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StrategyPa*ern

and

StatePa*ern

CSCI 3132 Summer 2011

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StrategyPa*ernRevisit

OODesignPrinciplesRevisitProgramtoaninterface,notanimplementa:onFavorobjectaggrega:onoverclassinheritanceFindwhatvariesandencapsulateit.

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StrategyPa*ernRevisit

Abehaviorpa*ern. Takesanalgorithmfromitshostandencapsulatesitintoaseparateclass.

Anobjectanditsbehaviorareseparatedandputintotwoclasses.

Thisallowsthealgorithmtobeswitchedatany:me. Thealgorithm/behaviorencapsulatedinitsownclassiscalledastrategy.

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StrategyPa*ern-ClassDiagram

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Trade-offs

Advantages Eliminateslargecondi:onal

statements.

Easiertokeeptrackofdifferentbehaviorbecause

theyareindifferentclasses.

Avarietyofimplementa:onsforthe

samebehavior.

Disadvantages Increasesthenumberof

objects.

Allalgorithmsusethesameinterface.

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TheStatePa*ern

AnothertypeofBehavioralpa*ern.Allowsanobjecttoalteritsbehaviorwhenitsinternalstatechanges.Theobjectwillappearto

changeitsclass.

UsesPolymorphismtodefinedifferentbehaviorsfordifferentstatesofanobject.

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WhentouseSTATEpa*ern?

Statepa*ernisusefulwhenthereisanobjectthatcanbeinoneofseveralstates,withdifferentbehaviorineachstate.

Tosimplifyopera:onsthathavelargecondi:onalstatementsthatdependontheobjectsstate.

if(myself=happy)then

{

eatIceCream();

.

}elseif(myself=sad)then

{

goToPub();

.

}

elseif(myself=ecstaAc)then

{

.

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Canweuseastate-transi:ontable?

Yes,buttheyaremorecomplextounderstandandimplement.

Upgradingfunc:onalityofprogramismoredifficult.

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BenefitsofusingSTATEpa*ern

Localizesallbehaviorassociatedwithapar3cularstateintooneobject. Newstateandtransi:onscanbeaddedeasilybydefiningnewsubclasses.

Simplifiesmaintenance. Itmakesstatetransi3onsexplicit.Separateobjectsforseparatestatesmakestransi:onexplicit

ratherthanusinginternaldatavaluestodefinetransi:onsin

onecombinedobject.

StateobjectscanbesharedContextcanshareStateobjectsiftherearenoinstance

variables.

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ExampleI

water StateOfWater

WaterVapor LiquidWater Ice

increaseTemp()decreaseTemp()

state variable

ClientincreaseTemp()

increaseTemp()decreaseTemp()

increaseTemp()decreaseTemp()

increaseTemp()

decreaseTemp()

increaseTemp()decreaseTemp()

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StatePa*ern

Context class:Represents the interface to the outside world.

State abstract class:Base class which defines the different states of

the state machine.Derived classes from the State class:

Defines the true nature of the state that the

state machine can be in.

Context class maintains a pointer to the current state. To changethe state of the state machine, the pointer needs to be changed.

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ExampleII

MyMood MoodState

doSomething()

mad angry happy

doSomething() doSomething() doSomething()

state variable

ClientdoSomething()

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WhyUseAStatePa*ern?

Dynamicallychangeanobjectsinternalstateduringrun:me. Thestatepa*erncreatesaseparateclassforeachcondi:onal

branch.Thiseliminatesif/elseorswitch/casestatements.

Codeismodulartoalloweasycrea:onofnewstates.

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StatePa*ern-ClassDiagram

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Trade-offs

Advantages Encapsulatesallbehaviorofa

stateintoanobject

Eliminatesmassivelinesofcodeinvolvingif/elseorswitch/casestatementswithstatetransi:onlogic

Statechangesoccurusingoneobject,therefore,avoidinginconsistentstates.

Disadvantages Increasednumberofobjects.

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Statev.s.Strategy

State pattern and Strategy pattern are very similar, but

their of intent are different.

- State object encapsulates a state-dependent behavior(and possibly state transitions)

- Strategy object encapsulates an algorithm- Both examples of Composition with Delegation!

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