VB2008从入门到精通(PDF格式英文版)-第55部分
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The parameter year could be justified because most countries do have specific tax rates
and implementations that are dependent on a year。 Yet the parameter province has no justifi
cation。 Imagine trying to implement a British tax system and needing to specify a province；
when Britain does not collect ine tax at a local level。
A solution might be to redefine the interface as follows：
Public Class Specifics
Public CanadianProvince As Province
Public AmericanState As State
End Class
Public Interface ITaxAccount
Sub AddDeduction（ByVal deduction As ITaxDeduction）
Sub AddIne（ByVal ine As ITaxIne）
Function GetTaxRate（ByVal ine As Double； ByVal year As Integer； _
ByVal specifics As Specifics） As Double
ReadOnly Property Deductions（） As ITaxDeduction（）
ReadOnly Property Ine（） As ITaxIne（）
End Interface
This new implementation has a specifics parameter； which is of type Specifics。 The
purpose of Specifics is to define a class that is a hodgepodge of information that is needed to
determine the correct tax rate。 However； the Specifics approach is wrong； for the following
reasons：
o It requires knowing the implementation； which in the case of the interface is a bad idea。
It is like going to a restaurant and saying you would like a waitress with blond hair。
o Even if the type Specifics were acceptable； you would be adding and removing data
members depending on how many tax systems you have implemented。 That is a bad
idea and introduces maintenance issues。
The proposed solutions are not acceptable。 Additionally； there is still the problem of
having to figure out which tax rate to use。
Implementing Ideas with Specifics
To implement a solution； let’s first start by fixing the TaxAccount class。 The modified version
will have some type of functionality that has data members that reference the year and prov
ince。 Here is the modified and correct implementation of TaxAccount：
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CH AP T E R 7 ■ L E AR N IN G AB O U T CO M P O N E N TS AN D C L AS S H I E R AR C HI E S 185
Friend Class TaxAccount
Inherits BaseTaxAccount
Private _province As Province
Private _year As Integer
Public Sub New （ByVal province As Province； ByVal year As Integer）
_province = province
_year = year
End Sub
Public Overrides Function GetTaxRate（ByVal ine As Double） As Double
If _year = 2008 Then
If _province = Province。Ontario Then
Return OntarioTax2008。TaxRate（ine）
End If
End If
Throw New NotSupportedException（〃Year 〃 & _year & 〃 Province 〃 & _
_province & 〃 not supported〃）
End Function
End Class
The fix is to add a constructor that has province and year as parameters。 This sort of fix is
quite mon； in that you don’t change the interfaces； rather you change how the implemen
tations are instantiated。 Remember that when you instantiate a specific implementation； you
know what functionality you want； and thus can give the additional parameters。 Once you are
at the interface level； you should need to use only general ideas。
Now the TaxEngine class needs to be fixed。 TaxEngine is responsible for instantiating
TaxAccount； and thus to instantiate a Canadian TaxAccount； TaxEngine needs additional
parameters； as follows：
Friend Class TaxEngine
Inherits BaseTaxEngine
Public Overrides Function CreateTaxAccount（） As ITaxAccount
Return New TaxAccount（Province。Ontario； 2008）
End Function
End Class
In the implementation of CreateTaxAccount（）； the province Ontario and year 2008 are
assumed。 Thus； whenever TaxEngine is instantiated； you need to make sure that the person is
in Ontario and paying taxes for the year 2008。 The implementation solves nothing and skirts
the issue of having to figure out how to deal with someone paying their taxes in British Columbia
and the year 2009。
If you look at the implementation of TaxEngine； you will notice it is short。 One obvious solu
tion would be to create a TaxEngine type for each province and each year。 Here are two examples：
