Polymorphism using Abstract Classes in Visual Basic.NET 2005 - Defining and implementing abstract classes in Visual Basic 2005: child classes continued

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Continuing from the previous section, I defined two more sub classes for the super class "Shape" as follows:

Public Class EquiTriangle

  Inherits Shape

 

  Public Sub New(ByVal b As Double, ByVal h As Double)

    MyBase.New(b, h)

  End Sub

 

  Public Overrides Function GetArea() As Double

    Return ((1.0 / 2.0) * Me.X * Me.Y)

  End Function

 

End Class

 

Public Class Square

  Inherits Shape

 

  Public Sub New(ByVal s As Double)

    MyBase.New(s)

  End Sub

 

  Public Overrides Function GetArea() As Double

    Return Me.X * Me.X

  End Function

 

End Class

If the developer doesn't implement "GetArea" in any of the classes inherited from the "Shape" class, it would be a compilation error. This is how we can enforce consistency among members of classes belonging to the same group.

Finally, to test the above classes, we can write the code as follows:

    Dim r As New Rectangle(10, 20)

    Dim s As New Square(20)

    Dim t As New EquiTriangle(5, 10)

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

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

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

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