Editing Factory method pattern

{{Short description|Object-oriented software design pattern}}
In [[object-oriented programming]], the '''factory method pattern''' is a [[software design pattern|design pattern]] that uses factory methods to deal with the problem of [[object creation|creating objects]] without having to specify their exact [[class (computer programming)|classes]]. Rather than by calling a [[Constructor (object-oriented programming)|constructor]], this is accomplished by invoking a factory method to create an object. Factory methods can be specified in an [[Interface (object-oriented programming)|interface]] and implemented by subclasses or implemented in a base class and optionally [[method overriding|overridden]] by subclasses. It is one of the 23 classic design patterns described in the book ''[[Design Patterns]]'' (often referred to as the "Gang of Four" or simply "GoF") and is subcategorized as a [[creational pattern]].{{sfn|Gamma|Helm|Johnson|Vlissides|1995|page=107}}

==Overview==
The factory method design pattern solves problems such as:
* How can an object's [[Subclass (computer science)|subclasses]] redefine its subsequent and distinct implementation? The pattern involves creation of a factory method within the [[Superclass (computer science)|superclass]] that defers the object's creation to a subclass's factory method.
* How can an object's instantiation be deferred to a subclass? Create an object by calling a factory method instead of directly calling a constructor.

This enables the creation of subclasses that can change the way in which an object is created (for example, by redefining which class to instantiate).

==Definition==
According to ''[[Design Patterns|Design Patterns: Elements of Reusable Object-Oriented Software]]'': "Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory method lets a class defer instantiation to subclasses."<ref name="gof">{{cite book |last1=Gamma |first1=Erich |author1-link=Erich Gamma |title=Design Patterns: Elements of Reusable Object-Oriented Software |title-link=Design Patterns |last2=Helm |first2=Richard |author2-link=Richard Helm |last3=Johnson |first3=Ralph |author3-link=Ralph Johnson (computer scientist) |last4=Vlissides |first4=John |author4-link=John Vlissides |publisher=Addison-Wesley |year=1995 |isbn=0-201-63361-2}}</ref>

Creating an object often requires complex processes not appropriate to include within a composing object. The object's creation may lead to a significant duplication of code, may require information inaccessible to the composing object, may not provide a sufficient level of abstraction or may otherwise not be included in the composing object's [[concern (computer science)|concerns]]. The factory method design pattern handles these problems by defining a separate [[method (computer science)|method]] for creating the objects, which subclasses can then override to specify the [[Subtyping|derived type]] of product that will be created.

The factory method pattern relies on inheritance, as object creation is delegated to subclasses that implement the factory method to create objects.<ref>{{cite book |last1=Freeman |first1=Eric |url=http://shop.oreilly.com/product/9780596007126.do |title=Head First Design Patterns: A Brain-Friendly Guide |last2=Robson |first2=Elisabeth |last3=Sierra |first3=Kathy |last4=Bates |first4=Bert |publisher=O'Reilly Media |year=2004 |isbn=978-0-596-00712-6 |editor-last1=Hendrickson |editor-first1=Mike |edition=1st |volume=1 |page=162 |format=paperback |access-date=2012-09-12 |editor-last2=Loukides |editor-first2=Mike}}</ref>
The pattern can also rely on the implementation of an [[Interface (object-oriented programming)|interface]].

==Structure==

=== UML class diagram ===

[[File:w3sDesign Factory Method Design Pattern UML.jpg|frame|none|A sample UML class diagram for the Factory Method design pattern.
<ref>{{cite web|title=The Factory Method design pattern - Structure and Collaboration|url=http://w3sdesign.com/?gr=c03&ugr=struct|website=w3sDesign.com|access-date=2017-08-12}}</ref>]]

In the above [[Unified Modeling Language|UML]] [[class diagram]], the <code>Creator</code> class that requires a <code>Product</code> object does not instantiate the <code>Product1</code> class directly. Instead, the <code>Creator</code> refers to a separate <code>factoryMethod()</code> to create a product object, which makes the <code>Creator</code> independent of the exact concrete class that is instantiated. Subclasses of <code>Creator</code> can redefine which class to instantiate. In this example, the <code>Creator1</code> subclass implements the abstract <code>factoryMethod()</code> by instantiating the <code>Product1</code> class.

