12/6/20041 the factory method pattern presenters 王世賀 f9006029 陳祐毓 f9006047 張峻銘...

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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

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Outline Motivation Problem In Real World Solution Intent Example Applicability Participants Structure

Collaborations Consequences Implementation Related

Patterns Reference

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Patterns Reference

Presenter 許書豪

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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

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Motivation （ 2/4 ）

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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

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Motivation （ 4/4 ） Particular Document subclass to

instantiate is application-specific when and what kind questions to

the Application class

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Problem

The framework must instantiate classes, but it only knows about abstract classes, which it cannot instantiate

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In Real World （ 1/2 ） A Bakery

0..1 0..*

0..1

0..*

Bakery

Johnson's Bakery

+ bake Bread () : void

bread

Baked by Johnson

A Factory Method

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In Real World （ 2/2 ） A Fruit Gardener

0..1

0..*

Fruit gardener

{abstract}

Apple gardener Strawberry gardener Grape gardener

Fruit

{abstract}

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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

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Solution （ 2/3 ） Product & Creator

Creator

{abstract} Product

{abstract}

ConcreteCreatorContreteProduct

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Solution （ 3/3 ） Example

0..10..*Document

{abstract}

Application

{abstract}

+++

CreateDocument ()NewDocument ()OpenDocument ()

: bool: bool: bool

DrawingApplication ImgzipApplication

The Factory Method

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Patterns Reference

Presenter 吳佩達

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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

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Example （ 1/2 ） Class Diagram in the book “Design

Patterns”

Factory Method

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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

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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 ）

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Patterns Reference

Presenter 張峻銘

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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 ）

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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

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Structure

Class Diagram form “Design Patterns”

To instantiate a Creator-specific Product instance

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Collaborations

Creator relies on its subclasses to define the factory method so that it returns an instance of the appropriate ConcreteProduct

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Patterns Reference

Presenter 林俊成

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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.

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Consequences （ 2/3 ） Two additional consequences of

the Factory Method pattern Provides hooks for subclasses Connects parallel class hierarchies

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Consequences （ 3/3 ） Connects parallel class hierarchies

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Patterns Reference

Presenter 鄭榮智

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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

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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

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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; }

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Patterns

Presenter 王世賀

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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

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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

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Implementation （ 3.3/5 ）

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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

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Patterns Reference

Presenter 陳祐毓

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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 ）

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Related Patterns

Abstract Factory Factory method: other example

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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

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The End