Overview
The Factory Method and Abstract Factory patterns are fundamental design patterns in software engineering, particularly important for creating objects in a way that increases the flexibility and reusability of code. Understanding the differences between these two patterns is crucial for designing robust, scalable, and maintainable software systems.
Key Concepts
- Creation without specifying exact class: Both patterns allow for the instantiation of objects without specifying the exact class of the object that will be created.
- Encapsulation of object creation: They encapsulate the logic of creating objects, allowing for more flexible and maintainable code.
- Dependency inversion: Both patterns promote dependency inversion by decoupling the client code from the concrete classes being instantiated.
Common Interview Questions
Basic Level
- What is the main intent of the Factory Method pattern?
- Can you give an example of where the Abstract Factory pattern might be more suitable than the Factory Method?
Intermediate Level
- How does the Factory Method pattern differ from the Simple Factory pattern?
Advanced Level
- When would you choose the Abstract Factory pattern over the Factory Method pattern, considering software design principles?
Detailed Answers
1. What is the main intent of the Factory Method pattern?
Answer: The main intent of the Factory Method pattern is to define an interface for creating an object, but let subclasses decide which class to instantiate. It lets a class defer instantiation to subclasses, thus providing flexibility in the decision of what objects to create.
Key Points:
- Allows for flexibility: Different subclasses can implement the factory method to create different types of objects.
- Promotes loose coupling: The client code depends on the abstract type/interface, not specific concrete classes.
- Supports the Open/Closed Principle: You can introduce new types of products without affecting the client code.
Example:
public abstract class DocumentCreator
{
// Factory Method
public abstract IDocument CreateDocument();
public void SomeOperation()
{
var document = CreateDocument();
document.Display();
}
}
public class PdfCreator : DocumentCreator
{
public override IDocument CreateDocument()
{
return new PdfDocument();
}
}
public interface IDocument
{
void Display();
}
public class PdfDocument : IDocument
{
public void Display()
{
Console.WriteLine("Displaying PDF document");
}
}
2. Can you give an example of where the Abstract Factory pattern might be more suitable than the Factory Method?
Answer: The Abstract Factory pattern is more suitable in scenarios where a system needs to create multiple families of related or dependent objects without specifying their concrete classes. For instance, when developing a cross-platform UI library where elements like Buttons and Textboxes can have different styles and behaviors on different operating systems, the Abstract Factory pattern provides a way to encapsulate the creation of these families of objects.
Key Points:
- Deals with families of related objects.
- Ensures that the created objects are compatible with each other.
- Promotes consistency among products.
Example:
public interface IGUIFactory
{
IButton CreateButton();
ITextbox CreateTextbox();
}
public class WinFactory : IGUIFactory
{
public IButton CreateButton()
{
return new WinButton();
}
public ITextbox CreateTextbox()
{
return new WinTextbox();
}
}
public interface IButton { void Paint(); }
public interface ITextbox { void Paint(); }
public class WinButton : IButton
{
public void Paint()
{
Console.WriteLine("Rendering a button in Windows style");
}
}
public class WinTextbox : ITextbox
{
public void Paint()
{
Console.WriteLine("Rendering a textbox in Windows style");
}
}
3. How does the Factory Method pattern differ from the Simple Factory pattern?
Answer: The Simple Factory isn't a true design pattern but rather a programming idiom. It involves a method that creates objects based on the provided parameters. On the other hand, the Factory Method pattern defines an interface for creating an object but leaves the choice of its type to the subclasses, creating a form of polymorphism.
Key Points:
- The Simple Factory encapsulates object creation in a single class, centralizing the decision-making process.
- The Factory Method pattern delegates the creation process to subclasses, which decide the type of the object to create.
- The Factory Method pattern supports the Open/Closed Principle better, as it allows for easier extension by introducing new subclasses.
Example:
// Simple Factory Example
public class SimpleDocumentFactory
{
public static IDocument CreateDocument(string type)
{
switch (type)
{
case "PDF":
return new PdfDocument();
default:
throw new ArgumentException("Unsupported document type");
}
}
}
// Factory Method in use (see previous example for PdfCreator)
4. When would you choose the Abstract Factory pattern over the Factory Method pattern, considering software design principles?
Answer: The Abstract Factory pattern should be chosen over the Factory Method when dealing with complex object creation that involves multiple families of products or when the system needs to be independent of how its products are created, composed, and represented. It's particularly useful when the system must be configured with one of multiple families of products.
Key Points:
- Use Abstract Factory when your code needs to work with various families of related products.
- Abstract Factory helps to ensure that a family of related products is used together correctly, maintaining consistency among them.
- Factory Method is preferable when there's only one product type to be created or when the creation logic can be encapsulated within one class with optional subclassing.
Example:
// Building on the previous Abstract Factory example with WinFactory
public class MacFactory : IGUIFactory
{
public IButton CreateButton()
{
return new MacButton();
}
public ITextbox CreateTextbox()
{
return new MacTextbox();
}
}
public class MacButton : IButton
{
public void Paint()
{
Console.WriteLine("Rendering a button in Mac style");
}
}
public class MacTextbox : ITextbox
{
public void Paint()
{
Console.WriteLine("Rendering a textbox in Mac style");
}
}
This example illustrates how the Abstract Factory pattern provides a mechanism to create families of related objects (Windows or Mac UI elements) without specifying their concrete classes, promoting flexibility and scalability in the design.
