Factory Method

Factory Method

A class pattern that defines an interface for creating an object but lets subclasses decide which class to instantiate lets a class defer instantiation to subclasses.

• Class Creational Pattern
• AKA: Virtual Constructor

Factory Method

• Frameworks use abstract classes to define and maintain relationships between objects
• Also responsible for creating these objects
• However, knowledge of what type of object to create may not be in the framework

Factory Method: Motivation

Factory Method: Applicability

• Classes can't anticipate the class of objects they must create
• Classes want subclasses to specify the objects they create
• Classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate

Factory Method: Structure

Factory Method: Product Participants

• Product (e.g., Document)
• Defines the interface of objects the factory method creates
• ConcreteProduct (e.g., MyDocument)
• Implements the Product interface

Factory Method: Creator Participants

• Creator (e.g., Application)
• Declares the factory method, which returns an object of type Product
• 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 (e.g., MyApplication)
• Overrides the factory method to return an instance of a ConcreteProduct.

Factory Method: Collaborations

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

Factory Method: Consequences

• Code only deals with Product interfaces so that it can work with any user-defined ConcreteProduct classes
• Does require a client to subclass Creator for each unique ConcreteProduct
• Can define a non-abstract default, e.g., CreateFileDialog, that subclasses can override if needed

Factory Method: Parallel Class Hierarchies

Implementation: Maze

Implementation: Factory

Implementation: Using Factory

Implementation: Subclassing

UML

Lazy Initialization

Known Uses

Factory methods pervade toolkits and frameworks

Related Patterns

• Abstract Factory
• Often implemented with factory methods
• Template Methods
• Typically called to create a proper object