Factory Method pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the Factory Method in the Creator class.

LLD

abstract class Creator {
    abstract [1] createProduct();
}

Drag options to blanks, or click blank then click option'

Avoid

BProduct

Cint

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using void as return type which means no object is returned.

Using primitive types like int or String which are unrelated.

The Factory Method returns a Product type object, so the return type must be Product.

2fill in blank

medium

Complete the code to implement the Factory Method in the ConcreteCreator class.

LLD

class ConcreteCreator extends Creator {
    @Override
    public [1] createProduct() {
        return new ConcreteProduct();
    }
}

Drag options to blanks, or click blank then click option'

AConcreteProduct

Bvoid

CProduct

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using ConcreteProduct as return type breaks abstraction.

Using void or primitive types causes compile errors.

The method must return the abstract type Product to follow the Factory Method pattern, even if it returns a ConcreteProduct instance.

3fill in blank

hard

Fix the error in the client code that uses the Factory Method to get a product.

LLD

Creator creator = new ConcreteCreator();
Product product = creator.[1]();
product.use();

Drag options to blanks, or click blank then click option'

AcreateProduct

BmakeProduct

CgetProduct

DnewProduct

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names not declared in Creator causes compile errors.

Trying to instantiate product directly instead of using factory method.

The Factory Method is named createProduct in the Creator class, so the client must call this method.

4fill in blank

hard

Fill both blanks to complete the product interface and its concrete implementation.

LLD

interface [1] {
    void [2]();
}

class ConcreteProduct implements Product {
    public void use() {
        System.out.println("Using ConcreteProduct");
    }
}

Drag options to blanks, or click blank then click option'

AProduct

Buse

Cexecute

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using different interface names breaks the pattern.

Method names that don't match cause implementation errors.

The interface is named Product and declares the method use() which the concrete class implements.

5fill in blank

hard

Fill all three blanks to complete the Creator class with a factory method and a business logic method.

LLD

abstract class Creator {
    public abstract [1] createProduct();

    public void [2]() {
        [3] product = createProduct();
        product.use();
    }
}

Drag options to blanks, or click blank then click option'

AProduct

Boperate

Dexecute

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching return and variable types causes errors.

Using incorrect method names breaks the pattern.

The factory method returns a Product. The business method is named operate and it calls createProduct() to get a product and use it.

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