Factory Pattern. When to use factory methods?

Question

I like thinking about design pattens in terms of my classes being 'people,' and the patterns are the ways that the people talk to each other.

So, to me the factory pattern is like a hiring agency. You've got someone that will need a variable number of workers. This person may know some info they need in the people they hire, but that's it.

So, when they need a new employee, they call the hiring agency and tell them what they need. Now, to actually hire someone, you need to know a lot of stuff - benefits, eligibility verification, etc. But the person hiring doesn't need to know any of this - the hiring agency handles all of that.

In the same way, using a Factory allows the consumer to create new objects without having to know the details of how they're created, or what their dependencies are - they only have to give the information they actually want.

public interface IThingFactory
{
    Thing GetThing(string theString);
}

public class ThingFactory : IThingFactory
{
    public Thing GetThing(string theString)
    {
        return new Thing(theString, firstDependency, secondDependency);
    }
}

So, now the consumer of the ThingFactory can get a Thing, without having to know about the dependencies of the Thing, except for the string data that comes from the consumer.

Factory methods should be considered as an alternative to constructors - mostly when constructors aren't expressive enough, ie.

class Foo{
  public Foo(bool withBar);
}

is not as expressive as:

class Foo{
  public static Foo withBar();
  public static Foo withoutBar();
}

Factory classes are useful when you need a complicated process for constructing the object, when the construction need a dependency that you do not want for the actual class, when you need to construct different objects etc.

One situation where I personally find separate Factory classes to make sense is when the final object you are trying to create relies on several other objects. E.g, in PHP: Suppose you have a House object, which in turn has a Kitchen and a LivingRoom object, and the LivingRoom object has a TV object inside as well.

The simplest method to achieve this is having each object create their children on their construct method, but if the properties are relatively nested, when your House fails creating you will probably spend some time trying to isolate exactly what is failing.

The alternative is to do the following (dependency injection, if you like the fancy term):

$TVObj = new TV($param1, $param2, $param3);
$LivingroomObj = new LivingRoom($TVObj, $param1, $param2);
$KitchenroomObj = new Kitchen($param1, $param2);
$HouseObj = new House($LivingroomObj, $KitchenroomObj);

Here if the process of creating a House fails there is only one place to look, but having to use this chunk every time one wants a new House is far from convenient. Enter the Factories:

class HouseFactory {
    public function create() {
        $TVObj = new TV($param1, $param2, $param3);
        $LivingroomObj = new LivingRoom($TVObj, $param1, $param2);
        $KitchenroomObj = new Kitchen($param1, $param2);
        $HouseObj = new House($LivingroomObj, $KitchenroomObj);

        return $HouseObj;
    }
}

$houseFactory = new HouseFactory();
$HouseObj = $houseFactory->create();

Thanks to the factory here the process of creating a House is abstracted (in that you don't need to create and set up every single dependency when you just want to create a House) and at the same time centralized which makes it easier to maintain. There are other reasons why using separate Factories can be beneficial (e.g. testability) but I find this specific use case to illustrate best how Factory classes can be useful.

It is important to clearly differentiate the idea behind using factory or factory method. Both are meant to address mutually exclusive different kind of object creation problems.

Let's be specific about "factory method":

First thing is that, when you are developing library or APIs which in turn will be used for further application development, then factory method is one of the best selections for creation pattern. Reason behind; We know that when to create an object of required functionality(s) but type of object will remain undecided or it will be decided ob dynamic parameters being passed.

Now the point is, approximately same can be achieved by using factory pattern itself but one huge drawback will introduce into the system if factory pattern will be used for above highlighted problem, it is that your logic of crating different objects(sub classes objects) will be specific to some business condition so in future when you need to extend your library's functionality for other platforms(In more technically, you need to add more sub classes of basic interface or abstract class so factory will return those objects also in addition to existing one based on some dynamic parameters) then every time you need to change(extend) the logic of factory class which will be costly operation and not good from design perspective. On the other side, if "factory method" pattern will be used to perform the same thing then you just need to create additional functionality(sub classes) and get it registered dynamically by injection which doesn't require changes in your base code.

interface Deliverable 
{
    /*********/
}

abstract class DefaultProducer 
{

    public void taskToBeDone() 
    {   
        Deliverable deliverable = factoryMethodPattern();
    }
    protected abstract Deliverable factoryMethodPattern();
}

class SpecificDeliverable implements Deliverable 
{
 /***SPECIFIC TASK CAN BE WRITTEN HERE***/
}

class SpecificProducer extends DefaultProducer 
{
    protected Deliverable factoryMethodPattern() 
    {
        return new SpecificDeliverable();
    }
}

public class MasterApplicationProgram 
{
    public static void main(String arg[]) 
    {
        DefaultProducer defaultProducer = new SpecificProducer();
        defaultProducer.taskToBeDone();
    }
}

They're also useful when you need several "constructors" with the same parameter type but with different behavior.

