Object-Oriented Design: Library Management System

📌 Problem Statement

"Design a library management system that supports searching, borrowing, returning, and tracking books for members and librarians."

This is one of the most commonly asked OOD interview questions. It tests your ability to design class relationships, manage entity lifecycles, and model role-based behavior with clarity and extensibility.

✅ Requirements

Functional Requirements

  • Members can search, borrow, and return books
  • Librarians can add, remove, and update book inventory
  • Track due dates, issue fines for overdue books
  • Support multiple copies per book
  • Allow users to reserve a book if it's currently checked out

Non-Functional Requirements

  • Support thousands of users and book records
  • Role-based access control (member vs. librarian)
  • Scalable and extendable system for future features

🎯 Key Object-Oriented Concepts

  • Inheritance: User types (Member, Librarian) share common logic
  • Encapsulation: Book copy availability and reservation logic is internal
  • Composition: Book is composed of copies; User has multiple loans
  • Interfaces/Strategy Pattern: Fine calculation logic can vary

🧱 Core Classes and Responsibilities

User (abstract)

  • userId, name, email, accountStatus
  • Common base class for Member and Librarian

Member extends User

  • Can searchBooks(), borrowBook(), returnBook(), reserveBook()
  • borrowedBooks: list of currently issued books
  • fines: outstanding dues

Librarian extends User

  • Can addBook(), removeBook(), updateBook(), manageFines()

Book

  • title, author, ISBN, genre, publisher
  • Contains a list of BookCopy instances

BookCopy

  • Unique identifier for a physical copy
  • status: available, checked out, reserved, lost
  • Tracks due date and borrower

Catalog

  • Provides search interface: by title, author, subject, ISBN
  • Internally manages book index (hash maps, search indexes)

Reservation

  • Represents a reservation made by a user on a specific book
  • Has timestamp and queue order

Loan

  • Issued when a user borrows a book copy
  • Tracks borrowDate, dueDate, returnDate, fineAmount

FinePolicy (interface/strategy)

  • Defines how overdue fines are calculated
  • Can support different fine strategies (flat rate, daily accrual, tiered)

🔁 Borrow & Return Flow

  1. Member searches catalog for a book
  2. System finds an available copy and assigns it
  3. A Loan object is created and associated with the member
  4. On return:
    • Calculate any fine (if overdue)
    • Update the book copy status
    • Mark loan as returned

If no copies are available:

  • The system creates a Reservation entry
  • When a copy is returned, the next reservation in the queue is fulfilled

🔐 Role-Based Access

  • Members can only borrow/return/search
  • Librarians have elevated privileges to manage inventory and resolve fines
  • Permissions are enforced through method access and class separation

🔍 Additional Features and Extensions

  • Email Notifications for due dates and availability of reserved books
  • Fine Payment Integration for resolving dues online
  • Digital Books Support with download tracking
  • Analytics: most borrowed books, user borrowing trends

🧠 Interviewer Evaluation Criteria

AreaWhat They’re Looking For
Class RelationshipsLogical structure and real-world mapping
ReusabilityCan common logic be shared through inheritance or interfaces?
ResponsibilitySeparation of concerns (searching, borrowing, managing)
ExtensibilityCan you add digital books or new roles easily?
State ModelingHow are loans, reservations, and fines handled?

✅ Summary

A Library Management System provides a full exercise in OOP best practices. A solid design includes:

  • Role-based user classes
  • Robust book and inventory tracking
  • Lifecycle management of borrowing and reservation
  • Pluggable fine and notification strategies

This question helps demonstrate your ability to model real-world domains cleanly and scalably.