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VISUAL BASIC.NET

Polymorphism using Abstract Classes in Visual Basic.NET 2005
By: Jagadish Chaterjee
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    2007-08-01

    Table of Contents:
  • Polymorphism using Abstract Classes in Visual Basic.NET 2005
  • Life without the Abstract class in Visual Basic 2005: continued
  • Defining and implementing abstract classes in Visual Basic 2005: abstract class
  • Defining and implementing abstract classes in Visual Basic 2005: child class
  • Defining and implementing abstract classes in Visual Basic 2005: child classes continued
  • Making use of Polymorphism using abstract classes in Visual Basic 2005

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    Polymorphism using Abstract Classes in Visual Basic.NET 2005 - Life without the Abstract class in Visual Basic 2005: continued
    (Page 2 of 6 )

    In the previous section, a "Rectangle" class was defined.  In this section, two more classes are introduced as follows:

    Public Class EquiTriangle

      Private _h As Double

      Private _b As Double

     

      Public Sub New(ByVal breadth As Double, ByVal height As Double) 'code removed for clarity

     

      Public Property H() As Double 'code removed for clarity

     

      Public Property B() As Double 'code removed for clarity

     

      Public Function GetArea() As Double

        Return (1.0 / 2.0) * _b * _h

      End Function

    End Class

     

    Public Class Square

      Private _s As Double

      Public Sub New(ByVal side As Double)

        Me.S = side

      End Sub

     

      Public Property S() As Double

        Get

          Return _s

        End Get

        Set(ByVal value As Double)

          _s = value

        End Set

      End Property

     

      Public Function GetArea() As Double

        Return _s * _s

      End Function

    End Class

    If you observe the three classes "Rectangle," "EquiTriangle" and "Square," you will see several similarities.  First of all, "GetArea" is available in all the classes (and it should be available for any of the shapes). In both "Rectangle" and "EquiTriangle," there exist two properties with almost the same signature along with constructors.

    The code to test the above classes would be the following:

        Dim r As New Rectangle(10, 20)

        Dim s As New Square(5)

        Dim t As New EquiTriangle(5, 20)

        MessageBox.Show("Area of rectangle: " & r.GetArea)

        MessageBox.Show("Area of square: " & s.GetArea)

        MessageBox.Show("Area of triangle: " & t.GetArea)

    Let us say I want the developers on my team to implement "GetArea" with the same signature as available in "Rectangle" (just to be consistent among all classes).  If the developers define their own methods like "GetEquiTriangleArea" or "GetSquareArea," the code would not give any compilation error (or warning) and somehow I lose consistency.

    To maintain signature consistency (without implementation) in our code and to enforce such rules, we can implement "abstract" classes, as defined in the next section.

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