Design a library system

1import java.time.*;2import java.util.*;3 4record Book(String isbn, String title, String author) {}5 6final class BookCopy {7    enum Status { AVAILABLE, ON_LOAN, ON_HOLD, LOST }8    final String barcode; final Book book;9    Status status = Status.AVAILABLE;10    String heldFor;                                   // member id when ON_HOLD11    BookCopy(String barcode, Book book) { this.barcode = barcode; this.book = book; }12}13 14final class Member {15    final String id; final String name;16    final List<Loan> activeLoans = new ArrayList<>();17    long finesCents;18    Member(String id, String name) { this.id = id; this.name = name; }19}20 21final class Loan {22    final BookCopy copy; final Member member; final LocalDate borrowed, due;23    LocalDate returned;24    Loan(BookCopy copy, Member member, LocalDate borrowed, LocalDate due) { this.copy = copy; this.member = member; this.borrowed = borrowed; this.due = due; }25}26 27interface FinePolicy { long fineCents(Loan loan, LocalDate returnedOn); }28final class PerDayFine implements FinePolicy {29    private final long perDay, cap;30    PerDayFine(long perDay, long cap) { this.perDay = perDay; this.cap = cap; }31    public long fineCents(Loan l, LocalDate on) { return Math.min(cap, Math.max(0, Duration.between(l.due.atStartOfDay(), on.atStartOfDay()).toDays()) * perDay); }32}33 34interface Notifier { void holdReady(Member m, Book b); }35 36final class Library {37    private static final int MAX_LOANS = 3, LOAN_DAYS = 14;38    private static final long MAX_FINES = 1_000;39 40    private final Map<String, Book> books = new HashMap<>();41    private final Map<String, List<BookCopy>> copiesByIsbn = new HashMap<>();42    private final Map<String, BookCopy> copiesByBarcode = new HashMap<>();43    private final Map<String, Deque<Member>> reservations = new HashMap<>();44    private final Map<String, Loan> loansByBarcode = new HashMap<>();45    private final FinePolicy fines;46    private final Notifier notifier;47    private final Clock clock;48 49    Library(FinePolicy fines, Notifier notifier, Clock clock) { this.fines = fines; this.notifier = notifier; this.clock = clock; }50 51    void addCopy(Book book, String barcode) {52        books.putIfAbsent(book.isbn(), book);53        BookCopy copy = new BookCopy(barcode, book);54        copiesByIsbn.computeIfAbsent(book.isbn(), k -> new ArrayList<>()).add(copy);55        copiesByBarcode.put(barcode, copy);56    }57 58    List<Book> search(String text) {59        String q = text.toLowerCase();60        return books.values().stream().filter(b -> b.title().toLowerCase().contains(q) || b.author().toLowerCase().contains(q) || b.isbn().equals(text)).toList();61    }62 63    Loan checkout(Member m, String isbn) {64        if (m.activeLoans.size() >= MAX_LOANS) throw new IllegalStateException("Borrow limit reached");65        if (m.finesCents > MAX_FINES) throw new IllegalStateException("Unpaid fines over limit");66        BookCopy copy = copiesByIsbn.getOrDefault(isbn, List.of()).stream()67            .filter(c -> c.status == BookCopy.Status.AVAILABLE || (c.status == BookCopy.Status.ON_HOLD && m.id.equals(c.heldFor)))68            .findFirst().orElseThrow(() -> new IllegalStateException("No copy available; reserve instead"));69        LocalDate today = LocalDate.now(clock);70        Loan loan = new Loan(copy, m, today, today.plusDays(LOAN_DAYS));71        copy.status = BookCopy.Status.ON_LOAN;72        copy.heldFor = null;73        m.activeLoans.add(loan);74        loansByBarcode.put(copy.barcode, loan);75        return loan;76    }77 78    long returnCopy(String barcode) {79        Loan loan = Optional.ofNullable(loansByBarcode.remove(barcode)).orElseThrow(() -> new IllegalArgumentException("Not on loan"));80        LocalDate today = LocalDate.now(clock);81        loan.returned = today;82        loan.member.activeLoans.remove(loan);83        long fine = fines.fineCents(loan, today);84        loan.member.finesCents += fine;85 86        BookCopy copy = loan.copy;87        Deque<Member> queue = reservations.getOrDefault(copy.book.isbn(), new ArrayDeque<>());88        Member next = queue.poll();89        if (next != null) {90            copy.status = BookCopy.Status.ON_HOLD;91            copy.heldFor = next.id;92            notifier.holdReady(next, copy.book);93        } else {94            copy.status = BookCopy.Status.AVAILABLE;95        }96        return fine;97    }98 99    void reserve(Member m, String isbn) {100        if (!books.containsKey(isbn)) throw new IllegalArgumentException("Unknown ISBN");101        Deque<Member> q = reservations.computeIfAbsent(isbn, k -> new ArrayDeque<>());102        if (q.contains(m)) throw new IllegalStateException("Already reserved");103        q.add(m);104    }105}