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Factory Method

Factory Method. Also known as “Virtual Constructor”. Chris Colasuonno. 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

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  1. Factory Method Also known as “Virtual Constructor” Chris Colasuonno

  2. 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)

  3. Factory Method: Defined • Creational

  4. Factory Method: Motivated Application Document CreateDocument() NewDocument() Open Document() Document* doc=CreateDocument(); docs.Add(doc); doc->Open(); Open() Close() Save() MyApplication MyDocument CreateDocument() Return new MyDocument

  5. Factory Method: Motivated

  6. Factory Method: Applied • When should we use Factory Method? • When a class: • Can’t 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

  7. 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)

  8. 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.

  9. Factory Method: The Consequences • In addition: • “Provides hooks for subclasses” • “Connects parallel class hierarchies” • P.109-110

  10. Factory Method: Implemented • Two major varieties • Parameterized factory methods • Language-specific issues • Using templates to avoid subclassing • Naming conventions

  11. Factory Method: Known Uses • Frameworks • Toolkits

  12. Factory Method: Related Patterns • Abstract Factory is often implemented with Factory Method. • Usually called in Template Methods • Prototypes don’t subclass the Creator, but often need to initialize the Product class. Creator would use Initialize on the object, but Factory Method doesn’t need this operation.

  13. 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

  14. Factory Method: Notes • Text Books pages 107-116 • Slides available at www.rpi.edu/~colasc

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