软件设计模式-体系结构-PPT讲稿 0-Software-Design-Patterns-20121030 Lec9_Visitor--Behavioral.ppt

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1、Lecture9.VisitorPatternBehavioral访问者模式1ContentsofthelectureIntroductionoftheVisitorPatternOfficialDiagramoftheVisitorDesignPatternDesignExampleUsingtheVisitorPatternFurtherdiscussions2IntroductionoftheVisitorPattern3Example1:taxcomputationproblem(税收问题)Example

2、1:taxcomputationproblem(税收问题)InUS,therearemanykindsoftaxes,includingenterprisetaxandindividualtax,etc.,.Thetaxescanbeexpressedbyusingahierarchyofclassesasbelow.Tocomputethetax,wecanuseaclasswithmainmethodstoinvokethetaxhierarchytogettaxrates,etc.4Example1:Tax

3、computationTaxgetData()calculateTax()Firstdesign-Foreachdifferentclass,thealgorithmisdifferentTaxComputationmainCharityTaxIndividuleTaxPoliticalOrgnBusinessTaxgetData()calculateTax()getData()calculateTax()getData()calculateTax()getData()calculateTax()Manufact

4、ureTaxEntertainmentTaxgetData()calculateTax()getData()calculateTax()FamilyTaxSingleTaxgetData()calculateTax()getData()calculateTax()ElectronicTaxAutomobileTaxgetData()calculateTax()getData()calculateTax()FamilyWithChildrenFamilyNoChildrengetData()calculateTax

5、()getData()calculateTax()5本设计的缺点。Problemsinthisdesign(drawbacks):Distributingalltheseoperationsacrossthevariousnodeclassesleadstoasystemthat'shardtounderstand,maintain,andchange.Example1:taxcomputationproblem(税收问题)6Problems(drawbacks)(cont):Forexample,Poormai

6、ntainability:Itwillbeconfusingtohavealotofvariouskindsofcodeinsomanynodes.Poorextendibility:addinganewoperationusuallyrequiresrecompilingalloftheseclasses,SeenextpageExample1:taxcomputationproblem(税收问题)7例如，在每个类中添加一个方法printTax()的时候，需要修改每一个并且重新编译每一个类TaxgetData(

7、)calculateTax()printTax()CharityTaxIndividuleTaxPoliticalOrgnBusinessTaxgetData()calculateTax()printTax()getData()calculateTax()printTax()getData()calculateTax()printTax()getData()calculateTax()printTax()FamilyTaxSingleTaxManufactureTaxEntertainmentTaxgetData

8、()calculateTax()printTax()getData()calculateTax()printTax()getData()calculateTax()printTax()getData()calculateTax()printTax()FamilyWithChildrenFamilyNoChildrenElectronicTaxAutomobileTaxge