Inheritance

A key feature of OOP is reusability. It's always time saving and useful if we can reuse something that already exists rather than trying to create the same thing again and again. Reusing the class that is tested, debugged and used many times can save us time and effort of developing and testing it again. Once a class has been written and tested, it can be used by other programs to suit the program's requirement. This is done by creating a new class from an existing class. The process of deriving a new class from an existing class is called Inheritance. The old class is called the base class and the new class is called derived class. The derived class inherits some or everything of the base class. In Visual Basic we use the Inherits keyword to inherit one class from other. The general form of deriving a new class from an existing class looks as follows:

Public Class One --- --- End Class Public Class Two      Inherits One --- --- End Class

Using Inheritance we can use the variables, methods, properties, etc, from the base class and add more functionality to it in the derived class. The following code demonstrates the process of Inheritance in Visual Basic.

Imports System.Console Module Module1 Sub Main() Dim ss As New Two() WriteLine(ss.sum()) Read() End Sub End Module Public Class One 'base class Public i As Integer = 10 Public j As Integer = 20 Public Function add() As Integer Return i + j End Function End Class Public Class Two     Inherits One 'derived class. class two inherited from class one Public k As Integer = 100 Public Function sum() As Integer 'using the variables, function from base class and adding more functionality Return i + j + k End Function End Class

Output of above code is sum of i, j, k as shown in the image below.