Abstract factory pattern

A software design pattern, the Abstract Factory Pattern provides a way to encapsulate a group of individual factories that have a common theme. In normal usage, the client software would create a concrete implementation of the abstract factory and then use the generic interfaces to create the concrete objects that are part of the theme. The client does not know (nor care) about which concrete objects it gets from each of these internal factories since it uses only the generic interfaces of their products. This pattern separates the details of implementation of a set of objects from its general usage.

An example of this would be an abstract factory class DocumentCreator that provides interfaces to create a number of products (eg. createLetter() and createResume()). The system would have any number of derived concrete versions of the DocumentCreator class like FancyDocumentCreator or ModernDocumentCreator, each with a different implementation of createLetter() and createResume() that would create a corresponding object like FancyLetter or ModernResume. Each of these products is derived from a simple abstract class like Letter or Resume of which the client is aware. The client code would get an appropriate instantiation of the DocumentCreator and call its factory methods. Each of the resulting objects would be created from the same DocumentCreator implementation and would share a common theme. (They would all be fancy or modern objects.) The client would need to know how to handle only the abstract Letter or Resume class, not the specific version that it got from the concrete factory.

In software development, a Factory is the location in the code at which objects are constructed. The intent in employing the pattern is to insulate the creation of objects from their usage. This allows for new derived types to be introduced with no change to the code that uses the base class.

Use of this pattern makes it possible to interchange concrete classes without changing the code that uses them, even at runtime. However, employment of this pattern, as with similar design patterns, incurs the risk of unnecessary complexity and extra work in the initial writing of code.

How to use it

The factory determines the actual concrete type of object to be created, and it is here that the object is actually created (in C++, for instance, by the new operator). However, the factory only returns an abstract pointer to the created concrete object.

This insulates client code from object creation by having clients ask a factory object to create an object of the desired abstract type and to return an abstract pointer to the object.

As the factory only returns an abstract pointer, the client code (which requested the object from the factory) does not know - and is not burdened by - the actual concrete type of the object which was just created. However, the type of a concrete object (and hence a concrete factory) has been known to the abstract factory, for instance, factory can read it from a configuration file. It just happened that the client has no need to specify the type, because it has been done already in the configuration file. In particular, this means:

• The client code has no knowledge whatsoever of the concrete type, not needing to include any header files or class declarations relating to the concrete type. The client code deals only with the abstract type. Objects of a concrete type are indeed created by the factory, but the client code accesses such objects only through their abstract interface.
• Adding new concrete types is done by modifying the client code to use a different factory, a modification which is typically one line in one file. (The different factory then creates objects of a different concrete type, but still returns a pointer of the same abstract type as before - thus insulating the client code from change.) This is significantly easier than modifying the client code to instantiate a new type, which would require changing every location in the code where a new object is created (as well as making sure that all such code locations also have knowledge of the new concrete type, by including for instance a concrete class header file). If all factory objects are stored globally in a singleton object, and all client code goes through the singleton to access the proper factory for object creation, then changing factories is as easy as changing the singleton object.

Structure

The class diagram of this design pattern is as shown below:

Examples

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 an 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 an OSXButton:"
}

C#

/*
* GUIFactory example
*/
/*
* Note how the abstract (base) class GUIFactory is the one
* that instantiates its own subclasses (WinFactory, OSXFactory).
* As those subclasses inherit from base and override base CreateButton method,
* it’s GUIFactory's job to disambiguate who’s overriding its CreateButton method,
* at the time of being instantiated by Client. 
* Adding more factory subclasses (e.g. LnxFactory) confines change
* to GUIFactory class only 
*/
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 an 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 an OSXButton: Play"
}

Visual Prolog

interface guiFactory
    predicates
        createButton : () -> button NewButton.
        createEdit : () -> edit Edit.
end interface guiFactory

class winFactory : guiFactory
end class winFactory

implement winFactory
    clauses
        createButton() = winButton::new().
    clauses
        createEdit() = winEdit::new().
end implement winFactory

class osxFactory : guiFactory
end class osxFactory

implement osxFactory
    clauses
        createButton() = osxButton::new().
    clauses
        createEdit() = osxEdit::new().
end implement osxFactory

(The goal here is just for illustrative purposes; in a real program the GUI system would have to be initialized, and the button must be drawn somewhere.)

goal
    if 1 = config::getAttribute("OS_TYPE") then
        GuiFactory = winFactory::new()
    else
        GuiFactory = osxFactory::new()
    end if,
    Button = GuiFactory:createButton(),
    Button:setText("Play"),
    Button:paint().

PHP5

<?php
/**
 * GUIFactory example
 * Taken from the Java example
 */
abstract class GUIFactory {

    /**
     * Returns a sub-factory
     *
     * @return GUIFactory
     */
    static function getFactory() {
        $win = strpos($_ENV['OS'], 'Windows') === 0;

        if ($win) {
            return new WinFactory();
        } else {
            return new OSXFactory();
        }

    }

    /**
     * Create a button
     * @return Button
     */
    abstract function createButton();
}


/**
 * Windows
 */
class WinFactory extends GUIFactory {
    /**
     * @return WinButton
     */
    function createButton() {

        return new WinButton();
    }
}

/**
 * OSX
 */
class OSXFactory extends GUIFactory {
     /**
      * @return OSXButton
      */
     function createButton() {
        return new OSXButton();
    }
}

/**
 * Button
 */
abstract class Button  {

    /** @var string */
    private $_caption = ;

    abstract function paint();

    /**
     * Get the caption

     * @return string
     */ 
    function getCaption(){
        return $this->_caption;
    }

    /**
     * Set the caption

     * @param string Caption
     */
    function setCaption($caption){
        $this->_caption = $caption;
    }
}

class WinButton extends Button {
    function paint() {
       print("I'm a WinButton: " . $this->getCaption() . "");

    }
}

class OSXButton extends Button {
   function paint() {
       print("I'm an OSXButton: " . $this->getCaption() . "");
     }
}

//--- main
$aFactory = GUIFactory::getFactory();
/** @var $aFactory GUIFactor */
$aButton = $aFactory->createButton();

$aButton->setCaption("Play");
$aButton->paint();

//output is
//I'm a WinButton: Play
//or
//I'm an OSXButton: Play
?>

See also

• object creation
• concrete class
• factory method pattern
• Design Pattern

External links

• Abstract Factory
• Abstract Factory on the Net Objectives Repository
• Abstract Factory pattern discussion with 1-page examples (C++ and Java)