Factory Method Pattern

Factory Method Pattern


Define an interface for creating an object but let sub-classes decide which
instance to instantiate. Factory Method lets a class defer instantiation to
subclasses.

UML class diagram

For better understanding follow the combined post on Abstract Factory and  Factory Method  http://devsid.blogspot.in/2013/09/abstract-factory.html

 participants

    The classes and/or objects participating in this pattern are:

• Product  (Page)
  • defines the interface of objects the factory method creates
• ConcreteProduct  (SkillsPage, EducationPage, ExperiencePage)
  • implements the Product interface
• Creator  (Document)
  • declares the factory method, which returns an object of type Product. Creator may also define a default implementation of the factory method that returns a default ConcreteProduct object.
  • may call the factory method to create a Product object.
• ConcreteCreator  (Report, Resume)
• overrides the factory method to return an instance of a ConcreteProduct.

sample code in C#

This structural code demonstrates the Factory method offering great flexibility in creating different objects. The Abstract class may provide a default object, but each subclass can instantiate an extended version of the object.

// Factory Method pattern -- Structural example

using System; namespace DoFactory.GangOfFour.Factory.Structural {   /// <summary>   /// MainApp startup class for Structural   /// Factory Method Design Pattern.   /// </summary>   class MainApp   {     /// <summary>     /// Entry point into console application.     /// </summary>     static void Main()     {       // An array of creators       Creator[] creators = new Creator[2];       creators[0] = new ConcreteCreatorA();       creators[1] = new ConcreteCreatorB();       // Iterate over creators and create products       foreach (Creator creator in creators)       {         Product product = creator.FactoryMethod();         Console.WriteLine("Created {0}",           product.GetType().Name);       }       // Wait for user       Console.ReadKey();     }   }   /// <summary>   /// The 'Product' abstract class   /// </summary>   abstract class Product   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class ConcreteProductA : Product   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class ConcreteProductB : Product   {   }   /// <summary>   /// The 'Creator' abstract class   /// </summary>   abstract class Creator   {     public abstract Product FactoryMethod();   }   /// <summary>   /// A 'ConcreteCreator' class   /// </summary>   class ConcreteCreatorA : Creator   {     public override Product FactoryMethod()     {       return new ConcreteProductA();     }   }   /// <summary>   /// A 'ConcreteCreator' class   /// </summary>   class ConcreteCreatorB : Creator   {     public override Product FactoryMethod()     {       return new ConcreteProductB();     }   } }

Output Created ConcreteProductA Created ConcreteProductB

This real-world code demonstrates the Factory method offering flexibility in creating different documents. The derived Document classes Report and Resume instantiate extended versions of the Document class. Here, the Factory Method is called in the constructor of the Document base class.

// Factory Method pattern -- Real World example

using System; using System.Collections.Generic; namespace DoFactory.GangOfFour.Factory.RealWorld {   /// <summary>   /// MainApp startup class for Real-World   /// Factory Method Design Pattern.   /// </summary>   class MainApp   {     /// <summary>     /// Entry point into console application.     /// </summary>     static void Main()     {       // Note: constructors call Factory Method       Document[] documents = new Document[2];       documents[0] = new Resume();       documents[1] = new Report();       // Display document pages       foreach (Document document in documents)       {         Console.WriteLine("\n" + document.GetType().Name + "--");         foreach (Page page in document.Pages)         {           Console.WriteLine(" " + page.GetType().Name);         }       }       // Wait for user       Console.ReadKey();     }   }   /// <summary>   /// The 'Product' abstract class   /// </summary>   abstract class Page   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class SkillsPage : Page   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class EducationPage : Page   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class ExperiencePage : Page   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class IntroductionPage : Page   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class ResultsPage : Page   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class ConclusionPage : Page   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class SummaryPage : Page   {   }   /// <summary>   /// A 'ConcreteProduct' class   /// </summary>   class BibliographyPage : Page   {   }   /// <summary>   /// The 'Creator' abstract class   /// </summary>   abstract class Document   {     private List<Page> _pages = new List<Page>();     // Constructor calls abstract Factory method     public Document()     {       this.CreatePages();     }     public List<Page> Pages     {       get { return _pages; }     }     // Factory Method     public abstract void CreatePages();   }   /// <summary>   /// A 'ConcreteCreator' class   /// </summary>   class Resume : Document   {     // Factory Method implementation     public override void CreatePages()     {       Pages.Add(new SkillsPage());       Pages.Add(new EducationPage());       Pages.Add(new ExperiencePage());     }   }   /// <summary>   /// A 'ConcreteCreator' class   /// </summary>   class Report : Document   {     // Factory Method implementation     public override void CreatePages()     {       Pages.Add(new IntroductionPage());       Pages.Add(new ResultsPage());       Pages.Add(new ConclusionPage());       Pages.Add(new SummaryPage());       Pages.Add(new BibliographyPage());     }   } }

Output Resume -------  SkillsPage  EducationPage  ExperiencePage Report -------  IntroductionPage  ResultsPage  ConclusionPage  SummaryPage  BibliographyPage