Factory Method Pattern

# Factory Method

- Purpose
- Structure
- Applications
- Consequences

## Purpose

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

## Structure

- 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.

## Applications

- 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.

## Consequences

- 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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