AbstractFactory

Abstract Factory

The Abstract Factory Pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes.

Problem

• Creating objects directly within the class that requires the object is inflexible
• Changing the instantiation later without chaging the class is impossible
• The created object cannot be reused
• The created object is tightly coupled with the class and it makes the class hard to test because real objects can't be replaced with mock objects.

Solution

• Encapsulate object creation in a separate (factory) object. That is, define an interface (AbstractFactory) for creating objects, and implement the interface.
• A class delegates object creation to a factory object instead of creating objects directly.

Common Structure

• AbstractFactory (PizzaIngredientFactory)
  • declares an interface for operations that create abstract products
• ConcreteFactory (ItalianPizzaIngredientFactory, AmericanPizzaIngredientFactory)
  • implements the operations to create concrete product objects
• AbstractProduct (Dough)
  • declares an interface for a type of product object
• ConcreteProduct(ThinCrustDough, ThickCrustDough)
  • defines a product object to be created by the corresponding concrete factory
  • implements the AbstractProduct interface

Collaboration

TODO - Add sequence diagram

• Normally a single instance of a ConcreteFactory class is created at runtime.

Benefits

• Avoids tight coupling between concrete products and code that uses them.
• Allows building families of product objects and guarantees their compatibility and consistency.
• Divides responsibilities between multiple classes.

Drawbacks

• Increases overall code complexity by creating multiple additional classes.
• Supporting new kinds of products is difficult.
  • It requires extending the AbstractFactory interface, which involves changing all of its subclasses.

Example

Definition - AbstractFactory and ConcreteFactory

publicinterfacePizzaIngredientFactory{DoughCreateDough();}publicclassAmericanPizzaIngredientFactory:PizzaIngredientFactory{publicDoughCreateDough(){returnnewThickCrustDough();}}publicclassItalianPizzaIngredientFactory:PizzaIngredientFactory{publicDoughCreateDough(){returnnewThinCrustDough();}}

Definition - AbstractProduct and ConcreteProduct

publicinterfaceDough{stringGetName();}publicclassThickCrustDough:Dough{publicstringGetName(){return"Extra thick crust dough";}}publicclassThinCrustDough:Dough{publicstringGetName(){return"Thin crust dough";}}

Usage

publicclassAmericanPizzaStore:PizzaStore{// AmericanPizzaStore creates the corresponding ingriedient factory.privatePizzaIngredientFactoryingredientFactory=newAmericanPizzaIngredientFactory();protectedoverridePizzaCreatePizza(PizzaTypetype){switch(type){casePizzaType.Cheese:// the correct ingredientFactory is transferred when creating a pizzareturnnewCheesePizza(ingredientFactory);casePizzaType.Veggie:returnnewVeggiePizza(ingredientFactory);default:thrownewNotImplementedException();}}}

Relations with Other Patterns

• Factory Method - AbstractFactory classes are often implemented with factory methods, but they can also be implemented using Prototype.

• Singleton - A concrete factory is often a singleton.