Differences between Abstract Factory Pattern and F

Question

I know there are many posts out there about the differences between these two patterns, but there are a few things that I cannot find.

From what I have been reading, I see that the factory method pattern allows you to define how to create a single concrete product but hiding the implementation from the client as they will see a generic product. My first question is about the abstract factory. Is its role to allow you to create families of concrete objects in (that can depend on what specific factory you use) rather than just a single concrete object? Does the abstract factory only return one very large object or many objects depending on what methods you call?

My final two questions are about a single quote that I cannot fully understand that I have seen in numerous places:

One difference between the two is that with the Abstract Factory pattern, a class delegates the responsibility of object instantiation to another object via composition whereas the Factory Method pattern uses inheritance and relies on a subclass to handle the desired object instantiation.

My understanding is that the factory method pattern has a Creator interface that will make the ConcreteCreator be in charge of knowing which ConcreteProduct to instantiate. Is this what it means by using inheritance to handle object instantiation?

Now with regards to that quote, how exactly does the Abstract Factory pattern delegate the responsibility of object instantiation to another object via composition? What does this mean? It looks like the Abstract Factory pattern also uses inheritance to do the construction process as well in my eyes, but then again I am still learning about these patterns.

Any help especially with the last question, would be greatly appreciated.

Answer 1

Real Life Example. (Easy to remember)

Factory

Imagine you are constructing a house and you approach a carpenter for a door. You give the measurement for the door and your requirements, and he will construct a door for you. In this case, the carpenter is a factory of doors. Your specifications are inputs for the factory, and the door is the output or product from the factory.

Abstract Factory

Now, consider the same example of the door. You can go to a carpenter, or you can go to a plastic door shop or a PVC shop. All of them are door factories. Based on the situation, you decide what kind of factory you need to approach. This is like an Abstract Factory.

I have explained here both Factory method pattern and abstract factory pattern beginning with not using them explaining issues and then solving issues by using the above patterns https://github.com/vikramnagineni/Design-Patterns/tree/master

Answer 2

Difference between AbstractFactory and Factory design patters are as follows:

• Factory Method is used to create one product only but Abstract Factory is about creating families of related or dependent products.
• Factory Method pattern exposes a method to the client for creating the object whereas in case of Abstract Factory they expose a family of related objects which may consist of these Factory methods.
• Factory Method pattern hides the construction of single object where as Abstract factory method hides the construction of a family of related objects. Abstract factories are usually implemented using (a set of) factory methods.
• AbstractFactory pattern uses composition to delegate responsibility of creating object to another class while Factory design pattern uses inheritance and relies on derived class or sub class to create object.
• The idea behind the Factory Method pattern is that it allows for the case where a 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 while AbstractFactory pattern is best utilised when your system has to create multiple families of products or you want to provide a library of products without exposing the implementation details.!

Factory Method Pattern Implementation:

AbstractFactory Pattern Implementation:

Answer 3

Consider this example for easy understanding.

What does telecommunication companies provide? Broadband, phone line and mobile for instance and you're asked to create an application to offer their products to their customers.

Generally what you'd do here is, creating the products i.e broadband, phone line and mobile are through your Factory Method where you know what properties you have for those products and it's pretty straightforward.

Now, the company wants to offer their customer a bundle of their products i.e broadband, phone line, and mobile altogether, and here comes the Abstract Factory to play.

Abstract Factory is, in other words, are the composition of other factories who are responsible for creating their own products and Abstract Factory knows how to place these products in more meaningful in respect of its own responsibilities.

In this case, the BundleFactory is the Abstract Factory, BroadbandFactory, PhonelineFactory and MobileFactory are the Factory. To simplify more, these Factories will have Factory Method to initialise the individual products.

