1. 12/6/20041 The Factory Method Pattern Presenters 王世賀 F9006029 陳祐毓 F9006047 張峻銘 F9006065 吳佩達 F9006084 林俊成 F9006097 鄭榮智 F9006100 許書豪 F9006205

2. 12/6/20042 Outline Motivation Problem In Real World Solution Intent Example Applicability Participants Structure Collaborations Consequences Implementation Related Patterns Reference

3. 12/6/20043 Outline Motivation Problem In Real World Solution Intent Example Applicability Participants Structure Collaborations Consequences Implementation Related Patterns Reference Presenter 許書豪

4. 12/6/20044 Motivation （ 1/4 ） Useful to Frameworks for applications Consider a framework for applications that can present multiple documents Two key abstractions Application managing Documents ： create Open or New from a menu Document application-specific implementations To subclass and to implement

5. 12/6/20045 Motivation （ 2/4 ）

6. 12/6/20046 Motivation （ 3/4 ） A drawing application Class DrawingApplication extends Application Class DrawingDocument extends Document A imgzip application Class ImgzipApplication extends Application Class ImgzipDocument extends Document particular Document subclass

7. 12/6/20047 Motivation （ 4/4 ） Particular Document subclass to instantiate is application-specific when and what kind questions to the Application class

8. 12/6/20048 Problem The framework must instantiate classes, but it only knows about abstract classes, which it cannot instantiate

9. 12/6/20049 In Real World （ 1/2 ） A Bakery

10. 12/6/200410 In Real World （ 2/2 ） A Fruit Gardener

11. 12/6/200411 Solution （ 1/3 ） The Factory Method pattern encapsulates the knowledge of which Document subclass to create and moves this knowledge out of the framework Lets a class （ abstractions ） defer instantiation to subclasses

12. 12/6/200412 Solution （ 2/3 ） Product & Creator

13. 12/6/200413 Solution （ 3/3 ） Example

14. 12/6/200414 Outline Motivation Problem In Real World Solution Intent Example Applicability Participants Structure Collaborations Consequences Implementation Related Patterns Reference Presenter 吳佩達

15. 12/6/200415 Intent Intent by the book “ Design Patterns ” Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. Also known as Virtual Constructor

16. 12/6/200416 Example （ 1/2 ） Class Diagram in the book “ Design Patterns ” Factory Method

17. 12/6/200417 Example （ 2/2 ） Application subclasses redefine an abstract CreateDocument operation on Application to return the appropriate Document subclass We call CreateDocument a factory method because it's responsible for "manufacturing" an application-specific object

18. 12/6/200418 Applicability Use the Factory Method pattern when a class can't anticipate the class of objects it must create a class wants its subclasses to specify the objects it creates classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate （ Composite ）

19. 12/6/200419 Outline Motivation Problem In Real World Solution Intent Example Applicability Participants Structure Collaborations Consequences Implementation Related Patterns Reference Presenter 張峻銘

20. 12/6/200420 Participants （ 1/2 ） Abstract Creator 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 （ see Consequences 1 ）

21. 12/6/200421 Participants （ 2/2 ） Abstract Product defines the interface of objects the factory method creates ConcreteCreator overrides the factory method to return an instance of a ConcreteProduct ConcreteProduct implements the Product interface

22. 12/6/200422 Structure Class Diagram form “ Design Patterns ” To instantiate a Creator-specific Product instance

23. 12/6/200423 Collaborations Creator relies on its subclasses to define the factory method so that it returns an instance of the appropriate ConcreteProduct

24. 12/6/200424 Outline Motivation Problem In Real World Solution Intent Example Applicability Participants Structure Collaborations Consequences Implementation Related Patterns Reference Presenter 林俊成

25. 12/6/200425 Consequences （ 1/3 ） Factory methods eliminate the need to bind application-specific classes into your code. A potential disadvantage of factory methods is that clients might have to subclass the Creator class just to create a particular ConcreteProduct object.

26. 12/6/200426 Consequences （ 2/3 ） Two additional consequences of the Factory Method pattern Provides hooks for subclasses Connects parallel class hierarchies

27. 12/6/200427 Consequences （ 3/3 ） Connects parallel class hierarchies

28. 12/6/200428 Outline Motivation Problem In Real World Solution Intent Example Applicability Participants Structure Collaborations Consequences Implementation Related Patterns Reference Presenter 鄭榮智

29. 12/6/200429 Implementation （ 1/5 ） Two major varieties Creator class is an abstract class and does not provide an implementation for the factory method it declares Creator class is a concrete class and provides a default implementation for the factory method

30. 12/6/200430 Implementation （ 2.1/5 ） Parameterized factory methods Another variation lets the factory method create multiple kinds of Products Takes a parameter that identifies the kind of object to create All objects the factory method creates will share the Product interface Application might support different kinds of Documents

31. 12/6/200431 Implementation （ 2.2/5 ） class Creator { public: virtual Product* Create( ProductId ); }; Product* Creator::Create ( ProductId id ) { if ( id == MINE ) return new MyProduct; if ( id == YOURS ) return new YourProduct; // repeat for remaining products... return 0; }

32. 12/6/200432 Outline Motivation Problem In Real World Solution Intent Example Applicability Participants Structure Collaborations Consequences Implementation Related Patterns Presenter 王世賀

33. 12/6/200433 Implementation （ 3.1/5 ） Language-specific variants and issues Factory methods in C++ are always virtual functions and are often pure virtual. Just be careful not to call factory methods in the Creator's constructor — the factory method in the ConcreteCreator won't be available yet

34. 12/6/200434 Implementation （ 3.2/5 ） Lazy initialization Instead of creating the concrete product in the constructor, the constructor merely initializes it to 0. The accessor returns the product. But first it checks to make sure the product exists, and if it doesn't, the accessor creates it

35. 12/6/200435 Implementation （ 3.3/5 ）

36. 12/6/200436 Implementation （ 4.1/5 ） Using templates to avoid subclassing Another potential problem with factory methods is that they might force you to subclass just to create the appropriate Product objects

37. 12/6/200437 Implementation （ 4.2/5 ）

38. 12/6/200438 Implementation （ 4.3/5 ）

39. 12/6/200439 Outline Motivation Problem In Real World Solution Intent Example Applicability Participants Structure Collaborations Consequences Implementation Related Patterns Reference Presenter 陳祐毓

40. 12/6/200440 Implementation （ 5/5 ） Naming conventions make it clear you're using factory methods For example, the MacApp Macintosh application framework （ declares the abstract operation that defines the factory method as Class* DoMakeClass(), where Class is the Product class ）

41. 12/6/200441 Related Patterns Abstract Factory Factory method: other example

42. 12/6/200442 Reference 閻宏, Java 與樣式理論, 碁峰資訊股份有 限公司, ISBN 9864214179, 2009.9.26 結城 浩, Design Patterns 於 Java 語言上的 實習應用, 博碩文化股份有限公司, ISBN 9575274644, 2002.2.26

43. 12/6/200443 The End