Differences between Abstract Factory Pattern and Factory Method
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.
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 (eg, 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
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 (ie 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.
Difference between AbstractFactory and Factory design patters are as follows:
Factory Method Pattern Implementation: 
AbstractFactory Pattern Implementation:
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