Factory Method

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Factory Method
Also known as Virtual Constructor

• Chris Colasuonno

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Factory Method Defined

• Define an interface for creating an object, but
  let the subclasses decide which class to
  instantiate.
• (Factory Method lets a class defer instantiation
  to subclasses)
• (Gamma et al. 107)

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Factory Method Defined

• Creational

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Factory Method Motivated
Application
Document
CreateDocument() NewDocument() Open Document()
Document docCreateDocument() docs.Add(doc) doc
-gtOpen()
Open() Close() Save()
MyApplication
MyDocument
CreateDocument()
Return new MyDocument
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Factory Method Motivated
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Factory Method Applied

• When should we use Factory Method?
• When a class
• Cant predict the class of the objects it needs
  to create
• Wants its subclasses to specify the objects that
  it creates
• Delegates responsibility to one of multiple
  helper subclasses, and you need to localize the
  knowledge of which helper is the delagte

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Factory Method The Consequences

• Advantage
• Elminates the need to bind application-specific
  classes to your code.
• Code deals only with the Product interface
  (Document), so it can work with any user-defined
  ConcreteProduct (MyDocument)

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Factory Method The Consequences

• Potential Disadvantages
• Clients may have to make a subclass of the
  Creator, just so they can create a certain
  ConcreteProduct.
• This would be acceptable if the client has to
  subclass the Creator anyway, but if not then the
  client has to deal with another point of
  evolution.

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Factory Method The Consequences

• In addition
• Provides hooks for subclasses
• Connects parallel class hierarchies
• P.109-110

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Factory Method Implemented

• Two major varieties
• Parameterized factory methods
• Language-specific issues
• Using templates to avoid subclassing
• Naming conventions

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Factory Method Known Uses

• Frameworks
• Toolkits

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Factory Method Related Patterns

• Abstract Factory is often implemented with
  Factory Method.
• Usually called in Template Methods
• Prototypes dont subclass the Creator, but often
  need to initialize the Product class. Creator
  would use Initialize on the object, but Factory
  Method doesnt need this operation.

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Sources

• Gammal et al. Design Patterns Elements of
  Reusable Object-Oriented Software.
  Addison-Wesley. 1995.
• Data and Object Factory. Image Factory.gif
  Internet. Available by HTTP http//www.dofactory
  .com/patterns/PatternFactory.aspx

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Factory Method Notes

• Text Books pages 107-116
• Slides available at www.rpi.edu/colasc