Factory
The job of the Factory design pattern is to create concrete sub classes. You can see the Factory design pattern used throughout the .NET Framework.
The essence of the Factory Pattern is to "Define an interface for creating an object, but let the subclasses decide which class to instantiate. The Factory method lets a class defer instantiation to subclasses." Factory methods encapsulate the creation of objects. This can be useful if the creation process is very complex, for example if it depends on settings in configuration files or on user input.
A C# example of the Factory Pattern
// A Simple Interface
public interface IVehicle
{
void Drive(int miles);
}
// The Vehicle Factory
public class VehicleFactory
{
public static IVehicle getVehicle(string Vehicle)
{
switch (Vehicle) {
case "Car":
return new Car();
case "Lorry":
return new Lorry();
default:
throw new ApplicationException(string.Format("Vehicle '{0}' cannot be created", Vehicle));
break;
}
}
}
// A Car Class that Implements the IVehicle Interface
public class Car : IVehicle
{
public void IVehicle.Drive(int miles)
{
// Drive the Car
}
}
// A Lorry Class that Implements the IVehicle Interface
public class Lorry : IVehicle
{
public void IVehicle.Drive(int miles)
{
// Drive the Lorry
}
}
A VB example of the Factory Pattern
' A Simple Interface
Public Interface IVehicle
Sub Drive(ByVal miles As Integer)
End Interface
' The Vehicle Factory
Public Class VehicleFactory
Public Shared Function getVehicle(ByVal Vehicle As String) As IVehicle
Select Case Vehicle
Case "Car"
Return New Car
Case "Lorry"
Return New Lorry
Case Else
Throw New ApplicationException(String.Format("Vehicle '{0}' cannot be created", Vehicle))
End Select
End Function
End Class
' A Car Class that Implements the IVehicle Interface
Public Class Car
Implements IVehicle
Public Sub Drive(ByVal miles As Integer) Implements IVehicle.Drive
' Drive the Car
End Sub
End Class
' A Lorry Class that Implements the IVehicle Interface
Public Class Lorry
Implements IVehicle
Public Sub Drive(ByVal miles As Integer) Implements IVehicle.Drive
' Drive the Lorry
End Sub
End Class
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Revision number 8, Friday, May 08, 2009 6:41:20 PM by Je_fait_le_logiciel