GoF之Abstract Factory

Factory Method

• Purpose
• Structure
• Applications
• Consequences

Purpose

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

Structure

 

• AbstractFactory : Declares an interface for operations that create abstract product objects.
• ConcreteFactory : Implements the operations to create concrete product objects.
• AbstractProduct : Declares an interface for a type of product object.
• ConcreteProduct: Defines a product object to be created by the corresponding c

• Product(Document) : defines the interface of objects the factory method creates.
• ConcreteProduct(Mydocument) : implements the Product interface.
• Creator(Application) : 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. may call the factory method to create a Product object.
• ConcreteCreator(MyApplication) : overrides the factory method to return an instance of a ConcreteProduct.

Applications

• 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.

Consequences

• Provides hooks for subclasses. Creating objects inside a class with a factory method is always more flexible than creating an object directly. Factory Method gives subclasses a hook for providing an extended version of an object.
• Connects parallel class hierarchies. In the examples we've considered so far, the factory method is only called by Creators. But this doesn't have to be the case; clients can find factory methods useful, especially in the case of parallel class hierarchies.

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