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

- Product(Document) : defines the interface of objects the factory
method creates.
- ConcreteProduct(Mydocument) : implements the Product interface.
- Creator(Application) : declares the factory method, which returns
an object of type Product. Creator may also define a default implementation
of the factory method that returns a default ConcreteProduct object. may
call the factory method to create a Product object.
- ConcreteCreator(MyApplication) : overrides the factory method to
return an instance of a ConcreteProduct.
- a class can't anticipate the class of objects it must create.
- a class wants its subclasses to specify the objects it creates.
- classes delegate responsibility to one of several helper subclasses, and
you want to localize the knowledge of which helper subclass is the delegate.
- Provides hooks for subclasses. Creating objects inside a class with
a factory method is always more flexible than creating an object directly.
Factory Method gives subclasses a hook for providing an extended version of
an object.
- Connects parallel class hierarchies. In the examples we've
considered so far, the factory method is only called by Creators. But this
doesn't have to be the case; clients can find factory methods useful,
especially in the case of parallel class hierarchies.
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