Se the code sample below:

public class BroadbandFactory : IFactory {
    public static Broadband CreateStandardInstance() {
        // broadband product creation logic goes here
    }
}

public class PhonelineFactory : IFactory {
    public static Phoneline CreateStandardInstance() {
        // phoneline product creation logic goes here
    }
}

public class MobileFactory : IFactory {
    public static Mobile CreateStandardInstance() {
        // mobile product creation logic goes here
    }
}

public class BundleFactory : IAbstractFactory {

    public static Bundle CreateBundle() {
        broadband = BroadbandFactory.CreateStandardInstance();
        phoneline = PhonelineFactory.CreateStandardInstance();
        mobile = MobileFactory.CreateStandardInstance();

        applySomeDiscountOrWhatever(broadband, phoneline, mobile);
    }

    private static void applySomeDiscountOrWhatever(Broadband bb, Phoneline pl, Mobile m) {
        // some logic here
        // maybe manange some variables and invoke some other methods/services/etc.
    }
}

Hope this helps.

Answer 4

The main difference between Abstract Factory and Factory Method is that Abstract Factory is implemented by Composition; but Factory Method is implemented by Inheritance.

Yes, you read that correctly: the main difference between these two patterns is the old composition vs inheritance debate.

I am not going to reproduce the UML diagrams here as those can be found in numerous places. I do want to provide code examples; however, because I think that combining the examples from the top two answers in this thread will give a better demonstration than either answer alone. Additionally, I have added terminology used in the (GoF) book to class and method names.

Abstract Factory

1. The most important point to grasp here is that the abstract factory is injected into the client. This is why we say that Abstract Factory is implemented by Composition. Often, a dependency injection framework would perform that task; but a framework is not required for DI.
2. The second critical point is that the concrete factories here are not Factory Method implementations! Example code for Factory Method is shown further below.
3. And finally, the third point to note is the relationship between the products: in this case the outbound and reply queues. One concrete factory produces Azure queues, the other MSMQ. The GoF refers to this product relationship as a "family" and it's important to be aware that family in this case does not mean class hierarchy.

public class Client {
    private final AbstractFactory_MessageQueue factory;

    public Client(AbstractFactory_MessageQueue factory) {
        // The factory creates message queues either for Azure or MSMQ.
        // The client does not know which technology is used.
        this.factory = factory;
    }

    public void sendMessage() {
        //The client doesn't know whether the OutboundQueue is Azure or MSMQ.
        OutboundQueue out = factory.createProductA();
        out.sendMessage("Hello Abstract Factory!");
    }

    public String receiveMessage() {
        //The client doesn't know whether the ReplyQueue is Azure or MSMQ.
        ReplyQueue in = factory.createProductB();
        return in.receiveMessage();
    }
}

public interface AbstractFactory_MessageQueue {
    OutboundQueue createProductA();
    ReplyQueue createProductB();
}

public class ConcreteFactory_Azure implements AbstractFactory_MessageQueue {
    @Override
    public OutboundQueue createProductA() {
        return new AzureMessageQueue();
    }

    @Override
    public ReplyQueue createProductB() {
        return new AzureResponseMessageQueue();
    }
}

public class ConcreteFactory_Msmq implements AbstractFactory_MessageQueue {
    @Override
    public OutboundQueue createProductA() {
        return new MsmqMessageQueue();
    }

    @Override
    public ReplyQueue createProductB() {
        return new MsmqResponseMessageQueue();
    }
}

Factory Method

1. The most important point to grasp here is that the ConcreteCreator is the client. In other words, the client is a subclass whose parent defines the factoryMethod(). This is why we say that Factory Method is implemented by Inheritance.
2. The second critical point is to remember that the Factory Method Pattern is nothing more than a specialization of the Template Method Pattern. The two patterns share an identical structure. They only differ in purpose. Factory Method is creational (it builds something) whereas Template Method is behavioral (it computes something).
3. And finally, the third point to note is that the Creator (parent) class invokes its own factoryMethod(). If we remove anOperation() from the parent class, leaving only a single method behind, it is no longer the Factory Method pattern. In other words, Factory Method cannot be implemented with less than two methods in the parent class; and one must invoke the other.

public abstract class Creator {
    public void anOperation() {
        Product p = factoryMethod();
        p.whatever();
    }

    protected abstract Product factoryMethod();
}

public class ConcreteCreator extends Creator {
    @Override
    protected Product factoryMethod() {
        return new ConcreteProduct();
    }
}

Misc. & Sundry Factory Patterns

Be aware that although the GoF define two different Factory patterns, these are not the only Factory patterns in existence. They are not even necessarily the most commonly used Factory patterns. A famous third example is Josh Bloch's Static Factory Pattern from Effective Java. The Head First Design Patterns book includes yet another pattern they call Simple Factory.

