Factory method pattern


In object-oriented programming, the factory method pattern is a design pattern that uses factory methods to deal with the problem of creating objects without having to specify their exact classes. Rather than by calling a constructor, this is accomplished by invoking a factory method to create an object. Factory methods can be specified in an interface and implemented by subclasses or implemented in a base class and optionally overridden by subclasses. It is one of the 23 classic design patterns described in the book Design Patterns and is subcategorized as a creational pattern.

Overview

The factory method design pattern solves problems such as:
  • How can an object's subclasses redefine its subsequent and distinct implementation? The pattern involves creation of a factory method within the 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.

Definition

According to 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."
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 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.
The pattern can also rely on the implementation of an interface.

Structure

UML class diagram

In the above UML class diagram, the Creator class that requires a Product object does not instantiate the Product1 class directly. Instead, the Creator refers to a separate factoryMethod to create a product object, which makes the Creator independent of the exact concrete class that 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.

Examples

Structure

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.
Room is the base class for a final product. 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. Factory methods thus decouple callers 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.

Example implementations

[C++]

This C++23 implementation is based on the pre C++98 implementation in the Design Patterns book.

import std;
using std::unique_ptr;
enum class ProductId: char ;
// defines the interface of objects the factory method creates.
class Product ;
// implements the Product interface.
class ConcreteProductMINE: public Product ;
// implements the Product interface.
class ConcreteProductYOURS: public Product ;
// declares the factory method, which returns an object of type Product.
class Creator ;
int main

The program output is like

this=0x6e5e90 print MINE
this=0x6e62c0 print YOURS

C#">C Sharp (programming language)">C#


// Empty vocabulary of actual object
public interface IPerson
public class Villager : IPerson
public class CityPerson : IPerson
public enum PersonType
///
/// Implementation of Factory - Used to create objects.
///
public class PersonFactory

The above code depicts the creation of an interface called IPerson and two implementations called Villager and CityPerson. Based on the type passed to the PersonFactory object, the original concrete object is returned as the interface IPerson.
A factory method is just an addition to the PersonFactory class. It creates the object of the class through interfaces but also allows the subclass to decide which class is instantiated.

public interface IProduct
public class Phone : IProduct
/* Almost same as Factory, just an additional exposure to do something with the created method */
public abstract class ProductAbstractFactory
public class PhoneConcreteFactory : ProductAbstractFactory

In this example, MakeProduct is used in concreteFactory. As a result, MakeProduct may be invoked in order to retrieve it from the IProduct. Custom logic could run after the object is obtained in the concrete factory method. GetObject is made abstract in the factory interface.

Java">Java (programming language)">Java

This Java example is similar to one in the book Design Patterns.
The MazeGame uses Room but delegates the responsibility of creating Room objects to its subclasses that create the concrete classes. The regular game mode could use this template method:

public abstract class Room
public class MagicRoom extends Room
public class OrdinaryRoom extends Room
public abstract class MazeGame

The MazeGame constructor is a template method that adds 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, the makeRoom method may be overridden:

public class MagicMazeGame extends MazeGame
public class OrdinaryMazeGame extends MazeGame
MazeGame ordinaryGame = new OrdinaryMazeGame;
MazeGame magicGame = new MagicMazeGame;

[PHP]

This PHP example shows interface implementations instead of subclassing. The factory method can also be defined as publicand called directly by the client code.

/* Factory and car interfaces */
interface CarFactory
interface Car
/* Concrete implementations of the factory and car */
class SedanFactory implements CarFactory
class Sedan implements Car
/* Client */
$factory = new SedanFactory;
$car = $factory->makeCar;
print $car->getType;

Python">Python (programming language)">Python

This Python example employs the same as did the previous Java example.

from abc import ABC, abstractmethod
class MazeGame:
def __init__ -> None:
self.rooms =
self._prepare_rooms
def _prepare_rooms -> None:
room1 = self.make_room
room2 = self.make_room
room1.connect
self.rooms.append
self.rooms.append
def play -> None:
print
@abstractmethod
def make_room:
raise NotImplementedError
class MagicMazeGame:
def make_room -> "MagicRoom":
return MagicRoom
class OrdinaryMazeGame:
def make_room -> "OrdinaryRoom":
return OrdinaryRoom
class Room:
def __init__ -> None:
self.connected_rooms =
def connect -> None:
self.connected_rooms.append
class MagicRoom:
def __str__ -> str:
return "Magic room"
class OrdinaryRoom:
def __str__ -> str:
return "Ordinary room"
ordinaryGame = OrdinaryMazeGame
ordinaryGame.play
magicGame = MagicMazeGame
magicGame.play

Uses

  • In ADO.NET, is an example of the use of factory method to connect parallel class hierarchies.
  • In Qt, is a factory method declared in a framework that can be overridden in application code.
  • In Java, several factories are used in the package, such as 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.