What are abstract classes?

The features of the abstract class are as follows:-

1.     You can not create an object of the abstract class.

2.     The Abstract class is designed to act as a base class (to be inherited by another classes). In the program development  the Abstract class is a design concept and provides a base upon which other classes are built.

3.     The Abstract classes are similar to the interfaces. After declaring an abstract class, it can't be instantiated on its own, it must be inherited.

4.     In VB.NET abstract classes are created by using the "MustInherit" keyword.In the C# we have "Abstract" keyword.

5.     The Abstract classes can have implementation or pure abstract methods which must be implemented in the child class.

Public MustInherit Class ClsAbstract

' use the mustinherit class to declare the class as abstract

Public Function Add(ByVal intnum1 As Integer, ByVal intnum2 As

Integer) As Integer

Return intnum1 + intnum2

End Function

' left this seconf function to be completed by the inheriting class

Public MustOverride Function MultiplyNumber(ByVal intnum1 As

Integer, ByVal intnum2 As Integer) As Integer

End Class

Public Class ClsChild

Inherits ClsAbstract

' class child overrides the Multiplynumber function

Public Overrides Function MultiplyNumber(ByVal intnum1 As

Integer, ByVal intnum2 As Integer) As Integer

Return intnum1 * intnum2

End Function

End Class