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Function CreateIne（ByVal amount As Double） As ITaxIne
Function CreateTaxAccount（） As ITaxAccount
End Interface
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174 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
We have four interfaces here： ITaxIne； ITaxDeduction； ITaxAccount； and ITaxEngine。
ITaxIne and ITaxDeduction are pure behavioral interfaces。 Pure behavioral means the inter
face does one thing； but it might be implemented in conjunction with other interfaces。 ITaxEngine
and ITaxAccount are behavioral functional interfaces。 Behavioral functional means that the
interfaces are typically implemented by themselves； and usually not with another interface。
For example； you might implement a Swiss tax system where there are two classes； defined
as follows：
Class SwissTaxEngine
Implements ITaxEngine
End Class
Class SwissTaxAccount
Implements ITaxAccount
End Class
And if you wish to implement an American tax system； the two classes would be defined
as follows：
Class AmericanTaxEngine
Implements ITaxEngine
End Class
Class AmericanTaxAccount
Implements ITaxAccount
End Class
The user of either the American or Swiss tax system will not know the specific details of
those tax systems。 Initially； users would need to determine which tax system they wished to
use。 This decision is made using something called a factory； as explained in the uping
“Abstracting Instantiations with Factories” section。
Implementing a Base Class Tax Engine
Whenever you define interfaces； you will need to implement them。 In most cases； you will
create a MustInherit base class that provides a certain amount of default functionality。 The
MustInherit base class serves the same purpose as outlined in the previous chapter： to provide
a certain amount of basic functionality。
In the case of the tax engine； we need to implement the ITaxEngine interface and provide
default implementations for some methods。 The following is the plete base class
implementation。
Public MustInherit Class BaseTaxEngine
Implements ITaxEngine
Protected _calculatedTaxable As Double
Public Overridable Function CalculateTaxToPay（ByVal account As ITaxAccount） _
As Double Implements ITaxEngine。CalculateTaxToPay
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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 175
_calculatedTaxable = 0。0
For Each ine As ITaxIne In account。Ine
If ine IsNot Nothing Then
_calculatedTaxable = _calculatedTaxable ＋ ine。TaxableAmount
End If
Next
For Each deduction As ITaxDeduction In account。Deductions
If deduction IsNot Nothing
_calculatedTaxable = _calculatedTaxable deduction。Amount
End If
Next
Return account。GetTaxRate（_calculatedTaxable） * _calculatedTaxable
End Function
Public Overridable Function CreateDeduction（ByVal amount As Double） _
As ITaxDeduction Implements ITaxEngine。CreateDeduction
Return New TaxDeduction（amount）
End Function
Public Overridable Function CreateIne（ByVal amount As Double） As ITaxIne _
Implements ITaxEngine。CreateIne
' Second argument is the tax rate
Return New TaxIne（amount； 1。0）
End Function
Public MustOverride Function CreateTaxAccount（） As ITaxAccount _
Implements ITaxEngine。CreateTaxAccount
End Class
The base class must implement all interface methods； regardless of whether or not the method
has an implementation。 The CalculateTaxToPay（）； CreateDeduction（）； and CreateIne（）
methods have implementations。 The CreateTaxAccount（） method does not have an implemen
tation and is declared as MustOverride。 The methods with implementations have an Overridable
keyword associated with them； indicating any class derived from BaseTaxEngine can override
the functionality if they don’t like the default functionality。
In the implementation of CalculateTaxToPay（）； the ine （account。Ine） is added
together and deductions （account。Deductions） are subtracted from the ine。 The resulting
total is used as a query amount （account。GetTaxRate（）） to retrieve the actual tax rate used to
calculate against the payable tax。
■Note The implementation of CalculateTaxToPay（） is a shared functionality； which implies that there
cannot be any code specific to a derived type。 All of the calculations and data manipulations are executed
against an interface； making it possible to generalize operations。 Whenever you implement base class methods or
shared pieces of code； you should try to keep the source code derived class…agnostic。
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176 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
Overriding for Specialized Functionality
In the base class implementation； the data member _calculatedTaxable is declared as Protected。
As you learned in the previous chapter； this means that _calculatedTaxable can be manipu
lated in a derived class。 However； if you look at how the data member is used； you will see that
only CalculateTaxToPay（） assigns the data