Polymorphism using Abstract Classes in Visual Basic.NET 2005 - Making use of Polymorphism using abstract classes in Visual Basic 2005
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Since you are now familiar with abstract methods and abstract classes, it is time to learn about polymorphism.
Let us modify the code for testing as follows:
Dim sh As Shape
sh = New Rectangle(10, 20)
MessageBox.Show("Area of rectangle: " & sh.GetArea)
sh = New Square(20)
MessageBox.Show("Area of square: " & sh.GetArea)
sh = New EquiTriangle(5, 10)
MessageBox.Show("Area of triangle: " & sh.GetArea)
In the above code, I am always using the same variable (reference variable, and it is not a concrete object) to hold the instances of all child classes -- "Rectangle," "Square" and "EquiTriangle."
You can also observe that I am using the same variable "sh" and calling the same method "GetArea" with different results. Using the same method with different implementations and different executions at run-time is nothing but polymorphism.
A final note! We can never create any concrete object (an instance with a "new" operator) or instantiate on any abstract class. For example, the following declaration is invalid:
Dim shp As New Shape(10, 20)
This is because any abstract class is never completely implemented (it contains abstract methods without any implementation). If there exists no implementation for a method, we will not be able to access it using an object (or instance). And hence, we will not be able to instantiate any abstract class.
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