Problem 4.4 — Library Management

Book vs BookCopy, a fine strategy, and the borrow flow that protects the 'one active loan per copy' rule.

public record Book(String isbn, String title, String author) {}

public final class BookCopy {
  private final String copyId;
  private final String isbn;
  private boolean onLoan = false;
  public BookCopy(String copyId, String isbn) { this.copyId = copyId; this.isbn = isbn; }
  public synchronized boolean checkout() {          // atomic claim, same idea as ParkingSpot
    if (onLoan) return false;
    onLoan = true;
    return true;
  }
  public synchronized void checkin() { onLoan = false; }
  public String copyId() { return copyId; }
}

public interface FinePolicy { Money finePerDay(); Duration loanPeriod(); }
public record StudentFinePolicy() implements FinePolicy {
  public Money finePerDay() { return new Money(1000, "INR"); }  // ₹10/day
  public Duration loanPeriod() { return Duration.ofDays(14); }
}

public final class Loan {
  private final BookCopy copy;
  private final Instant dueAt;
  private Instant returnedAt;
  private final FinePolicy policy;

  public Loan(BookCopy copy, Instant borrowedAt, FinePolicy policy) {
    this.copy = copy; this.policy = policy;
    this.dueAt = borrowedAt.plus(policy.loanPeriod());
  }

  public Money returnBook(Instant now) {                    // "tell, don't ask": Loan computes its own fine
    this.returnedAt = now;
    copy.checkin();
    long lateDays = Math.max(0, Duration.between(dueAt, now).toDays());
    return new Money(policy.finePerDay().minor() * lateDays, "INR");
  }
}

public final class LibraryService {
  public Loan borrow(BookCopy copy, FinePolicy policy, Instant now) {
    if (!copy.checkout()) throw new IllegalStateException("copy already on loan: " + copy.copyId());
    return new Loan(copy, now, policy);
  }
}