The Apartment , Commercial , and Home Derived Classes

The Apartment , Commercial , and Home Derived Classes

In this chapter you have created three derived classes from the base Building class. The code for these derived classes is presented in Listing 17.2. Note that each class is in the same file as the Building class. Being able to have multiple classes declared in a single file is something that previous versions of Visual Basic did not allow. This makes it easier to keep the base class and its derived classes collected together in one place.

Listing 17.2 The Derived Classes from the Building Class: Apartment , Commercial , and Home

 ' ++++++++++++++++++++++++++++ Apartment Class ++++++++++++++++++++++++ Public Class Apartment  Inherits Building  Private mUnits As Integer   ' The number of apartments  Private mRent As Double    ' Rent per unit  Private mOccupRate As Double ' Occupancy rate for building  ' ==================== Properties ======================  Public Property Units()    ' Units   Get    Return mUnits   End Get   Set(ByVal Value)    mUnits = Value   End Set  End Property  Public Property Rents()    ' Rents   Get    Return mRent   End Get   Set(ByVal Value)    mRent = Value   End Set  End Property  Public Property OccupancyRate() ' Occupancy rate   Get    Return mOccupRate   End Get   Set(ByVal Value)    mOccupRate = Value   End Set  End Property  ' ============== Methods ==================  Public Sub DisplayBuilding()   DisplayBaseInfo()   Console.WriteLine("Number of Units: " & mUnits)   Console.WriteLine("Rent per Unit: " & FormatMoney(mRent))   Console.WriteLine("Occupancy Rate: " & mOccupRate)  End Sub End Class ' ++++++++++++++++++++++++++++ Commercial Class ++++++++++++++++++++++++++++ Public Class Commercial  Inherits Building  Private mSquareFeet As Integer  ' Rentable square feet  Private mRentPerSF As Double   ' Rent per square foot  Private mParking As Integer    ' Parking spots  ' ==================== Properties ======================  Public Property SquareFeet()   ' Square feet   Get    Return mSquareFeet   End Get   Set(ByVal Value)    mSquareFeet = Value   End Set  End Property  Public Property RentPerSF()    ' Rent per square foot   Get    Return mRentPerSF   End Get   Set(ByVal Value)    mRentPerSF = Value   End Set  End Property  Public Property ParkingSpots()  ' Parking spots   Get    Return mParking   End Get   Set(ByVal Value)    mParking = Value   End Set  End Property  ' ============== Methods ==================  Public Sub DisplayBuilding()   DisplayBaseInfo()        ' Call base class   Console.WriteLine("Square Feet: " & mSquareFeet)   Console.WriteLine("Rent per SF: " & FormatMoney(mRentPerSF))   Console.WriteLine("Parking Spots: " & mParking)  End Sub End Class ' ++++++++++++++++++++++++++++ Home Class ++++++++++++++++++++++++++++ Public Class Home  Inherits Building  Private mSquareFeet As Integer   ' Home's square feet  Private mRentPerMonth As Double   ' Rent per month  Private mBedrooms As Integer    ' Number of bedrooms  Private mBaths As Integer      ' Number of bathrooms  ' ==================== Properties ======================  Public Property SquareFeet()    ' Square feet   Get    Return mSquareFeet   End Get   Set(ByVal Value)    mSquareFeet = Value   End Set  End Property  Public Property Rent()       ' Rent   Get    Return mRentPerMonth   End Get   Set(ByVal Value)    mRentPerMonth = Value   End Set  End Property  Public Property Bedrooms()     ' Bedrooms   Get    Return mBedrooms   End Get   Set(ByVal Value)    mBedrooms = Value   End Set  End Property  Public Property Baths()       ' Baths   Get    Return mBaths   End Get   Set(ByVal Value)    mBaths = Value   End Set  End Property  ' ============== Methods ==================  Public Sub DisplayBuilding()   DisplayBaseInfo()        ' Call base class   '  Console.WriteLine("Building Type: Home")   Console.WriteLine("Square Feet: " & mSquareFeet)   Console.WriteLine("Rent per Month: " & FormatMoney(mRentPerMonth))   Console.WriteLine("Bedrooms: " & mBedrooms)   Console.WriteLine("Baths: " & mBaths)  End Sub End Class 

The derived classes are very similar to one another, and the mechanics are the same for them all, so this section explains the code for just one of them.

Each derived class begins with the class name and the Inherits keyword:

 Public Class Apartment   Inherits Building 

The Inherits keyword tells Visual Basic .NET that the Apartment class will inherit all the allowable data members and methods of the base class. (Keep in mind that you can use different flavors of access specifiers to limit access of the derived classes to the member data and methods in the base class.)

Member Data and Property Accessors

The next several lines of each derived class list the member data for the class. The member data properties distinguish the derived class from the base class. For the Apartment class, these members are the number of apartment units ( mUnits ), the rent per month ( mRent ), and the occupancy rate for the apartment complex ( mOccupRate ). An apartment complex "is a" building, and these additional pieces of information supply details that allow you to identify this building as an apartment building.

After each of these member data items is declared, the property accessor methods are provided for each item. In this simple example, the Get method returns the value of each member, and the Set method assigns a value to the member. It's interesting to note that Visual Basic .NET is smart enough to make the argument to the Set method match the data type of the data member.

Methods of the Derived Classes

The final section of each class is a method named DisplayBuilding() that is used to display the member data of the class. Note that this method name is the same for all three derived classes. How is this possible? Isn't there going to be a namespace collision for the method if they appear in all three classes?

No, there is no namespace collision because the DisplayBuilding() method is encapsulated into each object. Therefore, if you define three objects, such as MyApt , MyOffice , and MyHome , the method can be accessed only with MyApt.DisplayBuilding() , MyOffice.DisplayBuilding() , and MyHome.DisplayBuilding() . In every case, you must prefix the Display method with the object name and dot operator, which prevents any namespace collision in Visual Basic .NET.