Creational Patterns

Factory Method Pattern

Definition

The Factory Method Pattern defines an interface for creating an object, but lets subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

Design Principle

• Dependency Inversion Principle: high-level components should not depend on our low-level components; rather, they should both depend on abstractions. Depend upon abstractions. Do not depend on concrete classes.

Best practices

• By placing all creation code in one object or method, we avoid duplication in the code and provide one place for maintenance.
• Clients depend only upon interfaces reather than the concrete classes required to instantiate objects => Allow to program to an interface, not an implementation => make the code more flexible and extensible in the future.

Example codes

Python Go

from abc import ABC, abstractmethod

# Abstract Product
class Pizza(ABC):
    def __init__(self):
        self.name = ""
        self.dough = ""
        self.sauce = ""
        self.toppings = []

    def prepare(self):
        print(f"Preparing {self.name}")
        print(f"Tossing {self.dough}")
        print(f"Adding {self.sauce}")
        print("Adding toppings:")
        for topping in self.toppings:
            print(f"  {topping}")

    @staticmethod
    def bake():
        print("Bake for 25 minutes at 350")

    @staticmethod
    def cut():
        print("Cutting the pizza into diagonal slices")

    @staticmethod
    def box():
        print("Place pizza in official PizzaStore box")

# Concrete Products
class NYStyleCheesePizza(Pizza):
    def __init__(self):
        super().__init__()
        self.name = "NY Style Sauce and Cheese Pizza"
        self.dough = "Thin Crust Dough"
        self.sauce = "Marinara Sauce"
        self.toppings.append("Grated Reggiano Cheese")

class ChicagoStyleCheesePizza(Pizza):
    def __init__(self):
        super().__init__()
        self.name = "Chicago Style Deep Dish Cheese Pizza"
        self.dough = "Extra Thick Crust Dough"
        self.sauce = "Plum Tomato Sauce"
        self.toppings.append("Shredded Mozzarella Cheese")

    @staticmethod
    def cut():
        print("Cutting the pizza into square slices")

# Abstract Creator
class PizzaStore(ABC):
    @abstractmethod
    def create_pizza(self, item):
        pass

    def order_pizza(self, pizza_type):
        pizza = self.create_pizza(pizza_type)
        print(f"--- Making a {pizza.name} ---")
        pizza.prepare()
        pizza.bake()
        pizza.cut()
        pizza.box()
        return pizza

# Concrete Creators
class NYPizzaStore(PizzaStore):
    def create_pizza(self, item):
        if item == "cheese":
            return NYStyleCheesePizza()
        # Add other pizza types here
        else:
            return None

class ChicagoPizzaStore(PizzaStore):
    def create_pizza(self, item):
        if item == "cheese":
            return ChicagoStyleCheesePizza()
        # Add other pizza types here
        else:
            return None

# Client code
if __name__ == "__main__":
    ny_store = NYPizzaStore()
    chicago_store = ChicagoPizzaStore()

    ny_pizza = ny_store.order_pizza("cheese")
    print(f"Ethan ordered a {ny_pizza.name}\n")

    chicago_pizza = chicago_store.order_pizza("cheese")
    print(f"Joel ordered a {chicago_pizza.name}\n")

"""Output:
--- Making a NY Style Sauce and Cheese Pizza ---
Preparing NY Style Sauce and Cheese Pizza
Tossing Thin Crust Dough
Adding Marinara Sauce
Adding toppings:
  Grated Reggiano Cheese
Bake for 25 minutes at 350
Cutting the pizza into diagonal slices
Place pizza in official PizzaStore box
Ethan ordered a NY Style Sauce and Cheese Pizza

--- Making a Chicago Style Deep Dish Cheese Pizza ---
Preparing Chicago Style Deep Dish Cheese Pizza
Tossing Extra Thick Crust Dough
Adding Plum Tomato Sauce
Adding toppings:
  Shredded Mozzarella Cheese
Bake for 25 minutes at 350
Cutting the pizza into square slices
Place pizza in official PizzaStore box
Joel ordered a Chicago Style Deep Dish Cheese Pizza
"""

package main

import "fmt"

// Pizza interface
type Pizza interface {
    Prepare()
    Bake()
    Cut()
    Box()
    GetName() string
}

