Why Factory Method pattern in LLD? - Purpose & Use Cases

The Big Idea

What if you could add new object types without rewriting your entire codebase?

The Scenario

Imagine you are building a software system that needs to create different types of objects, like various shapes or documents, by hand. Every time you want a new type, you write new code everywhere to create it.

The Problem

This manual approach is slow and confusing. You have to change many places in your code to add a new object type. It's easy to make mistakes and hard to keep track of all the creation logic scattered around.

The Solution

The Factory Method pattern solves this by centralizing object creation. It lets you define a method that creates objects, but lets subclasses decide which object to create. This keeps your code clean, flexible, and easy to extend.

Before vs After

✗ Before

if(type == 'circle') { return new Circle(); } else if(type == 'square') { return new Square(); }

✓ After

Shape shape = shapeFactory.createShape();

What It Enables

It enables easy addition of new object types without changing existing code, making your system scalable and maintainable.

Real Life Example

Think of a document editor that can create different document types like Word, PDF, or Excel. Using Factory Method, adding a new document type is simple and doesn't break existing code.

Key Takeaways

Manual object creation scatters code and causes errors.

Factory Method centralizes and delegates creation to subclasses.

This pattern makes adding new types easy and safe.

Practice

1. What is the main purpose of the Factory Method pattern in system design?

easy

A. To enforce a single instance of a class throughout the application

B. To create objects directly without any abstraction

C. To define an interface for creating objects but let subclasses decide which class to instantiate

D. To provide a global point of access to a resource

Solution

1. Step 1: Understand the Factory Method pattern goal

   The Factory Method pattern aims to separate object creation from usage by defining an interface for creating objects.

2. Step 2: Identify the role of subclasses

   Subclasses decide which specific class to instantiate, allowing flexibility and easy extension.

3. Final Answer:

   To define an interface for creating objects but let subclasses decide which class to instantiate -> Option C

4. Quick Check:

   Factory Method = Interface for object creation + subclass decides [OK]

Hint: Factory Method lets subclasses choose object types [OK]

Common Mistakes:

• Confusing Factory Method with Singleton pattern
• Thinking Factory Method creates objects directly without abstraction
• Mixing Factory Method with global access patterns

2. Which of the following is the correct way to declare a Factory Method in an interface in a typical object-oriented design?

easy

A. public Product createProduct();

B. public static Product createProduct();

C. public void createProduct();

D. private Product createProduct();

Solution

1. Step 1: Understand Factory Method signature

   The Factory Method returns an object of a product type, so it must have a return type matching the product interface or class.

2. Step 2: Check access and static modifiers

   Factory Methods are usually instance methods (not static) and public to allow subclass overriding and usage.

3. Final Answer:

   public Product createProduct(); -> Option A

4. Quick Check:

   Factory Method returns product instance, public and non-static [OK]

Hint: Factory Method returns product, not void or static [OK]

Common Mistakes:

• Using void return type instead of product
• Making Factory Method static which breaks subclass override
• Declaring Factory Method private so subclasses can't override

3. Given the following code snippet, what will be the output?

abstract class Creator {
  abstract Product factoryMethod();
  void someOperation() {
    Product product = factoryMethod();
    print(product.operation());
  }
}

class ConcreteProductA implements Product {
  String operation() => "Product A operation";
}

class ConcreteCreatorA extends Creator {
  Product factoryMethod() => ConcreteProductA();
}

void main() {
  Creator creator = ConcreteCreatorA();
  creator.someOperation();
}

medium

A. Product operation

B. No output

C. Error: factoryMethod not implemented

D. Product A operation

Solution

1. Step 1: Trace the factoryMethod call in ConcreteCreatorA

   ConcreteCreatorA overrides factoryMethod to return ConcreteProductA instance.

2. Step 2: Call operation() on ConcreteProductA instance

   ConcreteProductA's operation() returns "Product A operation" which is printed.

3. Final Answer:

   Product A operation -> Option D

4. Quick Check:

   ConcreteCreatorA creates ConcreteProductA, prints its operation [OK]

Hint: Factory Method returns ConcreteProductA, prints its operation [OK]

Common Mistakes:

• Assuming abstract Creator can be instantiated
• Forgetting ConcreteCreatorA overrides factoryMethod
• Expecting generic Product operation output

4. Identify the error in the following Factory Method implementation:

abstract class Creator {
  Product factoryMethod();
}

class ConcreteCreator extends Creator {
  void factoryMethod() {
    print("Creating product");
  }
}

medium

A. ConcreteCreator's factoryMethod has wrong return type

B. Creator class should not be abstract

C. factoryMethod should be static

D. No error, code is correct

Solution

1. Step 1: Check method signature in abstract Creator

   factoryMethod returns Product type, so subclass must match return type.

2. Step 2: Check ConcreteCreator's factoryMethod signature

   It returns void, which conflicts with abstract method's Product return type.

3. Final Answer:

   ConcreteCreator's factoryMethod has wrong return type -> Option A

4. Quick Check:

   Return type mismatch causes error [OK]

Hint: Return type must match abstract method in subclass [OK]

Common Mistakes:

• Ignoring return type mismatch
• Thinking abstract class can't have abstract methods
• Assuming static needed for Factory Method

5. You are designing a system that supports multiple types of notifications: Email, SMS, and Push. Using the Factory Method pattern, how should you organize your classes to allow easy addition of new notification types without modifying existing code?

hard

A. Create separate unrelated classes for each notification type without a common interface

B. Create an abstract NotificationCreator with a factoryMethod returning Notification; implement subclasses EmailCreator, SMSCreator, PushCreator each creating respective Notification objects

C. Use static methods in Notification class to create different notification types

D. Create a single Notification class with if-else to handle all notification types

Solution

1. Step 1: Apply Factory Method pattern structure

   Define an abstract creator class with a factoryMethod that returns a Notification interface or abstract class.

2. Step 2: Implement concrete creators for each notification type

   Each subclass (EmailCreator, SMSCreator, PushCreator) overrides factoryMethod to create its specific Notification object.

3. Step 3: Ensure easy extension without modifying existing code

   Adding new notification types means adding new creator subclasses without changing existing ones, following open-closed principle.

4. Final Answer:

   Create an abstract NotificationCreator with a factoryMethod returning Notification; implement subclasses EmailCreator, SMSCreator, PushCreator each creating respective Notification objects -> Option B

5. Quick Check:

   Factory Method enables easy extension via subclassing [OK]

Hint: Use abstract creator + subclasses for each notification type [OK]

Common Mistakes:

• Using if-else inside one class, violating open-closed principle
• Using static methods, reducing flexibility
• Not using a common interface for notifications