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186 CH AP T E R 7 ■ L E A R N IN G AB OU T CO M P O N E N TS AN D C L AS S H I E R AR C H IE S
Friend Class Ontario2008TaxEngine
Inherits BaseTaxEngine
Public Overrides Function CreateTaxAccount（） As ITaxAccount
Return New TaxAccount（Province。Ontario； 2008）
End Function
End Class
Friend Class BritishColumbia2009TaxEngine
Inherits BaseTaxEngine
Public Overrides Function CreateTaxAccount（） As ITaxAccount
Return New TaxAccount（Province。BritishColumbia； 2009）
End Function
End Class
This solution is not that bad； because to be able to instantiate the correct tax engine； you
just need to define a factory that knows which class to instantiate。 But for the problem at hand；
this solution is extremely tedious； as you could end up with hundreds； if not thousands； of
TaxEngine definitions。 You would use the specific implementation approach when you have
fewer than a dozen variations。
The better approach is to add an interface specific to the Canadian tax system。 Think of it
as follows。 When you are instantiating the tax engine； you will need to know which tax system
to use。 The factory protects you from needing to know which type to instantiate； but there is
nothing wrong with giving some extra information that could be used by a factory。
Thus； the solution is to define a new interface called ICanadaTaxEngine。 The purpose of
ICanadaTaxEngine is to add factory methods that are used to instantiate types with parameters
specific to the implementation。 Following is the definition of the ICanadaTaxEngine。
Public Enum Province
Alberta
BritishColumbia
Manitoba
NewBrunswick
NewfoundlandLabrador
NovaScotia
Nunavut
Ontario
PrinceEdwardIsland
Quebec
Saskatchewan
Yukon
End Enum
Public Interface ICanadaTaxEngine
Function CreateTaxAccount（ByVal province As Province； _
ByVal year As Integer） As ITaxAccount
Function CreateCapitalGain（ByVal amount As Double） As ITaxIne
End Interface
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CH AP T E R 7 ■ L E AR N IN G AB O U T CO M P O N E N TS AN D C L AS S H I E R AR C HI E S 187
The definition of ICanadaTaxEngine contains two additional methods：
o CreateTaxAccount（） is used to instantiate a tax account specific to a province and year。
o CreateCapitalGain（） is used to instantiate an ine using the Canadian capital gains
calculation。
Thus； the implementation of TaxEngine bees the following。
Friend Class TaxEngine
Inherits BaseTaxEngine
Implements ICanadaTaxEngine
Public Overrides Function CreateTaxAccount（） As ITaxAccount
Return New TaxAccount（Province。Ontario； 2008）
End Function
Public Overloads Function CreateTaxAccount（ByVal province As Province； _
ByVal year As Integer） _
As ITaxAccount _
Implements ICanadaTaxEngine。CreateTaxAccount
Return New TaxAccount（province； year）
End Function
Public Function CreateCapitalGain（ByVal amount As Double） As ITaxIne _
Implements ICanadaTaxEngine。CreateCapitalGain
Return New TaxIne（amount； 0。50）
End Function
End Class
In the modified implementation of TaxEngine； the class still derives from BaseTaxEngine；
fulfilling the requirement of being a general tax engine。 However； for the additional require
ments of the Canadian tax system； we implement the ICanadaTaxEngine interface。
Defining a specific interface that implies a certain implementation is fine； because the
specific interface is not bound to a certain implementation。 The better way to understand
this implementation technique is to consider the specific interface as a characteristic that
an implementation is capable of supporting。 This goes back to the shape example； where a
square can support both the ISquare and IRectangle interfaces。
Using the Tax Engine
The last step is to use the tax engine to calculate some taxes。 The following is an example of
calculating the taxes for Ontario in 2008。
Dim engine As ITaxEngine = EngineCreator。CreateCanadianTaxEngine（）
Dim canadaEngine As ICanadaTaxEngine = TryCast（engine； ICanadaTaxEngine）
Dim account As ITaxAccount = canadaEngine。CreateTaxAccount（Province。Ontario； 2008）
Dim ine As ITaxIne = engine。CreateIne（100）
Dim capitalGain As ITaxIne = ca
