IS437: Fall 2004 Instructor: Dr. Boris Jukic

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IS437 Fall 2004Instructor Dr. Boris Jukic

• Building Business Objects Part 2

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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
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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
  parents 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

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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

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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

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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