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Factory method pattern

In object oriented programming, the factory method pattern is a design pattern that uses factory methods to deal with the problem of creating objects without having to specify their exact class. Rather than by calling a constructor, this is done by calling a factory method to create an object. Factory methods can either be specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes. It is one of the 23 classic design patterns described in the book Design Patterns (often referred to as the "Gang of Four" or simply "GoF") and is sub-categorized as a creational pattern.

Overview

The Factory Method design pattern solves problems like:

How can an object be created so that subclasses can redefine its subsequent and distinct implementation?
How can an object's instantiation be deferred to a subclass?

The Factory Method design pattern describes how to solve such problems:

Define a factory method within the superclass that defers the object's creation to a subclass's factory method.
Create an object by calling a factory method instead of directly calling a constructor.

This enables the writing of subclasses that can change the way an object is created (e.g. by redefining which class to instantiate).
See also the UML class diagram below.

Definition

"Define an interface for creating an object, but let subclasses decide which class to instantiate. The Factory method lets a class defer instantiation it uses to subclasses." (Gang Of Four)

Creating an object often requires complex processes not appropriate to include within a composing object. The object's creation may lead to a significant duplication of code, may require information not accessible to the composing object, may not provide a sufficient level of abstraction, or may otherwise not be part of the composing object's concerns. The factory method design pattern handles these problems by defining a separate method for creating the objects, which subclasses can then override to specify the derived type of product that will be created.

The factory method pattern relies on inheritance, as object creation is delegated to subclasses that implement the factory method to create objects. As shown in the C# example below, the factory method pattern can also rely on an Interface - in this case IPerson - to be implemented.

Structure

UML class diagram

In the above UML class diagram, the Creator class that requires a Product object does not instantiate the Product1 class directly. Instead, the Creator refers to a separate factoryMethod() to create a product object, which makes the Creator independent of which concrete class is instantiated. Subclasses of Creator can redefine which class to instantiate. In this example, the Creator1 subclass implements the abstract factoryMethod() by instantiating the Product1 class.

Examples

This C++14 implementation is based on the pre C++98 implementation in the book.

The program output is like

A maze game may be played in two modes, one with regular rooms that are only connected with adjacent rooms, and one with magic rooms that allow players to be transported at random.

Structure

Room is the base class for a final product (MagicRoom or OrdinaryRoom). MazeGame declares the abstract factory method to produce such a base product. MagicRoom and OrdinaryRoom are subclasses of the base product implementing the final product. MagicMazeGame and OrdinaryMazeGame are subclasses of MazeGame implementing the factory method producing the final products. Thus factory methods decouple callers (MazeGame) from the implementation of the concrete classes. This makes the "new" Operator redundant, allows adherence to the Open/closed principle and makes the final product more flexible in the event of change.

Example implementations

C#

In the above code you can see the creation of one interface called IPerson and two implementations called Villager and CityPerson. Based on the type passed into the PersonFactory object, we are returning the original concrete object as the interface IPerson.

A factory method is just an addition to PersonFactory class. It creates the object of the class through interfaces but on the other hand, it also lets the subclass decide which class is instantiated.

You can see we have used MakeProduct in concreteFactory. As a result, you can easily call MakeProduct() from it to get the IProduct. You might also write your custom logic after getting the object in the concrete Factory Method. The GetObject is made abstract in the Factory interface.

Java

This Java example is similar to one in the book Design Patterns.

The MazeGame uses Rooms but it puts the responsibility of creating Rooms to its subclasses which create the concrete classes. The regular game mode could use this template method:

In the above snippet, the MazeGame constructor is a template method that makes some common logic. It refers to the makeRoom factory method that encapsulates the creation of rooms such that other rooms can be used in a subclass. To implement the other game mode that has magic rooms, it suffices to override the makeRoom method:

PHP

Another example in PHP follows, this time using interface implementations as opposed to subclassing (however, the same can be achieved through subclassing). It is important to note that the factory method can also be defined as public and called directly by the client code (in contrast with the Java example above).

Python

Same as Java example.

Uses

In ADO.NET, IDbCommand.CreateParameter is an example of the use of factory method to connect parallel class hierarchies.
In Qt, QMainWindow::createPopupMenu Archived 2015-07-19 at the Wayback Machine is a factory method declared in a framework that can be overridden in application code.
In Java, several factories are used in the javax.xml.parsers package. e.g. javax.xml.parsers.DocumentBuilderFactory or javax.xml.parsers.SAXParserFactory.
In the HTML5 DOM API, the Document interface contains a createElement factory method for creating specific elements of the HTMLElement interface.

Notes

References

Martin Fowler; Kent Beck; John Brant; William Opdyke; Don Roberts (June 1999). Refactoring: Improving the Design of Existing Code. Addison-Wesley. ISBN 0-201-48567-2.
Gamma, Erich; Helm, Richard; Johnson, Ralph; Vlissides, John (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2.
Cox, Brad J. (1986). Object-oriented programming: an evolutionary approach. Addison-Wesley. ISBN 978-0-201-10393-9.
Cohen, Tal; Gil, Joseph (2007). "Better Construction with Factories" (PDF). Journal of Object Technology. 6 (6). Bertrand Meyer: 103. doi:10.5381/jot.2007.6.6.a3. Retrieved 2007-03-12.