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.

UML class diagram


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


is the base class for a final product ( or  ). declares the abstract factory method to produce such a base product. and  are subclasses of the base product implementing the final product. and  are subclasses of   implementing the factory method producing the final products. Thus factory methods decouple callers 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.

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

A factory method is just an addition to  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  in concreteFactory. As a result, you can easily call  from it to get the. 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  constructor is a template method that makes some common logic. It refers to the  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  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 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.