It is good idea to use factory methods inside object when:

1. Object's class doesn't know what exact sub-classes it have to create
2. Object's class is designed so that objects it creates were specified by sub-classes
3. Object's class delegates its duties to auxiliary sub-classes and doesn't know what exact class will take these duties

It is good idea to use abstract factory class when:

1. Your object shouldn't depend on how its inner objects are created and designed
2. Group of linked objects should be used together and you need to serve this constraint
3. Object should be configured by one of several possible families of linked objects that will be a part of your parent object
4. It is required to share child objects showing interfaces only but not an implementation

UML from

Product: It defines an interface of the objects the Factory method creates.

ConcreteProduct: Implements Product interface

Creator: Declares the Factory method

ConcreateCreator: Implements the Factory method to return an instance of a ConcreteProduct

Problem statement: Create a Factory of Games by using Factory Methods, which defines the game interface.

Code snippet:

import java.util.HashMap;


/* Product interface as per UML diagram */
interface Game{
    /* createGame is a complex method, which executes a sequence of game steps */
    public void createGame();
}

/* ConcreteProduct implementation as per UML diagram */
class Chess implements Game{
    public Chess(){

    }
    public void createGame(){
        System.out.println("---------------------------------------");
        System.out.println("Create Chess game");
        System.out.println("Opponents:2");
        System.out.println("Define 64 blocks");
        System.out.println("Place 16 pieces for White opponent");
        System.out.println("Place 16 pieces for Black opponent");
        System.out.println("Start Chess game");
        System.out.println("---------------------------------------");
    }
}
class Checkers implements Game{
    public Checkers(){

    }
    public void createGame(){
        System.out.println("---------------------------------------");
        System.out.println("Create Checkers game");
        System.out.println("Opponents:2 or 3 or 4 or 6");
        System.out.println("For each opponent, place 10 coins");
        System.out.println("Start Checkers game");
        System.out.println("---------------------------------------");
    }
}
class Ludo implements Game{
    public Ludo(){

    }
    public void createGame(){
        System.out.println("---------------------------------------");
        System.out.println("Create Ludo game");
        System.out.println("Opponents:2 or 3 or 4");
        System.out.println("For each opponent, place 4 coins");
        System.out.println("Create two dices with numbers from 1-6");
        System.out.println("Start Ludo game");
        System.out.println("---------------------------------------");
    }
}

/* Creator interface as per UML diagram */
interface IGameFactory {
    public Game getGame(String gameName);
}

/* ConcreteCreator implementation as per UML diagram */
class GameFactory implements IGameFactory {

     HashMap<String,Game> games = new HashMap<String,Game>();
    /*  
        Since Game Creation is complex process, we don't want to create game using new operator every time.
        Instead we create Game only once and store it in Factory. When client request a specific game, 
        Game object is returned from Factory instead of creating new Game on the fly, which is time consuming
    */

    public GameFactory(){

        games.put(Chess.class.getName(),new Chess());
        games.put(Checkers.class.getName(),new Checkers());
        games.put(Ludo.class.getName(),new Ludo());        
    }
    public Game getGame(String gameName){
        return games.get(gameName);
    }
}

public class NonStaticFactoryDemo{
    public static void main(String args[]){
        if ( args.length < 1){
            System.out.println("Usage: java FactoryDemo gameName");
            return;
        }

        GameFactory factory = new GameFactory();
        Game game = factory.getGame(args[0]);
        if ( game != null ){                    
            game.createGame();
            System.out.println("Game="+game.getClass().getName());
        }else{
            System.out.println(args[0]+  " Game does not exists in factory");
        }           
    }
}

output:

java NonStaticFactoryDemo Chess
---------------------------------------
Create Chess game
Opponents:2
Define 64 blocks
Place 16 pieces for White opponent
Place 16 pieces for Black opponent
Start Chess game
---------------------------------------
Game=Chess

This example shows a Factory class by implementing a FactoryMethod.

1. Game is the interface for all type of games. It defines complex method: createGame()

2. Chess, Ludo, Checkers are different variants of games, which provide implementation to createGame()

3. public Game getGame(String gameName) is FactoryMethod in IGameFactory class

4. GameFactory pre-creates different type of games in constructor. It implements IGameFactory factory method.

5. game Name is passed as command line argument to NotStaticFactoryDemo

6. getGame in GameFactory accepts a game name and returns corresponding Game object.

Factory:

Creates objects without exposing the instantiation logic to the client.

FactoryMethod

Define an interface for creating an object, but let the subclasses decide which class to instantiate. The Factory method lets a class defer instantiation to subclasses

Use case:

When to use: Client doesn't know what concrete classes it will be required to create at runtime, but just wants to get a class that will do the job.