== Examples ==
This [[C++23]] implementation is based on the pre C++98 implementation in the ''[[Design Patterns]]'' book.{{sfn|Gamma|Helm|Johnson|Vlissides|1995|page=122}}
<syntaxhighlight lang="c++">
import std;

enum class ProductId {MINE, YOURS};

// defines the interface of objects the factory method creates.
class Product {
public:
  virtual void print() = 0;
  virtual ~Product() = default;
};

// implements the Product interface.
class ConcreteProductMINE: public Product {
public:
  void print() {
    std::println("this={} print MINE", this);
  }
};

// implements the Product interface.
class ConcreteProductYOURS: public Product {
public:
  void print() {
    std::println("this={} print YOURS", this);
  }
};

// declares the factory method, which returns an object of type Product.
class Creator {
public:
  virtual std::unique_ptr<Product> create(ProductId id) {
    if (ProductId::MINE == id) return std::make_unique<ConcreteProductMINE>();
    if (ProductId::YOURS == id) return std::make_unique<ConcreteProductYOURS>();
    // repeat for remaining products...

    return nullptr;
  }
  virtual ~Creator() = default;
};

int main() {
  // The unique_ptr prevent memory leaks.
  std::unique_ptr<Creator> creator = std::make_unique<Creator>();
  std::unique_ptr<Product> product = creator->create(ProductId::MINE);
  product->print();

  product = creator->create(ProductId::YOURS);
  product->print();
}
</syntaxhighlight>

The program output is like

<syntaxhighlight lang="c++">
this=0x6e5e90 print MINE
this=0x6e62c0 print YOURS
</syntaxhighlight>

A maze game may be played in two modes, one with regular rooms that are only connected with adjacent rooms, and one with magic rooms that allow players to be transported at random. 

=== Structure ===
[[File:New WikiFactoryMethod.png|734x734px|center]]

<code>Room</code> is the base class for a final product (<code>MagicRoom</code> or <code>OrdinaryRoom</code>). <code>MazeGame</code> declares the abstract factory method to produce such a base product. <code>MagicRoom</code> and <code>OrdinaryRoom</code> are subclasses of the base product implementing the final product. <code>MagicMazeGame</code> and <code>OrdinaryMazeGame</code> are subclasses of <code>MazeGame</code> implementing the factory method producing the final products. Factory methods thus decouple callers (<code>MazeGame</code>) from the implementation of the concrete classes. This makes the <code>new</code> operator redundant, allows adherence to the [[open–closed principle]] and makes the final product more flexible in the event of change.

=== Example implementations ===
====[[C Sharp (programming language)|C#]]====

<syntaxhighlight lang="csharp">
// Empty vocabulary of actual object
public interface IPerson
{
    string GetName();
}

public class Villager : IPerson
{
    public string GetName()
    {
        return "Village Person";
    }
}

public class CityPerson : IPerson
{
    public string GetName()
    {
        return "City Person";
    }
}

public enum PersonType
{
    Rural,
    Urban
}

/// <summary>
/// Implementation of Factory - Used to create objects.
/// </summary>
public class PersonFactory
{
    public IPerson GetPerson(PersonType type)
    {
        switch (type)
        {
            case PersonType.Rural:
                return new Villager();
            case PersonType.Urban:
                return new CityPerson();
            default:
                throw new NotSupportedException();
        }
    }
}
</syntaxhighlight>
The above code depicts the creation of an interface called <code>IPerson</code> and two implementations called <code>Villager</code> and <code>CityPerson</code>. Based on the type passed to the <code>PersonFactory</code> object, the original concrete object is returned as the interface <code>IPerson</code>.

A factory method is just an addition to the <code>PersonFactory</code> class. It creates the object of the class through interfaces but also allows the subclass to decide which class is instantiated.

