Sample Object-Oriented Design Interview Questions and Answers

Sample Object-Oriented Design Interview Questions and Answers

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1. Design a Parking Lot System

Question: Design a parking lot with multiple levels, each with multiple parking spots of different sizes (small, medium, large). Vehicles can be motorcycles, cars, or buses.

Answer:

Key Classes:

• ParkingLot

• Level

• ParkingSpot

• Vehicle (abstract)

  • Car

  • Motorcycle

  • Bus

Design Approach:

• Use inheritance for vehicle types.

• Each Level contains multiple ParkingSpots.

• ParkingSpot has a type (SMALL, MEDIUM, LARGE) and availability status.

• Vehicle has method canFitInSpot(ParkingSpot).

Design Considerations:

• A Bus needs 5 contiguous LARGE spots on the same level.

• Motorcycle fits in any spot, Car in medium or large.

• Use Strategy or Factory pattern for flexible spot assignment logic.

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2. Design an Online Book Reader System

Question: Design a system for an online book reader. Users can read books, bookmark pages, and make notes.

Answer:

Key Classes:

• OnlineReaderSystem

• User

• Book

• Library

• Bookmark

• Note

• Display

Design Approach:

• Library handles search and management of books.

• User can interact with Book, set Bookmark, and create Note.

• Display class shows book content, current page, notes, etc.

OOP Principles Used:

• Encapsulation: Each class maintains its own state.

• Composition: A User has multiple Bookmarks and Notes.

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3. Design a Hotel Management System

Question: Design a hotel reservation system to handle bookings, room availability, check-in/check-out, and billing.

Answer:

Key Classes:

• Hotel

• Room

• Reservation

• Customer

• Invoice

• Payment

• Receptionist / Admin / Guest (User Roles)

Design Considerations:

• Room has type, price, and availability.

• Reservation includes booking dates and room info.

• Use the State pattern for room status: Available, Reserved, Occupied, UnderMaintenance.

• Billing is associated with the reservation and finalized during checkout.

Additional Components:

• Notification service (e.g., SMS/email)

• Calendar integration for room availability

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4. Design a Ride-Sharing App (like Uber)

Question: Design the backend for a ride-hailing app including driver, passenger, trip management, and payments.

Answer:

Key Classes:

• User (abstract)

  • Driver

  • Passenger

• Trip

• Vehicle

• Location

• Payment

• RideService

Design Concepts:

• Observer pattern for location tracking and status updates.

• Strategy pattern for different pricing models (e.g., surge pricing).

• Each Trip contains origin, destination, driver, passenger, status, and cost.

• Vehicle types can be matched based on user preference and availability.

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5. Design a Social Media News Feed

Question: Design the news feed system for a social media app. Show personalized, recent posts from friends and followed users.

Answer:

Key Classes:

• User

• Post

• Feed

• Friendship

• NewsFeedService

Key Features:

• Posts have timestamps, privacy settings, and content types (text, image, video).

• Use a priority queue or min-heap to merge posts from multiple sources efficiently.

• Feed ranking algorithm based on relevance (likes, time decay, engagement).

Patterns & Principles:

• Dependency Injection for personalization service.

• Strategy pattern for ranking algorithm.

• Caching and background feed prefetching for performance.

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6. Design a File System

Question: Design a simplified in-memory file system with support for directories and files.

Answer:

Key Classes:

• FileSystem

• Directory

• File

• Node (abstract with name, path)

Approach:

• Composite pattern for Directory and File under Node.

• Use a tree structure to represent hierarchy.

• Add operations: mkdir, addFile, delete, move, list.

Advanced Considerations:

• Permissions and ownership

• File metadata (size, timestamps)

• Path resolution and symbolic links

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7. Design an ATM System

Question: Design an ATM system including card insertion, PIN validation, transactions, and cash dispensing.

Answer:

Key Classes:

• ATM

• Bank

• Account

• Card

• Transaction (abstract)

  • Withdrawal

  • Deposit

  • BalanceInquiry

• CashDispenser

• Keypad, Screen, Printer

Design Focus:

• Use State pattern for ATM states: Idle, CardInserted, Authenticated, TransactionInProgress.

• Strategy pattern for different transaction types.

• Card validation routed through the bank, which holds account data.

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8. Design a Chess Game

Question: Design an object-oriented chess game that can be played between two users.

Answer:

Key Classes:

• ChessGame

• Board

• Player

• Piece (abstract)

  • Pawn, Knight, Bishop, Rook, Queen, King

• Move

• Cell

Design Details:

• Use Factory pattern for initializing pieces.

• Each Piece has method isValidMove(Position from, Position to, Board board).

• Enforce game rules like check/checkmate, castling, en passant using Rule classes.

Additional Components:

• Game history tracking using a list of Moves

• Observer pattern for UI updates

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9. Design a Movie Ticket Booking System

Question: Design a system for booking movie tickets in multiple theaters.

Answer:

Key Classes:

• Movie

• Show

• Theater

• Seat

• Booking

• User

• Payment

Design Highlights:

• Each Theater has multiple Shows for a Movie.

• Seat has status: Available, Booked, Blocked.

• Concurrency handling for seat booking using locking or optimistic concurrency.

Additional Features:

• Search by location, movie, and showtime

• Loyalty program integration

• Cancellation and refund policy

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10. Design a Notification System

Question: Design a notification system supporting multiple channels: Email, SMS, Push.

Answer:

Key Classes:

• Notification

• NotificationService

• NotificationChannel (interface)

  • EmailChannel

  • SMSChannel

  • PushChannel

• UserPreference

Design Principles:

• Use Strategy pattern to send via different channels.

• Observer pattern for subscription-based updates.

• UserPreference determines preferred channel(s).

Scalability:

• Asynchronous delivery via message queues

• Retry and fallback mechanisms

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These sample object-oriented design interview questions and answers showcase how to approach problem decomposition, apply design principles, and use design patterns effectively. Mastering such cases will significantly improve your performance in software engineering interviews.