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ality of the BMW 530xi。 But here is the twist： the functionality of the 530xi Sports Wagon requires
you to get accustomed to the Sports Wagon。 For example； even though you press a single
button to open the trunk； the Sports Wagon’s trunk is slightly different from the trunk in the
530xi and 530i； and thus exposes the driver to a slightly different user interface and behavior。
When you use inheritance to overload functionality； you are adding functionality that is
called in the same way； but is used and behaves differently。 With this form of inheritance； you
don’t just change behavior； you also change the user interaction。
In our example； we will use inheritance to extend functionality； and not override or overload。
Using Visual Basic Properties
So far； the test code would call a data member reference； as in this line：
cls。ExchangeRate = 123。45
The data member could be implemented as follows：
Public MustInherit Class CurrencyTrader
Public ExchangeRate As Double
End Class
Exposing the data member using a public scope worked in previous chapters； but we don’t
really want to do this because we are exposing the internal state of the object。 In object…oriented
programming； exposing the internal state is a bad idea （as explained in more detail shortly）。
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Rather than expose the data member publicly； we will change the test code to use properties。
Properties also expose the internal state of an object； but they provide a layer of abstraction。 As
you will learn； some properties expose both internal state and external state。 This is the case
with the ExchangeRate property that we will use to access and modify the exchange rate。 If we
did not use the ExchangeRate property； we would need to create a method that assigns the
exchange rate and a method that retrieves the exchange rate。 The methods would behave like
the property； but they would not be as convenient to use。
Rewriting the Test Code to Use Properties
The interesting thing about Visual Basic properties is that they look and behave like data members。
This means with Visual Basic properties； the test code does not need to be rewritten； as the test
code still assumes a direct access to the variable。 Consider the rewritten CurrencyTrader that
exposes ExchangeRate as a Visual Basic property； shown in Figure 6…3。
Data member ExchangeRate is relabeled to
Public MustInherit Class CurrencyTrader _exchangeRate； and the scope is changed
Private _exchangeRate As Double from public to private
Public Property ExchangeRate（） As Double
Get
Return _exchangeRate A Visual Basic property declaration looks like a
End Get function declaration without the parameters； and
Set（ByVal value As Double） instead of Function； Property is used
_exchangeRate = value
End Set
End Property
End Class What makes a property uniquely
different from a function are the Get
and Set keywords used to return and
assign the value of the property
Figure 6…3。 Exposing ExchangeRate as a Visual Basic property
To pletely fool the test； the name of the property must be identical to the name of the
previously declared data member。 So that there is no identifier collision； the data member
ExchangeRate is renamed to _exchangeRate； and the scope changes from public to private。
Properties look like methods without parameters； but have a return value。 Also； each prop
erty must have at least a Get code block or a Set block （it can have both）； which are called getters
and setters。 If a property has only a Get code block； you have created a read…only property。 If a
property has only a Set code block； you have created a write…only property。 Within the context
of a property； you can have only the Get or Set keyword。 You cannot add any other code。
The following code will execute the code defined in the Get code block and retrieve the
state from the object to the variable。
value = cls。ExchangeRate
And the following code will execute the code in the Set code block and assign the state
from the variable to the object。
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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 143
cls。ExchangeRate = value
Each code block has special features。 The Get code block must always return data to the
caller and thus requires the use of the Return keyword。 The Set code block has no requirements
to do anything。 However； if you want to know what data is being passed to the property； you
can use the value parameter。
Understanding the Problems with Properties
Many programmers feel that properties promote bad programming practices。 Properties expose
the internal state of an object。 Looking at the previous source code； you will see that the data
member _exchangeRate has a one…to…one relationship to the property ExchangeRate。 If the caller
assigns the property ExchangeRate； then the private data member _exchangeRate is immediately
assigned。 This exposes the internal state； even though the exposure is indirect。
As an example； say that you want to preheat an oven to a certain temperature to bake
something。 The simplest way to preheat the oven is to monitor the temperature and create a
property Temperature； like this：
Class Oven
Private _temperature As Integer
Public Property Temperature（） As Integer
Get
Return _temperature
End Get
Set（ByVal Value As Integer）
_temperature = Value
End Set
End Property
End Class
The class Oven exposes its temperature as a property； which is a direct reference to the vari
able _temperature。 The caller of Oven would periodically ask the oven for its temperature and
decide when the oven has been preheated。
So； is Oven in its current implementation actually a structural class？ The user of Oven has
quite a bit of responsibility in that it must periodically query the temperature and decide if the
oven is preheated。 A better implementation is to define a class that takes care of itself。 The caller
would then just need to ask； “Are you ready？” Here’s how that code would look：
Class Oven
Private _temperature As Integer
Public Sub SetTemperature（ByVal temperature As Integer）
_temperature = temperature
End Sub
Public Function AreYouPreheated（） As Boolean
' Check if oven temperature matches prescribed temperature
Return False
End Function
End Class
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In the mod