Library Management System

public final class BookCopy {
    private final String barcode;
    private final String isbn;
    private boolean available = true;

    public BookCopy(String barcode, String isbn) {
        this.barcode = barcode;
        this.isbn = isbn;
    }

    public boolean isAvailable() { return available; }
    public String isbn() { return isbn; }

    void checkout() {
        if (!available) throw new IllegalStateException("out");
        available = false;
    }

    void checkin() { available = true; }
}

public final class Member {
    private final String id;
    private int activeLoans;
    private final int limit;

    public Member(String id, int limit) {
        this.id = id;
        this.limit = limit;
    }

    public boolean canBorrow() { return activeLoans < limit; }
    public String id() { return id; }
    public void onLoanCreated() { activeLoans++; }
    public void onLoanClosed() { activeLoans--; }
}

public record Loan(String id, String memberId, String barcode, LocalDate due) {}

public final class LoanService {
    private final Map<String, BookCopy> copies = new HashMap<>();
    private final Map<String, Loan> openLoans = new HashMap<>();

    public Loan checkout(Member member, String isbn) {
        if (!member.canBorrow()) throw new IllegalStateException("limit");
        BookCopy copy = copies.values().stream()
            .filter(c -> c.isbn().equals(isbn) && c.isAvailable())
            .findFirst()
            .orElseThrow(() -> new IllegalStateException("none available"));
        copy.checkout();
        member.onLoanCreated();
        Loan loan = new Loan(UUID.randomUUID().toString(), member.id(), copy.barcode(), LocalDate.now().plusWeeks(2));
        openLoans.put(loan.id(), loan);
        return loan;
    }

    public Money returnBook(String loanId) {
        Loan loan = openLoans.remove(loanId);
        copies.get(loan.barcode()).checkin();
        if (LocalDate.now().isAfter(loan.due())) {
            return Money.of(1).multiply(ChronoUnit.DAYS.between(loan.due(), LocalDate.now()));
        }
        return Money.ZERO;
    }
}