Factory Method Design Pattern Example

overview

A factory method is a method in a subclass that creates an object. Factory Method pattern defines a Creator interface with Factory Methods (“create” methods) that are implemented by sub-classes. According to Design Patterns by GoF, the intent of Factory Method is to,

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

source: GoF

In Factory Method pattern, the class that is responsible for creating concrete product objects is said to be a Creator. In our example below, MazeGame, BombedMazeGame, and EnchantedMazeGame are Creator classes. In order to create different types of concrete Products (Wall, Room, Door), different type of concrete Creator classes with their own version of factory methods to create their own version of Wall, Room, or Door will be used. All concrete Creators derived from a base Creator interface or class (e.g. MazeGame) with default way of how to create a Wall, Room, or Door. Only if the default is not suited, the subclass can override the instantiation logic.

Factory Method pattern is often built upon by Abstract Factory design pattern. Both differs in their intent. In Factory pattern, a client class delegates the responsibility of object instantiation to another object (a Factory object) via composition whereas the Factory Method pattern uses inheritance and relies on a subclass to handle the desired object instantiation.

example

Creator

[1] A factory method (createMaze()) that creates the final end product resides in the Creator class of the Factory Method pattern.
[2-5] Factory methods to be overridden/redefined by subclasses to decide how to instantiate products. 

comparison

Factory Method vs. Factory

• Factory Method use subclasses for creating different types of objects, whereas Factory  which contains one or more create methods is just a plain class and doesn’t implement any interface.​
  

Factory Method vs. Abstract Factory

• Differ in their intent. In Abstract Factory pattern, a client class delegates the responsibility of object instantiation to another object (an Factory object) via composition whereas the Factory Method pattern uses inheritance and relies on a subclass to handle the desired object instantiation.
• A Factory Method class offers a method (e.g. createMaze()) to create the final product. In Abstract Factory pattern, the Abstract Factory class does offer a method that creates the final end product. Instead, a Factory is passed to (referenced by) other client code that has this factory method to create the final product based on the Factory. 
• An Abstract Factory class builds on top of Factory Method pattern as seen in Abstract Factory's derived Factory classes. An Abstract Factory is then being passed to (or referenced by) other client code to create the final product using a specific Concrete Factory.

references

• Factory vs Factory Method vs Abstract Factory
• Design Patterns: Factory vs Factory method vs Abstract Factory​