<syntaxhighlight lang="csharp">
public interface IProduct
{
    string GetName();
    bool SetPrice(double price);
}

public class Phone : IProduct
{
    private double _price;

    public string GetName()
    {
        return "Apple TouchPad";
    }

    public bool SetPrice(double price)
    {
        _price = price;
        return true;
    }
}

/* Almost same as Factory, just an additional exposure to do something with the created method */
public abstract class ProductAbstractFactory
{
    protected abstract IProduct MakeProduct();

    public IProduct GetObject() // Implementation of Factory Method.
    {
        return this.MakeProduct();
    }
}

public class PhoneConcreteFactory : ProductAbstractFactory
{
    protected override IProduct MakeProduct()
    {
        IProduct product = new Phone();
        // Do something with the object after receiving it
        product.SetPrice(20.30);
        return product;
    }
}
</syntaxhighlight>
In this example, <code>MakeProduct</code> is used in <code>concreteFactory</code>. As a result, <code>MakeProduct()</code> may be invoked in order to retrieve it from the <code>IProduct</code>. Custom logic could run after the object is obtained in the concrete factory method. <code>GetObject</code> is made abstract in the factory interface.

====[[Java (programming language)|Java]]====
This [[Java (programming language)|Java]] example is similar to one in the book ''[[Design Patterns]].''

[[File:Maze game UML.svg]]

The <code>MazeGame</code> uses <code>Room</code> but delegates the responsibility of creating <code>Room</code> objects to its subclasses that create the concrete classes. The regular game mode could use this template method:

<syntaxhighlight lang="java">
public abstract class Room {
    abstract void connect(Room room);
}

public class MagicRoom extends Room {
    public void connect(Room room) {}
}

public class OrdinaryRoom extends Room {
    public void connect(Room room) {}
}

public abstract class MazeGame {
     private final List<Room> rooms = new ArrayList<>();

     public MazeGame() {
          Room room1 = makeRoom();
          Room room2 = makeRoom();
          room1.connect(room2);
          rooms.add(room1);
          rooms.add(room2);
     }

     abstract protected Room makeRoom();
}
</syntaxhighlight>

The <code>MazeGame</code> constructor is a [[Template method pattern|template method]] that adds some common logic. It refers to the <code>makeRoom()</code> factory method that encapsulates the creation of rooms such that other rooms can be used in a subclass. To implement the other game mode that has magic rooms, the <code>makeRoom</code> method may be overridden:

<syntaxhighlight lang="java">
public class MagicMazeGame extends MazeGame {
    @Override
    protected MagicRoom makeRoom() {
        return new MagicRoom();
    }
}

public class OrdinaryMazeGame extends MazeGame {
    @Override
    protected OrdinaryRoom makeRoom() {
        return new OrdinaryRoom();
    }
}

MazeGame ordinaryGame = new OrdinaryMazeGame();
MazeGame magicGame = new MagicMazeGame();
</syntaxhighlight>

====[[PHP]]====
This [[PHP]] example shows interface implementations instead of subclassing (however, the same can be achieved through subclassing). The factory method can also be defined as <code>public</code>and called directly by the client code (in contrast to the previous Java example).

<syntaxhighlight lang="php">
/* Factory and car interfaces */

interface CarFactory
{
    public function makeCar(): Car;
}

interface Car
{
    public function getType(): string;
}

/* Concrete implementations of the factory and car */

class SedanFactory implements CarFactory
{
    public function makeCar(): Car
    {
        return new Sedan();
    }
}

class Sedan implements Car
{
    public function getType(): string
    {
        return 'Sedan';
    }
}

/* Client */

$factory = new SedanFactory();
$car = $factory->makeCar();
print $car->getType();
</syntaxhighlight>

==== [[Python (programming language)|Python]] ====
This Python example employs the same as did the previous Java example.

<syntaxhighlight lang="python3">
from abc import ABC, abstractmethod


class MazeGame(ABC):
    def __init__(self) -> None:
        self.rooms = []
        self._prepare_rooms()

    def _prepare_rooms(self) -> None:
        room1 = self.make_room()
        room2 = self.make_room()

        room1.connect(room2)
        self.rooms.append(room1)
        self.rooms.append(room2)

    def play(self) -> None:
        print(f"Playing using {self.rooms[0]}")

    @abstractmethod
    def make_room(self):
        raise NotImplementedError("You should implement this!")


