Factory Method

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Factory Method

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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

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UML class diagram

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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.

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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.

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// 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.

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// 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

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