1. IS437: Fall 2004 Instructor: Dr. Boris Jukic
Building Business Objects: Part 2

2. Inheritance Creating a new class, based on existing class
All non-private members (elements) are inherited in the new , derived class
In addition, all methods can be inherited (public or private)
Public Class StudentBookSale
Inherits BookSale

3. Inheritance: Constructor
Sub New(ByVal Title As String, ByVal Quantity As Integer, ByVal Price As Decimal)
MyBase.New(Title, Quantity, Price)
CalculateStudentTotals()
End Sub
Base class constructor
A new method, specific to the subclass only
Derived subclass does not explicitly inherit the parent’s class constructor method
It needs its own, within it can call the base class constructor
Additional methods can be specified in the constructor class of the derived class

4. Inheritance: Polymorphism through Overriding Methods
Protected Overrides Sub CalculateItemPrices() …
End Sub
Method as defined in derived class
Protected Overridable Sub CalculateItemPrices() …
End Sub
Method as defined in base class
Derived subclass can implement a method from the base (parent) class in a different way
The method as described in the derived class overrides the method as defined in the base class when an instance of the inherited class is instantiated and used

5. Inheritance: Overriding Methods
Protected Overrides Sub CalculateItemPrices()
ExtendedPrice = Quantity * Price
Discount = ExtendedPrice * STUDENT_DISCOUNT_RATE
DiscountedPrice = ExtendedPrice - Discount
End Sub
Base class properties are used in calculations, but not base class private variables
The method as defined in the derived (child) class can access the values of the properties defined in the base class
The methods in children class do not have access to the private variables of the parent class

6. Inheritance: New Methods and Properties in Derived Class
Private Shared decimalTotalStudentDiscount As Decimal
Property TotalStudentDiscount() As Decimal
Get
Return decimalTotalStudentDiscount
End Get
Set(ByVal Value As Decimal)
decimalTotalStudentDiscount = Value
End Set
End Property
Private Sub CalculateStudentTotals()
decimalTotalStudentDiscount += Discount
End Sub
Derived classes can have their own original (non-inherited methods and properties
The values of those original properties are contained in the private variables defined in the inherited class module