Factory Method

Intent, Motivation, Applicability, Structure, Participants, Collaborations, Consequences, Implementation, Sample Code, Known Uses, Related Patterns

Known Uses

Factory methods pervade toolkits and frameworks. The preceding document example is a typical use in MacApp and ET++. The manipulator example is from Unidraw.

Class View in the Smalltalk-80 Model/View/Controller framework has a method defaultController that creates a controller, and this might appear to be a factory method. But subclasses of View specify the class of their default controller by defining defaultControllerClass, which returns the class from which defaultController creates instances. So defaultControllerClass is the real factory method, that is, the method that subclasses should override.

A more esoteric example in Smalltalk-80 is the factory method parserClass defined by Behavior (a superclass of all objects representing classes). This enables a class to use a customized parser for its source code. For example, a client can define a class SQLParser to analyze the source code of a class with embedded SQL statements. The Behavior class implements parserClass to return the standard Smalltalk Parser class. A class that includes embedded SQL statements overrides this method (as a class method) and returns the SQLParser class.

The Orbix ORB system from IONA Technologies uses Factory Method to generate an appropriate type of proxy when an object requests a reference to a remote object. Factory Method makes it easy to replace the default proxy with one that uses client-side caching, for example.

Known Uses of Abstract Factory, Adapter, Bridge, Builder, Chain of Responsibility, Command, Composite, Decorator, Facade, Factory Method, Flyweight, Interpreter, Iterator, Mediator, Memento, Observer, Prototype, Proxy, Singleton, State, Strategy, Template Method, Visitor