Overview
The Factory Method design pattern is a creational pattern that provides an interface for creating objects in a superclass but allows subclasses to alter the type of objects that will be created. This pattern is particularly important in scenarios where a system needs to be independent of how its products are created, composed, or represented. It is widely used in real-world applications for decoupling the construction of objects from their usage, making the system more modular, easier to extend, and maintain.
Key Concepts
- Encapsulation of Object Creation: Encapsulates object creation by allowing objects to be created at runtime depending on the logic and criteria that are dynamic.
- Subclassing: The pattern promotes subclassing, where subclasses implement the factory method to create objects rather than creating objects directly using a constructor.
- Single Responsibility Principle: Adheres to the Single Responsibility Principle by separating the product construction code from the code that uses the product.
Common Interview Questions
Basic Level
- What is the Factory Method design pattern and why is it used?
- Can you provide a simple implementation of the Factory Method pattern?
Intermediate Level
- How does the Factory Method pattern differ from the Abstract Factory pattern?
Advanced Level
- In what scenarios would you optimize the use of the Factory Method pattern, and how would you implement these optimizations?
Detailed Answers
1. What is the Factory Method design pattern and why is it used?
Answer:
The Factory Method design pattern is a creational design pattern that defines an interface for creating an object but lets subclasses decide which class to instantiate. It is used to achieve loose coupling and to adhere to the principle of open/closed principle, making a system easier to extend without modifying its existing code. The pattern is particularly useful when the exact types and dependencies of the objects being created cannot be known until runtime.
Key Points:
- Encapsulates object creation, improving code maintainability and flexibility.
- Promotes subclassing, allowing for dynamic instantiation logic.
- Supports the open/closed principle, as new types can be added with minimal changes to existing code.
Example:
public abstract class DocumentFactory
{
// The Factory Method
public abstract Document CreateDocument();
public void SomeOperation()
{
var document = CreateDocument();
document.Open();
}
}
public class PdfDocumentFactory : DocumentFactory
{
public override Document CreateDocument()
{
return new PdfDocument();
}
}
public abstract class Document
{
public abstract void Open();
}
public class PdfDocument : Document
{
public override void Open()
{
Console.WriteLine("PDF Document Opened.");
}
}
2. Can you provide a simple implementation of the Factory Method pattern?
Answer:
A simple implementation of the Factory Method pattern involves creating an abstract creator class that defines a factory method, concrete creator classes that implement the factory method to instantiate specific products, and an abstract product class with concrete product implementations.
Key Points:
- The abstract creator offers the factory method interface.
- Concrete creators override the factory method to return an instance of a concrete product.
- The pattern facilitates the addition of new product types with minimal changes to the client code.
Example:
public abstract class AnimalFactory
{
// Factory Method
public abstract Animal CreateAnimal();
}
public class DogFactory : AnimalFactory
{
public override Animal CreateAnimal()
{
return new Dog();
}
}
public class CatFactory : AnimalFactory
{
public override Animal CreateAnimal()
{
return new Cat();
}
}
public abstract class Animal
{
public abstract void Speak();
}
public class Dog : Animal
{
public override void Speak()
{
Console.WriteLine("Woof!");
}
}
public class Cat : Animal
{
public override void Speak()
{
Console.WriteLine("Meow!");
}
}
3. How does the Factory Method pattern differ from the Abstract Factory pattern?
Answer:
The Factory Method pattern focuses on a single product, providing a method for creating an instance of a product, whereas the Abstract Factory pattern deals with families of related or dependent products without specifying their concrete classes. The Factory Method is a class-level creation pattern, whereas Abstract Factory is an object-level creation pattern, offering an interface for creating families of related objects.
Key Points:
- Factory Method is about creating a single product, while Abstract Factory is about creating families of products.
- Factory Method uses inheritance and relies on subclasses to handle the object instantiation, whereas Abstract Factory uses composition to delegate to multiple factory methods.
- Abstract Factory can be implemented using multiple Factory Methods.
Example:
// Factory Method within Abstract Factory
public interface IAnimalFactory
{
Dog CreateDog();
Cat CreateCat();
}
public class PetAnimalFactory : IAnimalFactory
{
public Dog CreateDog()
{
return new Dog(); // Uses Factory Method internally
}
public Cat CreateCat()
{
return new Cat(); // Uses Factory Method internally
}
}
4. In what scenarios would you optimize the use of the Factory Method pattern, and how would you implement these optimizations?
Answer:
Optimizations for the Factory Method pattern can be considered when there are performance issues due to frequent object creation, a large number of subclasses, or complex initialization logic. Utilizing caching, lazy initialization, or pooling strategies can optimize the pattern's usage.
Key Points:
- Caching: Reuse instances instead of creating new ones if instantiation is expensive and the objects are often repeated.
- Lazy Initialization: Delay the creation of an object until it's actually needed to reduce startup time and resource usage.
- Pooling: Pre-instantiate and pool objects that are expensive to create and reuse them.
Example:
public abstract class VehicleFactory
{
private static Dictionary<string, Vehicle> _cache = new Dictionary<string, Vehicle>();
public abstract Vehicle CreateVehicle(string type);
// Caching optimization
public Vehicle GetVehicle(string type)
{
if (!_cache.ContainsKey(type))
{
_cache[type] = CreateVehicle(type);
}
return _cache[type];
}
}
public class CarFactory : VehicleFactory
{
public override Vehicle CreateVehicle(string type)
{
return new Car();
}
}
public abstract class Vehicle
{
}
public class Car : Vehicle
{
}
This example demonstrates a simple caching mechanism within a factory to avoid repeated instantiation of the same type of objects, optimizing the use of the Factory Method pattern for scenarios where object creation is costly.
