Design a library management system. What are the key classes?

Direct Answer

A library management system needs entities for catalog (Book), physical inventory (BookItem), users (Member, Librarian), and transactions (BookLending, Reservation, Fine). The key distinction is Book represents the catalog entry while BookItem represents each physical copy with its own barcode and availability status.

Deep Dive & Architecture

Book title and physical-copy identities

Book represents the catalogue record: ISBN, title, authors, and subject. BookCopy represents one barcode that can be available, reserved, loaned, lost, or under repair. A library may own five copies of one title, so borrowing a Book directly loses the state of each copy.

Loans and reservations record changing relationships

Loan connects one member to one copy and stores issue, due, and return dates. Reservation connects a member to a title or copy, records queue order and expiry, and has its own status. Member owns identity and eligibility rules; Librarian performs privileged catalogue or account actions.

Services coordinate use cases

Catalog handles search. BorrowingService checks eligibility, claims a copy, and creates the loan. FinePolicy calculates any overdue charge, while NotificationService sends hold or due-date messages. For example, returning barcode C-17 closes its loan, then either marks C-17 available or holds it for the next valid reservation. This split keeps catalogue facts, copy state, and transaction history distinct.

Code Example

enum BookStatus { AVAILABLE, LOANED, RESERVED, LOST }

class Book {                          // catalog entry (one per title)
    private final String isbn;
    private final String title;
    private final String author;
    private final List<BookItem> copies = new ArrayList<>();   // 1 -> many
}

class BookItem {                      // one physical copy
    private final String barcode;
    private BookStatus status = BookStatus.AVAILABLE;
    boolean isAvailable() { return status == BookStatus.AVAILABLE; }
}

class Member {
    private final String memberId;
    private final List<BookLending> activeLoans = new ArrayList<>();
    private double outstandingFine = 0;
    boolean canBorrow() { return outstandingFine == 0 && activeLoans.size() < 5; }
}

record BookLending(BookItem item, Member member, LocalDate issued, LocalDate due) {}

A Member borrows a BookItem, not a Book — you can only check out a physical copy.

Common Mistakes & Pitfalls

• Merging Book and BookItem — then you can't track which physical copy is out, or have 5 copies of one title.
• Borrowing a Book — you borrow a BookItem; the catalog entry isn't checkout-able.
• A God Library class doing search, lending, and fines — split by responsibility (SRP).
• No member borrowing rules — canBorrow() should gate on outstanding fines and loan count.