How do you handle fines in the library management system?

Direct Answer

Calculate fine during book return by comparing return date to due date. If overdue, fine equals overdue days multiplied by daily rate. Use a FineCalculator strategy to encapsulate rate logic (standard, premium members might have different rates). Member entity tracks outstanding fine balance that must be cleared before new borrowing is allowed.

Deep Dive & Architecture

The loan supplies the dates

A Loan records its due date and actual return date. A FinePolicy receives those values and the member type, calculates overdue days, applies any grace period or maximum cap, and returns a money value. Keeping the policy outside Member prevents borrowing state from becoming mixed with pricing rules.

A small calculation makes the rule visible

Suppose a copy was due on 10 June and returned on 13 June. With no grace period and a daily rate of ₹5, the overdue period is three days and the fine is ₹15. A return on or before 10 June produces zero. Boundary tests should cover the due date itself, the first overdue day, a grace period, and a capped fine.

The balance is a ledger, not a loose number

The system should record why each charge or payment changed the member's balance. BigDecimal is appropriate for money, and an injected clock makes date tests repeatable. Borrowing rules can consult the outstanding balance, but paying a fine should create a payment entry instead of silently replacing the total.

Code Example

interface FineStrategy { BigDecimal dailyRate(); }
class StandardFine implements FineStrategy { public BigDecimal dailyRate() { return new BigDecimal("5"); } }
class PremiumFine  implements FineStrategy { public BigDecimal dailyRate() { return new BigDecimal("2"); } }

class ReturnService {
    BigDecimal returnBook(BookLending loan, LocalDate returnDate, FineStrategy strategy) {
        long overdue = ChronoUnit.DAYS.between(loan.due(), returnDate);
        BigDecimal fine = BigDecimal.ZERO;
        if (overdue > 0) {
            fine = strategy.dailyRate().multiply(BigDecimal.valueOf(overdue));
            loan.member().addFine(fine);            // updates outstanding balance
        }
        loan.item().markAvailable();
        return fine;
    }
}

Rate is pluggable per member tier; BigDecimal keeps money exact.

Common Mistakes & Pitfalls

• Hard-coding one rate — different tiers need different rates; use a FineStrategy.
• double for money — rounding errors. Use BigDecimal.
• Computing fines on a schedule — compute on return (or on demand), not via a nightly job for the interview scope.
• Letting members borrow with unpaid fines — canBorrow() must check the outstanding balance.