Factory Method pattern in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the Factory Method pattern?

The Factory Method pattern provides a way to create objects without specifying the exact class of the object that will be created. It lets subclasses decide which class to instantiate.

beginner

In the Factory Method pattern, who is responsible for creating the product objects?

The subclasses of the creator class are responsible for creating the product objects by overriding the factory method.

intermediate

How does the Factory Method pattern help in system design?

It promotes loose coupling by separating object creation from usage, making the system easier to extend and maintain.

intermediate

Which of the following best describes the Factory Method pattern structure?

It has a Creator class with a factory method that returns a Product interface. Subclasses override the factory method to instantiate concrete products.

beginner

Give a real-life example of the Factory Method pattern.

Consider a document editor app that can create different types of documents (like text or spreadsheet). The app uses a factory method to create the right document type without knowing the exact class.

What does the Factory Method pattern primarily help with?

A Handling user interface events

B Managing database connections

C Optimizing memory usage

D Creating objects without specifying their exact class

The Factory Method pattern focuses on creating objects without specifying their exact class.

Who overrides the factory method in the Factory Method pattern?

A The base Creator class

B The subclasses of the Creator class

C The client code

D The product classes

Subclasses of the Creator class override the factory method to create specific product instances.

Which benefit does the Factory Method pattern provide?

A Separates object creation from usage

B Tight coupling between classes

C Forces use of global variables

D Eliminates need for interfaces

It separates object creation from usage, promoting loose coupling.

In the Factory Method pattern, what does the factory method return?

A A concrete product instance

B A configuration file

C A product interface or abstract product

D A database connection

The factory method returns a product interface or abstract product, allowing flexibility.

Which scenario best fits the Factory Method pattern?

A Creating different types of documents in an editor app

B Sorting a list of numbers

C Sending an email notification

D Calculating mathematical formulas

Creating different document types is a classic example of the Factory Method pattern.

Explain the Factory Method pattern and how it helps in designing flexible systems.

Think about how subclasses decide which object to create.

Describe a real-world example where the Factory Method pattern can be applied and why.

Consider apps that handle multiple related object types.

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