用于面向对象编程的Association和Aggregation和Composition三个概念之间的区别外教英文版

Exercise#4:

ExplainthedifferencebetweenthreeconceptsusedinObjectOrientedProgramming.

Association

Aggregation

Composition

Firstlookatassociation.Belowisatypicalassoicationclassdiagram.

Associationistherepresentationofthesolidlinewitharrows,therearetwokey,navigabilityandmultiplicity,navigabilityisthearrow,referstotheotherclass,saidshallberesponsibleforthemaintenanceofassociationrelationship,inthisclassofdatamemberisreferstotheclasshavethereference(pointer),multiplicityistheuppernumber,fromhisclassisfaraway,saystheotherclassandtherelationshipbetweenthe

class,suchasabove,aSchoolcanhavemorethanoneStudent,anda

StudentcanonlyhaveaSchool,andaStudentcanchoose1to6ratsa

Coursecanbe1tomultipleStudentchoose.

Associationcommonproblemis,exactlyhowthearrowtoreferto?

Generallyspeaking,ifacouplemorerelations,isbyapointtomany,becausethechurchaclasswouldhaveanarrayorvectorstoremultipleobjects,butthisisnotabsolute,canonlysaythatusuallyso,ifthe

StudentisapairofCourse,andCourseofStudentisalsoapairof,buthechosetheCourseisresponsibleforassociationrelationship,sofullyseedesignneed,inpractice,SuggestionssuchasUMLforJavaProgrammer

Chinesep.3-15that,plusstereotype,detailednarrationisbelongtowhichaassociation,inthefutureI'llintroducethenegativity.

IfC++says,associationCodeasfollows

日classA(

private:

B*itsB;

2

3

4};

Againtoseeaggregation

connpMfqr

corTipSh

Systenisystcti'lameipAddresslocation

Therepresentationofaggregationbyemptydiamondandarrowsinthegraphicalrepresentation(chosenthispictureismoreoldUMLformat,sothereisnoarrows),emptydiamondsaidwhole,arrowpartsaid.IfEnglishsays,istherelationshipbetweenhasa,isamoretypicaloftheaboverepresentations.

IfC++says,associationCodeasfollows

日

classNode(

private:

vector<Node*>itsNodes;

};

Thecompositionisafter,representationandaggregationdifferenceofdiamondintoreality,otherexactlythesame,withthedifferenceinmeaningaggregationcompositionemphasison"liveanddietogether",whentheSystemobjectsdeath,Componentobjectalsowantwiththedeath,buttheaggregationis"lifeanddeathislife",whentheSystemobjectsdeath,Componentobjectisnotparticularlytodealwith.

IfC++says,compositionCodeasfollows

classCar(

public:

3白virtual~Car()(deleteitsCarb;}

private:

J…一一

Carburetor*itsCarb

}；

Byaboveprograms,itisknownthatthecompositionshouldbemadebydestructortodealwith,andaggregationarenot.Byabovethatprogram,compositionShouldbemadeBydestructorTodealwith.Inc#,andJavaisgarbagecollectionofthiskindoflanguage,compositionalmostneverused,butinc++,aslongasthepointerused,willhaveTo

goTodelete,soincompositionToac++isveryimportant.

InseveralrelationsUMLobject:association,aggregationand

compositionisas"has-a"relationship,includingfinedistinctionrelatedmaterialprovidethefollowingsomeexplanation:

Associationassociation

Associationrepresentstheabilityofoneinstancetosendamessagetoanotherinstance.Thisistypicallyimplementedwithapointerorreferenceinstancevariable,althoughitmightalsobeimplementedasamethodargument,orthecreationofalocalvariable.

[Example:]

IAI >IBI

classA

{

private:

B*itsB;

};

Sometimesarolehasanarrowindicatingitsnavigability.Thisshowswhichclasshastheresponsibilityformaintainingtherelationshipbetweentheclasses.Intheexampleabove,theschoolclasshasaresponsibilityforknowingwhichstudentsattend.Therefore,youdon't

havetoexplicitlydefineagetStudentsoperationintheschoolclass(althoughyoumay)sinceitisalreadyimplicitlydefinedbythenavigabilityoftheassociation.

Component

compMfqrsmpWi如cnmpSh

Systeni

system4ameipAddresslocation

Aggregationpolymerization

Co

Aggregationisthetypicalwhole/partrelationship.Thisisexactlythesameasanassociationwiththeexceptionthatinstancescannothavecyclicaggregationrelationships(i.e.apartcannotcontainitswhole).

[Example:]

INodel<> >INodel

classNode

{

private:

vector<Node*>itsNodes;

};

ThefactthatthisisaggregationmeansthattheinstancesofNodecannotformacycle.Thus,thisisaTreeofNodesnotagraphofNodes.

Composition[...]isexactlylikeAggregationexceptthatthelifetimeofthe'part'iscontrolledbythe'whole'.Thiscontrolmaybedirectortransitive.Thatis,the'whole'maytakedirectresponsibilityforcreatingordestroyingthe'part',oritmayacceptanalreadycreatedpart,andlaterpassitontosomeotherwholethatassumesresponsibilityforit.

[Example:]

|Car|<#> >|Carburetor|

classCar

{

public:

virtual~Car(){deleteitsCarb;}

private:

Carburetor*itsCarb

}；

Compositionindicatesthatoneclassbelongstotheother.Apolygonism