Java – Design Patterns: Factory vs Factory method vs Abstract Factory

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I was reading design patterns from a website

There I read about Factory, Factory method and Abstract factory but they are so confusing, am not clear on the definition. According to definitions

Factory – Creates objects without exposing the instantiation logic to the client and Refers to the newly created object through a common interface. Is a simplified version of Factory Method

Factory Method – Defines an interface for creating objects, but let subclasses to decide which class to instantiate and Refers to the newly created object through a common interface.

Abstract Factory – Offers the interface for creating a family of related objects, without explicitly specifying their classes.

I also looked the other stackoverflow threads regarding Abstract Factory vs Factory Method but the UML diagrams drawn there make my understanding even worse.

Can anyone please tell me

  1. How are these three patterns different from each other?
  2. When to use which?
  3. And also if possible, any java examples related to these patterns?

Best Answer

All three Factory types do the same thing: They are a "smart constructor".

Let's say you want to be able to create two kinds of Fruit: Apple and Orange.

Factory

Factory is "fixed", in that you have just one implementation with no subclassing. In this case, you will have a class like this: 
class FruitFactory {

  public Apple makeApple() {
    // Code for creating an Apple here.
  }

  public Orange makeOrange() {
    // Code for creating an orange here.
  }

}

Use case: Constructing an Apple or an Orange is a bit too complex to handle in the constructor for either.

Factory Method

Factory method is generally used when you have some generic processing in a class, but want to vary which kind of fruit you actually use. So: 
abstract class FruitPicker {

  protected abstract Fruit makeFruit();

  public void pickFruit() {
    private final Fruit f = makeFruit(); // The fruit we will work on..
    <bla bla bla>
  }
}

...then you can reuse the common functionality in FruitPicker.pickFruit() by implementing a factory method in subclasses:

class OrangePicker extends FruitPicker {

  @Override
  protected Fruit makeFruit() {
    return new Orange();
  }
}

Abstract Factory

Abstract factory is normally used for things like dependency injection/strategy, when you want to be able to create a whole family of objects that need to be of "the same kind", and have some common base classes. Here's a vaguely fruit-related example. The use case here is that we want to make sure that we don't accidentally use an OrangePicker on an Apple. As long as we get our Fruit and Picker from the same factory, they will match. 
interface PlantFactory {
  
  Plant makePlant();

  Picker makePicker(); 

}

public class AppleFactory implements PlantFactory {
  Plant makePlant() {
    return new Apple();
  }

  Picker makePicker() {
    return new ApplePicker();
  }
}

public class OrangeFactory implements PlantFactory {
  Plant makePlant() {
    return new Orange();
  }

  Picker makePicker() {
    return new OrangePicker();
  }
}
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