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Factory method patternIn 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 (often referred to as the "Gang of Four" or simply "GoF") and is subcategorized as a creational pattern.[1] OverviewThe factory method design pattern solves problems such as:
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). DefinitionAccording 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."[2] 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.[3] The pattern can also rely on the implementation of an interface. StructureUML class diagram In the above UML class diagram, the ExamplesThis C++23 implementation is based on the pre C++98 implementation in the book.[5][which?] 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();
}
The program output is like this=0x6e5e90 print MINE
this=0x6e62c0 print YOURS
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
Example implementations// 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();
}
}
}
The above code depicts the creation of an interface called A factory method is just an addition to the 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;
}
}
In this example, This Java example is similar to one in the book Design Patterns.
The 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();
}
The 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();
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 /* 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();
This Python example employs the same as did the previous 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(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()
Uses
See also
Notes
References
External links The Wikibook Computer Science Design Patterns has a page on the topic of: Factory method examples
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