimplementa7on Reduc7oninthenumberofsubclasses. CleanercodeandReduc7oninexecutablesize. Interfaceandimplementa7oncanbevariedindependently.

Abstrac7onandimplementa7oncanbeextendedindependently.

Looselycoupledclientcode.

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BridgeandAdapter

TheBridgepa1ernisdesignedtoseparateaclasssinterfacefromitsimplementa7on,sothatyoucanvaryorreplacetheimplementa7onwithoutchangingtheclientcode.

AdaptermakesthingsworkaZertheyredesigned;Bridgemakesthemworkbeforetheyare.

Bridgeisdesignedup-fronttolettheabstrac7onandtheimplementa7onvaryindependently.Adapterisretrofi1edto

makeunrelatedclassesworktogether.

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BridgeandStrategy

OZen,theStrategyPa1ernisconfusedwiththeBridgePa1ern.

Eventhough,thesetwopa1ernsaresimilarinstructure,theyaretryingtosolvetwodifferentdesignproblems.

Strategyismainlyconcernedinencapsula7ngalgorithms,whereasBridgedecouplesthe

abstrac7onfromtheimplementa7on,toprovidedifferentimplementa7onforthesameabstrac7on.

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TheAbstractFactorypa1erncanbeusedbytheBridgepa1erntodecidewhich

implementa7onclasstoinstan7ateforanabstrac7onobject.

BridgeandAbstractFactory

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Flyweight

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Problemofredundantobjects

Flyweight:aclassthathasonlyoneinstanceforeachuniquestate

problem:existenceofredundantobjectscanbogdownsystem

manyobjectshavesamestate example:Fileobjectsthatrepresentthesamefileondisk

newFile("mobydick.txt") newFile("mobydick.txt") newFile("mobydick.txt")...

newFile("notes.txt") newFile("notes.txt")

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Flyweightpa1ern

flyweight:anassurancethatnomorethanoneinstanceofaclasswillhaveiden7calstate

achievedbycachingiden7calinstancesofobjectstoreduceobjectconstruc7on

similartosingleton,buthasmanyinstances,oneforeachunique-stateobject usefulforcaseswhentherearemanyinstancesofatypebutmanyarethesame

canbeusedinconjunc7onwithFactorypa1erntocreateaveryefficientobject-builder

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FlyweightandStrings

Flyweightedstrings JavaStringsareflyweightedbythecompilerwherever

possible

canbeflyweightedatrun7mewiththeinternmethodpublic class StringTest {public static void main(String[] args) {

String fly = "fly", weight = "weight";String fly2 = "fly", weight2 = "weight";

System.out.println(fly == fly2); // trueSystem.out.println(weight == weight2); // true

String distinctString = fly + weight;System.out.println(distinctString == "flyweight"); // false

String flyweight = (fly + weight).intern();System.out.println(flyweight == "flyweight"); // true

}}

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Implemen7ngaFlyweight

flyweigh7ngworksbeston immutableobjects immutable:cannotbechangedonceconstructed

classpseudo-codesketch:

publicclassFlyweighted{

sta7cmaportableofinstances

privateconstructor

sta7cmethodtogetaninstance

ifwehavecreatedthistypeofinstancebefore,getitfrommapandreturnit

otherwise,makethenewinstance,storeandreturnit

}

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Flyweightsequencediagram

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Classbeforeflyweigh7ng

Aclasstobeflyweightedpublic class Point {

private int x, y;

public Point(int x, int y) {this.x = x; this.y = y;

}

public int getX() { return this.x; }

public int getY() { return this.y; }

public String toString() {

return "(" + this.x + ", " + this.y + ")";

}

}

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ClassaZerflyweigh7ng

Aclassthathasbeenflyweighted!public class Point {

private static Map instances = new HashMap();

public static Point getInstance(int x, int y) {String key = x + ", " + y;if (instances.containsKey(key)) // re-use

existing pt

return (Point)instances.get(key);

Point p = new Point(x, y);

instances.put(key, p);return p;

}private final int x, y; // immutable

private Point(int x, int y) {...

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GoFPa1ernsSummary

Crea3onalPa7ernsabstracttheinstan7a7onprocess.Theyhelpmakeasystemindependentofhowitsobjectsare

created,composed,andrepresented.

StructuralPa7ernsareconcernedwithhowclassesandobjectsarecomposedtoformlargerstructures.

BehavioralPa7ernsareconcernedwithalgorithmsandtheassignmentofresponsibili7esbetweenobjects.Behavioral

pa1ernsdescribenotjustpa1ernsofobjectsorclassesbut

alsothepa1ernsofcommunica7onbetweenthem.

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DesignSpaceforGoFPa1erns

Scope: domain over which a pattern appliesPurpose: reflects what a pattern does

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GoFPa1ernRela7onships

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designPatterns-11.pdf