// BasePizza struct
type BasePizza struct {
    name     string
    dough    string
    sauce    string
    toppings []string
}

func (b *BasePizza) Prepare() {
    fmt.Printf("Preparing %s\n", b.name)
    fmt.Printf("Tossing %s\n", b.dough)
    fmt.Printf("Adding %s\n", b.sauce)
    fmt.Println("Adding toppings:")
    for _, topping := range b.toppings {
        fmt.Printf("  %s\n", topping)
    }
}

func (b *BasePizza) Bake() {
    fmt.Println("Bake for 25 minutes at 350")
}

func (b *BasePizza) Cut() {
    fmt.Println("Cutting the pizza into diagonal slices")
}

func (b *BasePizza) Box() {
    fmt.Println("Place pizza in official PizzaStore box")
}

func (b *BasePizza) GetName() string {
    return b.name
}

// NYStyleCheesePizza struct
type NYStyleCheesePizza struct {
    BasePizza
}

func NewNYStyleCheesePizza() *NYStyleCheesePizza {
    pizza := &NYStyleCheesePizza{}
    pizza.name = "NY Style Sauce and Cheese Pizza"
    pizza.dough = "Thin Crust Dough"
    pizza.sauce = "Marinara Sauce"
    pizza.toppings = []string{"Grated Reggiano Cheese"}
    return pizza
}

// ChicagoStyleCheesePizza struct
type ChicagoStyleCheesePizza struct {
    BasePizza
}

func NewChicagoStyleCheesePizza() *ChicagoStyleCheesePizza {
    pizza := &ChicagoStyleCheesePizza{}
    pizza.name = "Chicago Style Deep Dish Cheese Pizza"
    pizza.dough = "Extra Thick Crust Dough"
    pizza.sauce = "Plum Tomato Sauce"
    pizza.toppings = []string{"Shredded Mozzarella Cheese"}
    return pizza
}

func (c *ChicagoStyleCheesePizza) Cut() {
    fmt.Println("Cutting the pizza into square slices")
}

// PizzaStore interface
type PizzaStore interface {
    CreatePizza(pizzaType string) Pizza
    OrderPizza(pizzaType string) Pizza
}

// BasePizzaStore struct
type BasePizzaStore struct {
    createPizza func(pizzaType string) Pizza
}

func (b *BasePizzaStore) CreatePizza(pizzaType string) Pizza {
    return b.createPizza(pizzaType)
}

func (b *BasePizzaStore) OrderPizza(pizzaType string) Pizza {
    pizza := b.CreatePizza(pizzaType)
    if pizza == nil {
        fmt.Printf("Sorry, we don't have %s pizza.\n", pizzaType)
        return nil
    }
    fmt.Printf("--- Making a %s ---\n", pizza.GetName())
    pizza.Prepare()
    pizza.Bake()
    pizza.Cut()
    pizza.Box()
    return pizza
}

// NYPizzaStore struct
type NYPizzaStore struct {
    BasePizzaStore
}

func NewNYPizzaStore() *NYPizzaStore {
    store := &NYPizzaStore{}
    store.createPizza = func(pizzaType string) Pizza {
        if pizzaType == "cheese" {
            return NewNYStyleCheesePizza()
        }
        // Add other pizza types here
        return nil
    }
    return store
}

// ChicagoPizzaStore struct
type ChicagoPizzaStore struct {
    BasePizzaStore
}

func NewChicagoPizzaStore() *ChicagoPizzaStore {
    store := &ChicagoPizzaStore{}
    store.createPizza = func(pizzaType string) Pizza {
        if pizzaType == "cheese" {
            return NewChicagoStyleCheesePizza()
        }
        // Add other pizza types here
        return nil
    }
    return store
}

func main() {
    nyStore := NewNYPizzaStore()
    chicagoStore := NewChicagoPizzaStore()

    pizza := nyStore.OrderPizza("cheese")
    if pizza != nil {
        fmt.Printf("Ethan ordered a %s\n\n", pizza.GetName())
    }

    pizza = chicagoStore.OrderPizza("cheese")
    if pizza != nil {
        fmt.Printf("Joel ordered a %s\n\n", pizza.GetName())
    }

    // Test with a non-existent pizza type
    pizza = nyStore.OrderPizza("pepperoni")
    if pizza != nil {
        fmt.Printf("Ethan ordered a %s\n\n", pizza.GetName())
    }
}

/* OUTPUT:
--- Making a NY Style Sauce and Cheese Pizza ---
Preparing NY Style Sauce and Cheese Pizza
Tossing Thin Crust Dough
Adding Marinara Sauce
Adding toppings:
  Grated Reggiano Cheese
Bake for 25 minutes at 350
Cutting the pizza into diagonal slices
Place pizza in official PizzaStore box
Ethan ordered a NY Style Sauce and Cheese Pizza

--- Making a Chicago Style Deep Dish Cheese Pizza ---
Preparing Chicago Style Deep Dish Cheese Pizza
Tossing Extra Thick Crust Dough
Adding Plum Tomato Sauce
Adding toppings:
  Shredded Mozzarella Cheese
Bake for 25 minutes at 350
Cutting the pizza into square slices
Place pizza in official PizzaStore box
Joel ordered a Chicago Style Deep Dish Cheese Pizza

Sorry, we don't have pepperoni pizza.
*/