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Creational Design Patterns-Factory Method Pattern


Design Patterns can be categorized under the following Heads
  1. Creational
  2. Structural
  3. Behavioral


This section deals with the creational design patterns,

Creational Patterns
The creational patterns aim to separate a system from how its objects are created, composed, and represented.
Creational Patterns can be classified as follows
1. Abstract Factory
2. Builder
3. Factory
4. Prototype
5. Singleton

Creational patterns encapsulate the knowledge about which classes a system uses, but they hide the details of how the instances of these classes are created and put together.


Factory Method Pattern

The Factory Method pattern is a way of creating objects, but letting subclasses decide exactly which class to instantiate.

Various subclasses might implement the interface; the Factory Method instantiates the appropriate subclass based on information supplied by the client or extracted from the current state.

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

Example:
The Example presented below is a basic document creation factory, where different types of documents(Resumes, Reports etc.) can be created using the Factory Pattern

public abstract class DocPage
  {
     public string Header
     {

         set { Header = "P.Solutions"; }
    
     }
     public string Footer
     {

         set { Footer = "Confidential Document"; }

     }

  }

  class SkillSetPage : DocPage
  {
   
  }

  class EducationalDetailsPage : DocPage

  {

  }

  class ExperienceSummaryPage : DocPage

  {

  }
  class BriefInrtoPage : DocPage

  {

  }

  class FindingsPage : DocPage

  {

  }
 
  class ConclusionPage : DocPage

  {

  }

 class SummaryPage : DocPage

  {

  }

 class BibliographyPage : DocPage
  {

  }



//The 'Creator' abstract class. This is class that acts as creator of  Documents, depending upon the requirements


 public abstract class Document
  {

    private List<DocPage> _pages = new List<DocPage>();

     // Constructor calls abstract Factory method

    public Document()
    {

      this.CreatePages();

    }

  public List<DocPage> Pages
    {
      get { return _pages; }
    }
    // Factory Method

    public abstract void CreatePages();

  }

  // 'ConcreteCreator' classes. These Classes  are called in by Client , hieding the details as to how the classes produce documnents

 public class Resume : Document
  {
    // Factory Method implementation

    public override void CreatePages()
    {
      Pages.Add(new SkillsPage());
      Pages.Add(new EducationPage());
      Pages.Add(new ExperiencePage());
    }

  }

 
  /// A 'ConcreteCreator' class

  ///

  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());

    }

  }

All in Place, Let’s now call these classes to get the documents.

protected void Page_Load(object sender, EventArgs e)
    {
          
        Resume rs = new Resume();
        Response.Write("");
        foreach( DocPage dp in rs.Pages)
        {
           Response.Write("
");
            Response.Write(dp.GetType().Name);
       
        }
       
     
   }

 
When called, as shown above, Following pages become part of Resume document.
SkillsPage
EducationPage
ExperiencePage 


Please note that instead of the abstract classes, Interface is generally considered for implementing the Factory pattern. In case you feel the need, feel free to modify the code for using interfaces.

For further understanding  implementation of the pattern in .Net, go to the following link.
 http://aspalliance.com/1751_Exemplifying_the_Factory_Method_Pattern_inside_the_NET_Framework.all

Hope this was helpful,

Till next time. Happy Coding!!
 



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