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method with a parameter of type HotelCurrencyTrader。
This problem is a classical polymorphism issue and is solved by using MustOverride methods
（as you’ll see in Chapter 7； the Overridable keyword also allows polymorphism）。 Consider the
following rewritten ConvertToTextField（） method。
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CH A PT E R 6 ■ L E A R N I N G T HE B AS IC S O F O B J E CT OR I E N TE D P R O G R AM M IN G 161
Public Function ConvertToTextField（ByVal cls As CurrencyTrader） As Double
Return cls。ConvertTo（Double。Parse（text1。Text））
End Function
This implementation of ConvertToTextField（） uses the CurrencyTrader base class； and
since ConvertTo（） and ConvertFrom（） are declared； they can be referenced。
How you call ConvertToTextField（） and how you instantiate HotelCurrencyTrader or
ActiveCurrencyTrader will not be covered in this chapter； because polymorphism is the subject
of the next chapter。 Just keep what you’ve learned about the MustInherit and MustOverride
keywords in mind as you read that chapter； which covers concepts such as interfaces； because
you can do the same types of things with them。
The Important Stuff to Remember
In this chapter； you learned some of the basics of object…oriented programming。 Here are the
key points to remember：
o Your code will be posed of structural （or base class functionality） and architectural
business…related functionality。
o Base class functionality is focused on a particular problem。 Base class functionality
requires specific knowledge of the field。 Think of it as implementing a calculator where
your main concern is ensuring that the calculations are correct。
o Architectural business…related functionality is higher…level functionality where general
business knowledge is needed。 The idea is to take the base classes and use them to solve
a business problem。 Think of it as using a calculator and your main concern is getting
the results of the calculations。
o Properties should be used instead of exposing data members publicly。
o Many developers dislike properties because they promote bad programming practices。
You need to get beyond those ments and think in terms of the credit card and the
cashier。 Properties have their time and place。
o Classes should； in general； not expose their state。 To get around exposing state； create
methods that implement the general intent of the purpose of the class。
o Inheritance is a fundamental part of Visual Basic and you will need to know how to use it。
o Overriding is when you keep the interface the same and change behavior。
o Overloading is when you derive a class and define an identifier that happens to be the
same as some base class identifier。 Your overloaded identifier may change behavior and
usage。
o Conditional pilation statements can be effective when you want to include code in
certain configurations。
o Partial classes are very useful when you want to separate functionality that serves a
specific purpose。 In this chapter’s example； it meant having the ability to add test code
without violating the rule of exposing state。
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162 CH AP T E R 6 ■ L E A R N IN G T HE B AS IC S O F OB J E CT OR I E N T E D P R O G R AM M IN G
Some Things for You to Do
To apply what you’ve learned in this chapter； you can do the following exercises。
1。 The code for HotelCurrencyTrader。ConvertTo（） and HotelCurrencyTrader。
ConvertFrom（） contains a potentially big bug。 Identify the bug and fix it。
2。 In the examples； the exchange rate is assigned by the caller of the HotelCurrencyTrader
and ActiveCurrencyTrader。 Implement functionality such that the exchange rate can be
retrieved dynamically。
3。 The currency ExchangeRate property is a Double。 Implement ExchangeRate using the
Decimal type。
4。 Write test code for ActiveCurrencyTrader and HotelCurrencyTrader。
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C H A P T E R 7
■ ■ ■
Learning About ponents
and Class Hierarchies
The previous chapter covered the basics of object…oriented programming。 You learned how
one class can subclass another class； thus building a hierarchy。 Using a class hierarchy； classes
can share functionality。
This chapter will focus on the details of class hierarchies； including how to extend them so
that derived classes can specialize shared functionality。 To demonstrate these concepts； we will
build a tax application。 This type of application is a good example of using class hierarchies；
because the general idea of paying taxes is identical； regardless of the country； but the details
are different。
From a technical perspective； we’ll cover the following topics：
o Interfaces； which are the basis of software ponents
o Details of overloading and overriding methods
o How and when to use factories
We’ll begin with some basic tax concepts that relate to our sample application。
Understanding Some Basic Tax Concepts
Taxes and death are two sure things in life。 Regardless of where you live； the government has
managed to create a system where you pay taxes （for the good of the people； of course）。 As a
result； people hate paying taxes； and many figure out creative ways to not pay taxes。 Honest people
who would normally not break the law find clever ways of bending the rules in their favor。 In
some countries； they even expect that you will cheat； and will calculate your “true” tax figure。
Taxes seem like a plex topic； but they are rather simple。 The volumes of rules associ
ated with taxes are what make them plicated。 Taxes boil down to the fact that if you buy or
do X； then you can deduct or charge Y。 Sometimes； the taxes make sense； sometimes they don’t。
Overall though； the tax rules are similar in logic； and that means a hierarchy of classes can
be designed。
Taxable ine is the sum of monies that you earned for which a tax rate is applied。 The
taxable ine may or may not be equal to the sum of money that you earned during a year。
163
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164 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
Taxable ine can be less due to deductions and ine deferment。 And in some cases；
taxable ine can be more due to deferment。
Deductions are items that can be charged against taxable ine。 People mistakenly believe
that deductions can be charged directly against the tax payable amount。 Or they mistakenly
believe a tax deduction is always an advantage。 For example； let’s say you have 1；000 of tax
deductions。 If your ine is 2；000；000； then the deduction is not worth much； since your
taxable ine is still 1；999；000。
Partial taxation is when a partial amount of earned monies is added to the total taxable
ine。 Often； partial taxation is applied to capital gains。 Capital gains refer to when you have
bought something like a property and then sold it at a higher price than you paid for it。 The
difference between the two prices is subje