FactoryMethod

Factory Method

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

Problem

• You should create a particular object (e.g. AmericanCheesePizza or ItalianVeggiePizza) depending on the type.
• Creating a new object using new keyword and if-else conditionals in every place where you need is very inflexible.

Solution

• Define a separate operation factory method in the Creator class (e.g. CreatePizza() method in the AmericanPizzaStore class)
• Move the constructor call (new AmericanCheesePizza()) inside the factory method.

Definition

        // A facotry method
        //    1. is abstract so the subclass are counted on to handle object creation.
        //    2. returns a Product.
        //    3. isolates the client from knowing which concrete Product is actually created.
        //    4. may be parameterized (or not) to select among several variations of a product.
        protected abstract Pizza CreatePizza(PizzaType type);

Common Structure

• Product (Pizza)
  • defines the interface of objects the factory method creates
• ConcreteProduct (ItalianCheesePizza, AmericanCheesePizza etc.)
  • implements the Product interface
• Creator (PizzaStore)
  • declares the abstract factory method which returns an instance of type Product.
  • may call the factory method to return an instance (OrderPizza calls CreatePizza)
• ConcreteCreator(AmericanPizzaStore)
  • overrides the abstract factory method to return an instance of a ConcreteProduct (eg. AmericanCheesePizza)

Collaborations

Source: static.dzone.com

• Creator/Factory relies on its subclasses to implement the factory method so that it returns an instance of the appropriate ConcreteProduct.

Benefits

• Avoids tight coupling between concrete products and code that uses them.
• Simplifies code due to moving all creational code to one place.
• Simplifies adding new products to the program.

Drawbacks

• Requires extra subclasses.

Example Usage

            PizzaStore americanStore = new AmericanPizzaStore();
            Pizza pizza = americanStore.OrderPizza(PizzaType.Veggie);
            Console.WriteLine("Esposito ordered a " + pizza.Name);
            
            // Italian Pizza Store
            PizzaStore italianStore = new ItalianPizzaStore();
            Pizza pizza = italianStore.OrderPizza(PizzaType.Cheese);
            Console.WriteLine("Esposito ordered a " + pizza.Name);

Relations with Other Patterns

• Abstract Factory is often implemented with Factory Methods.

• Templated Methods - Factory methods are usually called within Template Methods.

• Prototype doesn't require subclassing, but it does require an initialize() operation. On the other hand, Factory Method requires subclassing, but doesn't require initialization step.