Factory Method

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patterns    C#, ASP.NET, VB.NET, XML, SOAP     Factory Method  < back to list of patterns  definition  UML diagram  participants  sample code in C# Definition Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. Frequency of use:  medium high return to top UML class diagram return to top 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. return to top Sample code in C# download C# source code zip file 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. Hide code // Factory Method pattern -- Structural example using System; using System.Collections; // "Product" abstract class Product { } // "ConcreteProductA" class ConcreteProductA : Product { } // "ConcreteProductB" class ConcreteProductB : Product { } // "Creator" abstract class Creator {   // Methods   abstract public Product FactoryMethod(); } // "ConcreteCreatorA" class ConcreteCreatorA : Creator {   // Methods   override public Product FactoryMethod()   {     return new ConcreteProductA();   } } // "ConcreteCreatorB" class ConcreteCreatorB : Creator {   // Methods   override public Product FactoryMethod()   {     return new ConcreteProductB();   } } /// <summary> /// Client test /// </summary> class Client {   public static void Main( string[] args )   {     // FactoryMethod returns ProductA     Creator c = new ConcreteCreatorA();     Product p = c.FactoryMethod();     Console.WriteLine( "Created {0}", p );     // FactoryMethod returns ProductB     c = new ConcreteCreatorB();     p = c.FactoryMethod();     Console.WriteLine( "Created {0}", p );     } } 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. Hide code // Factory Method pattern -- Real World example using System; using System.Collections; // "Product" abstract class Page { } // "ConcreteProduct" class SkillsPage : Page { } // "ConcreteProduct" class EducationPage : Page { } // "ConcreteProduct" class ExperiencePage : Page { } // "ConcreteProduct" class IntroductionPage : Page { } // "ConcreteProduct" class ResultsPage : Page { } // "ConcreteProduct" class ConclusionPage : Page { } // "ConcreteProduct" class SummaryPage : Page { } // "ConcreteProduct" class BibliographyPage : Page { } // "Creator" abstract class Document {   // Fields   protected ArrayList pages = new ArrayList();   // Constructor   public Document()   {     this.CreatePages();   }   // Properties   public ArrayList Pages   {     get{ return pages; }   }   // Factory Method   abstract public void CreatePages(); } // "ConcreteCreator" class Resume : Document {   // Factory Method implementation   override public void CreatePages()   {     pages.Add( new SkillsPage() );     pages.Add( new EducationPage() );     pages.Add( new ExperiencePage() );   } } // "ConcreteCreator" class Report : Document {   // Factory Method implementation   override public void CreatePages()   {     pages.Add( new IntroductionPage() );     pages.Add( new ResultsPage() );     pages.Add( new ConclusionPage() );     pages.Add( new SummaryPage() );     pages.Add( new BibliographyPage() );   } } /// <summary> ///  FactoryMethodApp test /// </summary> class FactoryMethodApp {   public static void Main( string[] args )   {     Document[] docs = new Document[ 2 ];     // Note: constructors call Factory Method     docs[0] = new Resume();     docs[1] = new Report();     // Display document pages     foreach( Document document in docs )     {       Console.WriteLine( "\n" + document + " ------- " );       foreach( Page page in document.Pages )         Console.WriteLine( " " + page );     }   } } Output Resume ------- SkillsPage EducationPage ExperiencePage Report ------- IntroductionPage ResultsPage ConclusionPage SummaryPage BibliographyPage return to top top of document home | about us | courses | curriculum | schedule | registration | careers | contact us | site map Copyright © 2002 - Data & Object Factory. 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