•          A human  and a parrot can both whistle, however it would not make sense to represent Humans and Parrots as subclasses of a Whistler class, rather they would most likely be subclasses of an Animal class (likely with intermediate classes), but would both implement the Whistler interface

     What is interface

• The behavior of classes 

• All methods of an interface are abstract methods 

• They cannot have bodies 

• You cannot create an instance from an interface 

• An interface can only be implemented 

• This is the concept of encapsulation

• During runtime, actual object instance is associated

  with the interface type

• It needs only the interface at the compile time

• Interface does not contain constructor.

• It not supports multiple inheritance directly but using interfaces we can achieve multiple inheritance. 

Defining Classes and Interfaces in VB .NET

            Public Class AClass

            End Class

            Public Interface AnInterface

            End Interface 

Implementing Interfaces

             Public Interface AnInterface   

                  Function WhoAmI() As String     

            End Interface  

          Public Class AClass

                   Implements AnInterface

                     Public Function WhoAmI() As String Implements AnInterface.WhoAmI

                            Return “AClass”

                     End Function

         End Class