Don't fall into the trap of assuming every Factory pattern must match one from the GoF.

Answer 5

The Difference Between The Two

The main difference between a "factory method" and an "abstract factory" is that the factory method is a single method, and an abstract factory is an object. I think a lot of people get these two terms confused, and start using them interchangeably. I remember that I had a hard time finding exactly what the difference was when I learnt them.

Because the factory method is just a method, it can be overridden in a subclass, hence the second half of your quote:

... the Factory Method pattern uses inheritance and relies on a subclass to handle the desired object instantiation.

The quote assumes that an object is calling its own factory method here. Therefore the only thing that could change the return value would be a subclass.

The abstract factory is an object that has multiple factory methods on it. Looking at the first half of your quote:

... with the Abstract Factory pattern, a class delegates the responsibility of object instantiation to another object via composition ...

What they're saying is that there is an object A, who wants to make a Foo object. Instead of making the Foo object itself (e.g., with a factory method), it's going to get a different object (the abstract factory) to create the Foo object.

Code Examples

To show you the difference, here is a factory method in use:

class A {
    public void doSomething() {
        Foo f = makeFoo();
        f.whatever();   
    }

    protected Foo makeFoo() {
        return new RegularFoo();
    }
}

class B extends A {
    protected Foo makeFoo() {
        //subclass is overriding the factory method 
        //to return something different
        return new SpecialFoo();
    }
}

And here is an abstract factory in use:

class A {
    private Factory factory;

    public A(Factory factory) {
        this.factory = factory;
    }

    public void doSomething() {
        //The concrete class of "f" depends on the concrete class
        //of the factory passed into the constructor. If you provide a
        //different factory, you get a different Foo object.
        Foo f = factory.makeFoo();
        f.whatever();
    }
}

interface Factory {
    Foo makeFoo();
    Bar makeBar();
    Aycufcn makeAmbiguousYetCommonlyUsedFakeClassName();
}

//need to make concrete factories that implement the "Factory" interface here

Answer 6

Abstract factory creates a base class with abstract methods defining methods for the objects that should be created. Each factory class which derives the base class can create their own implementaton of each object type.

Factory method is just a simple method used to create objects in a class. It's usually added in the aggregate root (The Order class has a method called CreateOrderLine)

Abstract factory

In the example below we design an interface so that we can decouple queue creation from a messaging system and can therefore create implementations for different queue systems without having to change the code base.

interface IMessageQueueFactory
{
  IMessageQueue CreateOutboundQueue(string name);
  IMessageQueue CreateReplyQueue(string name);
}

public class AzureServiceBusQueueFactory : IMessageQueueFactory
{
      IMessageQueue CreateOutboundQueue(string name)
      {
           //init queue
           return new AzureMessageQueue(/*....*/);
      }

      IMessageQueue CreateReplyQueue(string name)
      {
           //init response queue
           return new AzureResponseMessageQueue(/*....*/);
      }

}

public class MsmqFactory : IMessageQueueFactory
{
      IMessageQueue CreateOutboundQueue(string name)
      {
           //init queue
           return new MsmqMessageQueue(/*....*/);
      }

      IMessageQueue CreateReplyQueue(string name)
      {
           //init response queue
           return new MsmqResponseMessageQueue(/*....*/);
      }
}

Factory method

The problem in HTTP servers are that we always need an response for every request.

public interface IHttpRequest
{
    // .. all other methods ..

    IHttpResponse CreateResponse(int httpStatusCode);
}

Without the factory method, the HTTP server users (i.e. programmers) would be forced to use implementation specific classes which defeat the purpose of the IHttpRequest interface.

Therefore we introduce the factory method so that the creation of the response class also is abstracted away.

Summary

The difference is that the intended purpose of the class containing a factory method is not to create objects, while an abstract factory should only be used to create objects.

One should take care when using factory methods since it's easy to break the LSP (Liskovs Substitution principle) when creating objects.