Identifying classes from requirements in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identifying the main class from a simple requirement

You have a requirement: "A library system needs to manage books and members." Which class should be the main focus to represent the core entity?

A Book

B Author

C Member

D Library

💡 Hint

Think about what represents the whole system managing books and members.

The 'Library' class represents the system managing books and members, so it is the main focus.

❓ Architecture

intermediate

Choosing classes for an online shopping system

Given the requirement: "The system should allow customers to place orders for products." Which set of classes best represents the core entities?

A Customer, Payment, Delivery

B Order, Invoice, Shipment

C Customer, Order, Product

D Product, Supplier, Inventory

💡 Hint

Focus on who places orders and what is ordered.

Customers place Orders for Products, so these classes represent the core entities.

❓ scaling

advanced

Identifying classes for a scalable ride-sharing app

For a ride-sharing app that must handle millions of users and rides, which class design best supports scalability?

A User, Driver, Ride, Vehicle, Location, Payment, Rating

B User, Ride, Vehicle, Location, Payment

C Driver, Vehicle, Route, Payment

D User, Driver, Ride, Payment

💡 Hint

Consider all entities needed to support features and scale.

Option A includes all relevant classes to handle users, rides, vehicles, locations, payments, and ratings, supporting scalability.

❓ tradeoff

advanced

Tradeoff in class granularity for a hotel booking system

Which class design shows a tradeoff between simplicity and detailed representation?

A Booking, Room, Customer, Payment, Invoice, CancellationPolicy

B Booking, Customer

C Booking, Room, Customer

D Room, Customer, Payment

💡 Hint

More classes mean more detail but more complexity.

Option A balances detailed representation with complexity by including payment, invoice, and cancellation policy classes.

❓ estimation

expert

Estimating number of classes for a social media platform

Estimate the minimum number of classes needed to represent a social media platform with users, posts, comments, likes, and messages.

A 9

B 5

C 7

D 3

💡 Hint

Think about one class per main entity mentioned.

Users, Posts, Comments, Likes, and Messages each need a class, totaling 5 classes.

Practice

1. Which of the following is most likely a class when designing a library management system?

easy

A. Return

B. Borrow

C. Book

D. Search

Solution

1. Step 1: Identify nouns in the requirement

   Classes usually come from nouns. Here, 'Book' is a noun representing an object.

2. Step 2: Differentiate between nouns and verbs

   'Borrow', 'Return', and 'Search' are actions (verbs), so they are methods, not classes.

3. Final Answer:

   Book -> Option C

4. Quick Check:

   Class = Noun = Book [OK]

Hint: Pick the noun as class, verbs become methods [OK]

Common Mistakes:

• Choosing verbs as classes
• Confusing actions with objects
• Ignoring nouns in requirements

2. In a ride-sharing app, which of these is the correct way to represent a class in code?

easy

A. class Driver {}

B. function Driver() {}

C. var Driver = 5;

D. let driver = 'John';

Solution

1. Step 1: Understand class syntax in LLD context

   Classes are defined with the keyword 'class' followed by the class name.

2. Step 2: Check options for class declaration

   class Driver {} uses 'class Driver {}' which is correct syntax for a class.

3. Final Answer:

   class Driver {} -> Option A

4. Quick Check:

   Class syntax uses 'class' keyword [OK]

Hint: Look for 'class' keyword to identify class declaration [OK]

Common Mistakes:

• Using function instead of class
• Assigning class name to variable
• Confusing variables with classes

3. Given the requirement: "A hotel booking system must manage Guests, Rooms, and Bookings." Which of these is the correct set of classes identified?

medium

A. [Booking, Book, GuestList]

B. [Guest, Room, Booking]

C. [Guest, Book, RoomBooking]

D. [Room, Guest, ReservationList]

Solution

1. Step 1: Extract nouns from the requirement

   The nouns are Guests, Rooms, and Bookings, which represent objects.

2. Step 2: Match nouns to class names

   Classes should be singular nouns representing entities: Guest, Room, Booking.

3. Final Answer:

   [Guest, Room, Booking] -> Option B

4. Quick Check:

   Classes = Nouns = Guest, Room, Booking [OK]

Hint: Pick singular nouns as class names from requirements [OK]

Common Mistakes:

• Using verbs as classes
• Using plural or combined names incorrectly
• Adding unrelated terms

4. You have a requirement: "The system should allow Users to login and logout." Which of the following is an error in identifying classes and methods?

medium

A. Class: User; Methods: login(), logout()

B. Class: UserAccount; Methods: login(), logout()

C. Class: User; Methods: authenticate(), logout()

D. Class: Login; Methods: user(), logout()

Solution

1. Step 1: Identify nouns and verbs

   'User' is a noun (class), 'login' and 'logout' are actions (methods).

2. Step 2: Check option correctness

   Class: Login; Methods: user(), logout() incorrectly uses 'Login' as a class and 'user()' as a method, which reverses noun and verb roles.

3. Final Answer:

   Class: Login; Methods: user(), logout() -> Option D

4. Quick Check:

   Class = noun, Method = verb [OK]

Hint: Classes are nouns; methods are verbs [OK]

Common Mistakes:

• Using verbs as classes
• Naming methods as nouns
• Mixing class and method roles

5. A food delivery app requirement states: "Customers place Orders, Restaurants prepare Meals, and DeliveryAgents deliver Orders." Which classes should be created to model this system correctly?

hard

A. [Customer, Order, Restaurant, Meal, DeliveryAgent]

B. [Customer, PlaceOrder, Restaurant, Meal, Deliver]

C. [CustomerOrder, RestaurantMeal, Delivery]

D. [Order, Meal, DeliveryAgent, Place]

Solution

1. Step 1: Identify all nouns in the requirement

   Nouns are Customers, Orders, Restaurants, Meals, DeliveryAgents.

2. Step 2: Use nouns as class names and verbs as methods

   Classes should be Customer, Order, Restaurant, Meal, DeliveryAgent. Actions like 'place' and 'deliver' become methods.

3. Final Answer:

   [Customer, Order, Restaurant, Meal, DeliveryAgent] -> Option A

4. Quick Check:

   Classes = Nouns only, verbs become methods [OK]

Hint: Classes = nouns; actions = methods in those classes [OK]

Common Mistakes:

• Using verbs as classes
• Combining nouns incorrectly
• Omitting important entities