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
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:
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.
See the UML class diagram below.

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."
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.

Structure

UML class diagram

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. 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 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#">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 Factory

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
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

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">Java (programming language)">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
public class MagicRoom extends Room
public class OrdinaryRoom extends Room
public abstract class MazeGame

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

[PHP]

Another example in PHP follows, this time using interface implementations as opposed to subclassing. It is important to note that the factory method can also be defined as public and 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

Same as 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:
return MagicRoom
class OrdinaryMazeGame:
def make_room:
return OrdinaryRoom
class Room:
def __init__ -> None:
self.connected_rooms =
def connect -> None:
self.connected_rooms.append
class MagicRoom:
def __str__:
return "Magic room"
class OrdinaryRoom:
def __str__:
return "Ordinary room"
ordinaryGame = OrdinaryMazeGame
ordinaryGame.play
magicGame = MagicMazeGame
magicGame.play

Uses