Factory method pattern

In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. This is done by creating objects by calling a factory method—either specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling a constructor.

Overview

The Factory Method [1] design pattern is one of the "Gang of Four" design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

The Factory Method design pattern is used instead of the regular class constructor for keeping within the SOLID principles of programming, decoupling the construction of objects from the objects themselves. This has the following advantages and is useful for the following cases, among others: [2]

  • Allows construction of classes with a component of a type that has not been predetermined, but only defined in an "interface", or which is defined as a dynamic type.
Thus, for example, a class Vehicle that has a member Motor of interface IMotor, but no concrete type of Motor defined in advance, can be constructed by telling the Vehicle constructor to use an ElectricMotor or a GasolineMotor. The Vehicle constructor code then calls a Motor factory method, to create the desired Motor that complies with the IMotor interface.
  • Allows construction of subclasses to a parent whose component type has not been predetermined, but only defined in an interface, or which is defined as a dynamic type.
For example, a class Vehicle with a member Motor defined with a dynamic type, can have subclasses of type ElectricPlane and OldCar each constructed with a different type of Motor. This can be accomplished by constructing the subclasses with a Vehicle factory method, while supplying the motor type. In cases like this, the constructor may be hidden.
  • Allows for more readable code in cases where multiple constructors exist, each for a different reason.
For example if there are two constructors Vehicle(make:string, motor:number) and Vehicle(make:string, owner:string, license:number, purchased:date) a more readable construction of the classes would be to use Vehicle.CreateOwnership(make:string, owner:string, license:number, purchased: date) vs Vehicle.Create(make:string, motor:number)
  • Allows a class to defer instantiation to subclasses, and to prevent direct instantiation of an object of the parent class type.
For example, a Vehicle can be prevented from being instantiated directly since it has no constructor, and only subclasses like ElectricPlane or OldCar can be created by calling the Vehicle (static) factory method in the subclass constructor or initializer.

Creating an object directly within the class that requires or uses the object is inflexible because it commits the class to a particular object and makes it impossible to change the instantiation independently of the class. A change to the instantiator would require a change to the class code which we would rather not touch. This is referred to as code coupling and the Factory method pattern assists in decoupling the code.

The Factory Method design pattern is used by first defining a separate operation, a factory method, for creating an object, and then using this factory method by calling it to create the object. This enables writing of subclasses that decide how a parent object is created and what type of objects the parent contains.

Definition

"Define an interface for creating an object, but let subclasses decide which class to instantiate. The Factory method lets a class defer instantiation it uses to subclasses." (Gang Of Four)

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 not accessible to the composing object, may not provide a sufficient level of abstraction, or may otherwise not be part of the composing object's concerns. The factory method design pattern handles these problems by defining a separate method for creating the objects, which subclasses can then override to specify the 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.[3]

Structure

UML class diagram
A sample UML class diagram for the Factory Method design pattern. [4]

In the above UML class diagram, the Creator class that requires a Product object doesn't instantiate the Product1 class directly. Instead, the Creator refers to a separate factoryMethod() to create a product object, which makes the Creator independent of which concrete class is instantiated. Subclasses of Creator can redefine which class to instantiate. In this example, the Creator1 subclass implements the abstract factoryMethod() by instantiating the Product1 class.

Example

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

Room is the base class for a final product (MagicRoom or OrdinaryRoom). MazeGame declares the abstract factory method to produce such a base product. MagicRoom and OrdinaryRoom are subclasses of the base product implementing the final product. MagicMazeGame and OrdinaryMazeGame are subclasses of MazeGame implementing the factory method producing the final products. Thus factory methods decouple callers (MazeGame) from the implementation of the concrete classes. This makes the "new" Operator redundant, allows adherence to the Open/closed principle and makes the final product more flexible in the event of change.

C#

 
// 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 Factory
{
    public IPerson GetPerson(PersonType type)
    {
        switch (type)
        {
            case PersonType.Rural:
                return new Villager();
            case PersonType.Urban:
                return new CityPerson();
            default:
                throw new NotSupportedException();
        }
    }
}

In the above code you can see the creation of one interface called IPerson and two implementations called Villager and CityPerson. Based on the type passed into the Factory object, we are returning the original concrete object as the interface IPerson.

A factory method is just an addition to Factory class. It creates the object of the class through interfaces but on the other hand, it also lets the subclass decide which class is instantiated.

public interface IProduct
{
    string GetName();
    string 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 you get the object.
        product.SetPrice(20.30);
        return product;
    }
}

You can see we have used MakeProduct in concreteFactory. As a result, you can easily call MakeProduct() from it to get the IProduct. You might also write your custom logic after getting the object in the concrete Factory Method. The GetObject is made abstract in the Factory interface.

Java

This Java example is similar to one in the book Design Patterns.

The MazeGame uses Rooms but it puts the responsibility of creating Rooms to its subclasses which create the concrete classes. The regular game mode could use this template method:

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();
}

In the above snippet, the MazeGame constructor is a template method that makes some common logic. It refers to the makeRoom 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, it suffices to override the makeRoom method:

public class MagicMazeGame extends MazeGame {
    @Override
    protected Room makeRoom() {
        return new MagicRoom();
    }
}

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

MazeGame ordinaryGame = new OrdinaryMazeGame();
MazeGame magicGame = new MagicMazeGame();

PHP

Another example in PHP follows, this time using interface implementations as opposed to subclassing (however, the same can be achieved through subclassing). It is important to note that the factory method can also be defined as public and called directly by the client code (in contrast with the Java example above).

/* 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();

Python

Same as Java example.

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('Playing using "{}"'.format(self.rooms[0]))

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


class MagicMazeGame(MazeGame):
    def make_room(self):
        return MagicRoom()


class OrdinaryMazeGame(MazeGame):
    def make_room(self):
        return OrdinaryRoom()


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

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


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


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


ordinaryGame = OrdinaryMazeGame()
ordinaryGame.play()

magicGame = MagicMazeGame()
magicGame.play()

Uses
  • In ADO.NET, IDbCommand.CreateParameter is an example of the use of factory method to connect parallel class hierarchies.
  • In Qt, QMainWindow::createPopupMenu is a factory method declared in a framework that can be overridden in application code.
  • In Java, several factories are used in the javax.xml.parsers package. e.g. javax.xml.parsers.DocumentBuilderFactory or javax.xml.parsers.SAXParserFactory.
  • In the HTML5 DOM API, the Document interface contains a createElement factory method for creating specific elements of the HTMLElement interface.

See also

References
  1. Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp. 107ff. ISBN 0-201-63361-2.CS1 maint: multiple names: authors list (link)
  2. "The Factory Method design pattern - Problem, Solution, and Applicability". w3sDesign.com. Retrieved 2017-08-17.
  3. Freeman, Eric; Freeman, Elisabeth; Kathy, Sierra; Bert, Bates (2004). Hendrickson, Mike; Loukides, Mike (eds.). Head First Design Patterns (paperback). 1. O'REILLY. p. 162. ISBN 978-0-596-00712-6. Retrieved 2012-09-12.
  4. "The Factory Method design pattern - Structure and Collaboration". w3sDesign.com. Retrieved 2017-08-12.
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