class MagicMazeGame(MazeGame):
    def make_room(self) -> "MagicRoom":
        return MagicRoom()


class OrdinaryMazeGame(MazeGame):
    def make_room(self) -> "OrdinaryRoom":
        return OrdinaryRoom()


class Room(ABC):
    def __init__(self) -> None:
        self.connected_rooms = []

    def connect(self, room: "Room") -> None:
        self.connected_rooms.append(room)


class MagicRoom(Room):
    def __str__(self) -> str:
        return "Magic room"


class OrdinaryRoom(Room):
    def __str__(self) -> str:
        return "Ordinary room"


ordinaryGame = OrdinaryMazeGame()
ordinaryGame.play()

magicGame = MagicMazeGame()
magicGame.play()
</syntaxhighlight>

==Uses==
* In [[ADO.NET]], [https://docs.microsoft.com/en-us/dotnet/api/system.data.idbcommand.createparameter?view=netframework-4.8 IDbCommand.CreateParameter] is an example of the use of factory method to connect parallel class hierarchies.
* In [[Qt (toolkit)|Qt]], [http://qt-project.org/doc/qt-5.0/qtwidgets/qmainwindow.html#createPopupMenu QMainWindow::createPopupMenu] {{Webarchive|url=https://web.archive.org/web/20150719014739/http://qt-project.org/doc/qt-5.0/qtwidgets/qmainwindow.html#createPopupMenu |date=2015-07-19 }} is a factory method declared in a framework that can be overridden in [[application code]].
* In [[Java (programming language)|Java]], several factories are used in the [http://download.oracle.com/javase/1.5.0/docs/api/javax/xml/parsers/package-summary.html javax.xml.parsers] package, such as javax.xml.parsers.DocumentBuilderFactory or javax.xml.parsers.SAXParserFactory.
* In the [[HTML5]] [[Document Object Model|DOM]] [[application programming interface|API]], the Document interface contains a createElement() factory method for creating specific elements of the HTMLElement interface.

==See also==
* ''[[Design Patterns]]'', the highly influential book
* [[Design pattern]], overview of design patterns in general
* [[Abstract factory pattern]], a pattern often implemented using factory methods
* [[Builder pattern]], another creational pattern
* [[Template method pattern]], which may call factory methods
* [[Joshua Bloch]]'s idea of a [[static factory method]] for which Bloch claims there is no direct equivalent in ''[[Design Patterns]]''.

==Notes==
{{Reflist}}
==References==
*{{cite book
 | author1=Martin Fowler | author2-link=Kent Beck | author2=Kent Beck | author3-link=John Brant (author) | author3=John Brant | author4-link=William Opdyke | author4=William Opdyke | author5-link=Don Roberts (author) | author5=Don Roberts
 | title = Refactoring: Improving the Design of Existing Code
 | publisher = Addison-Wesley
 | date=June 1999
 | isbn = 0-201-48567-2
| author1-link=Martin Fowler (software engineer) }}
*{{cite book |author1=Cox, Brad J. |
       title=Object-oriented programming: an evolutionary approach |
       publisher=Addison-Wesley |
       year=1986 |
       isbn=978-0-201-10393-9
       |
 title-link=Object-oriented programming }}
*{{cite journal |
       last=Cohen |
       first=Tal |author2=Gil, Joseph  |
       title=Better Construction with Factories |
       journal=Journal of Object Technology |
 volume=6 |
 issue=6 |
 pages=103 |
       year=2007 |
       publisher=[[Bertrand Meyer]] |
       url=http://tal.forum2.org/static/cv/Factories.pdf |
       access-date=2007-03-12
       |
 doi=10.5381/jot.2007.6.6.a3 |
       doi-access=free }}

==External links==
{{Wikibooks
 | 1 = Computer Science Design Patterns
 | 2 = Factory method examples
}}
* [http://designpattern.co.il/Factory.html Factory Design Pattern] {{Webarchive|url=https://web.archive.org/web/20180110103637/http://designpattern.co.il/Factory.html |date=2018-01-10 }} Implementation in Java

* [https://web.archive.org/web/20080503082912/http://www.lepus.org.uk/ref/companion/FactoryMethod.xml Factory method in UML and in LePUS3] (a Design Description Language)
* [http://drdobbs.com/java/208403883 Consider static factory methods] by Joshua Bloch

{{Design Patterns Patterns}}

{{DEFAULTSORT:Factory Method Pattern}}
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