Adapter Pattern

The gang of four definition is "Convert the interface of a class into another interface clients expect. Adpater lets the classes work together that couldn't otherwise because of incompatible interfaces". Below is an example that uses a third party payment system, this also uses the Factory design pattern.

A VB example of the Adapter Pattern

' This code would be run at page code behind levelDim aPayment As IPaymentDim paymentType As String ' this value would be populated elsewhere
aPayment = PaymentFactory.getPayment(paymentType)
aPayment.takePayment(10.5)

' The payment interface
Public Interface IPayment
    Sub takePayment(ByVal Amount As Double)End Interface
' PayPal Payment class that implements the IPayment InterfacePublic Class PayPalPayment
    Implements IPayment    

    Public Sub takePayment(ByVal Amount As Double) ImplementsIPayment.takePayment
        ' Code to take payment via PayPal merchant
    End SubEnd Class
' Credit Card Payment class that implements the IPayment InterfacePublic Class CreditCardPayment
    Implements IPayment    

    Public Sub takePayment(ByVal Amount As Double) ImplementsIPayment.takePayment
       ' Code to take payment via a credit card
    End SubEnd Class
' Adapter Class to ensure third party code implements IPaymentPublic Class ThirdPartyPaymentAdapter
    Inherits ThirdPartyPayment
    Implements IPayment    

    Public Sub takePayment(ByVal Amount As Double) ImplementsIPayment.takePayment
       ' This methods calls the actual thrid party code.
       ' If we couldn't inherit we could instantiate the object here or in the
       ' constructor method.
       MyBase.makeTransasction(Amount, False)
    End SubEnd Class
' Third Party Payment ClassPublic Class ThirdPartyPayment   

   Public Sub makeTransasction(ByVal amount As Double, ByVal refund AsBoolean)
      ' Third party code...
   End SubEnd Class
' Factory Class to return Concrete Payment objectPublic Class PaymentFactory   

   Public Shared Function getPayment(ByVal PaymentType As String) AsIPayment
       Select Case PaymentType
           Case "CreditCard"
               Return New CreditCardPayment
           Case "PayPal"
               Return New PayPalPayment
           Case Else
               Return New ThirdPartyPaymentAdapter
       End Select
   End FunctionEnd Class