The Difference Between The Two
The main difference between a "factory method" and an "abstract factory" is that the factory method is a 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 usesinheritance and relies on a subclassto handle the desired objectinstantiation.
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 classdelegates the responsibility of objectinstantiation to another object viacomposition ...
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