Pattern - Abstract Factory

Java:

/*
* GUIFactory example
*/

abstract class GUIFactory
{
    public static GUIFactory getFactory()
    {
        int sys = readFromConfigFile("OS_TYPE");
        if (sys == 0)
        {
            return new WinFactory();
        }
        else
        {
            return new OSXFactory();
        }
   }

   public abstract Button createButton();
}

class WinFactory extends GUIFactory
{
    public Button createButton()
    {
        return new WinButton();
    }
}

class OSXFactory extends GUIFactory
{
    public Button createButton()
    {
        return new OSXButton();
    }
}

abstract class Button
{
    public abstract void paint();
}

class WinButton extends Button
{
    public void paint()
    {
       System.out.println("I'm a WinButton: ");
    }
}

class OSXButton extends Button
{
    public void paint()
    {
       System.out.println("I'm a OSXButton: ");
    }
}

public class Application
{
    public static void main(String[] args)
    {
        GUIFactory factory = GUIFactory.getFactory();
        Button button = factory.createButton();
        button.paint();
    }
    // Output is either:
    //   "I'm a WinButton:"
    // or:
    //   "I'm a OSXButton:"
}

[edit] C#

/*
* GUIFactory example
*/

abstract class GUIFactory
{
    public static GUIFactory GetFactory()
    {
        int sys = ReadFromConfigFile("OS_TYPE");
        if (sys == 0)
        {
            return new WinFactory();
        }
        else
        {
            return new OSXFactory();
        }
   }

   public abstract Button CreateButton();
}

class WinFactory : GUIFactory
{
    public override Button CreateButton()
    {
        return new WinButton();
    }
}

class OSXFactory : GUIFactory
{
    public override Button CreateButton()
    {
        return new OSXButton();
    }
}

abstract class Button
{
    public string Caption;
    public abstract void Paint();
}

class WinButton : Button
{
    public override void Paint()
    {
       Console.WriteLine("I'm a WinButton: " + Caption);
    }
}

class OSXButton : Button
{
    public override void Paint()
    {
       Console.WriteLine("I'm a OSXButton: " + Caption);
    }
}

class Application
{
    static void Main()
    {
        GUIFactory factory = GUIFactory.GetFactory();
        Button button = factory.CreateButton();
        button.Caption = "Play";
        button.Paint();
    }
    // Output is either:
    //   "I'm a WinButton: Play"
    // or:
    //   "I'm a OSXButton: Play"
}

Scheme:

;; this is the straight port:

(define (make-factory)
  (case (magically-read-os-symbol)
    ((win95) (win95-factory))
    ((plan9) (plan9-factory))))

(define (win95-factory)
  make-win95-button)

(define (plan9-factory)
  make-plan9-button)

(define (make-plan9-button)
  (lambda () (print "I'm button on plan 9 from bell labs")))

(define (make-win95-button)
  (lambda () (print "I'm button on windows 95")))


;; app usage:

(((make-factory))) ; the first call makes the factory, the second makes the button, the third calls the button.

This is another version with duplication removed:

(define (make-factory)
 (lambda ()
  (make-button (magically-read-os-symbol))))


;; the button-maker can be separated on any level; to remove
;; duplication bring it closer (like the example below, to add
;; separation bring it further apart (like dispatching to return
;; separate top-level defines instead of one lambda)

(define (make-button os)
  (lambda ()
    (print "I'm a button on "
	   (case os
	     ((win95) "windows 95")
	     ((plan9) "plan 9 from bell labs")))))

;;; app usage:

(((make-factory))) ; ; the first call makes the factory, the second makes the button, the third calls the button.