Java code review

import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.Paths; import java.util.ArrayList; import java.util.Date; import java.util.HashMap; import java.util.List; import java.util.Map; @SuppressWarnings("serial") public class library implements Serializable { private static final String LIBRARY_FILE = "library.obj"; private static final int LOAN_LIMIT = 2; private static final int LOAN_PERIOD = 2; private static final double FINE_PER_DAY = 1.0; private static final double MAX_FINES_OWED = 5.0; private static final double DAMAGE_FEE = 2.0; private static library self; private int BID; private int MID; private int LID; private Date loadDate; private Map<Integer, book> catalog; private Map<Integer, member> members; private Map<Integer, loan> loans; private Map<Integer, loan> currentLoans; private Map<Integer, book> damagedBooks; private library() { catalog = new HashMap<>(); members = new HashMap<>(); loans = new HashMap<>(); currentLoans = new HashMap<>(); damagedBooks = new HashMap<>(); BID = 1; MID = 1; LID = 1; } public static synchronized library INSTANCE() { if (self == null) { Path path = Paths.get(LIBRARY_FILE); if (Files.exists(path)) { try (ObjectInputStream lof = new ObjectInputStream(new FileInputStream(LIBRARY_FILE));) { self = (library) lof.readObject(); Calendar.getInstance().setDate(self.loadDate); lof.close(); } catch (Exception e) { throw new RuntimeException(e); } } else self = new library(); } return self; } public static synchronized void SAVE() { if (self != null) { self.loadDate = Calendar.getInstance().Date(); try (ObjectOutputStream lof = new ObjectOutputStream(new FileOutputStream(LIBRARY_FILE));) { lof.writeObject(self); lof.flush(); lof.close(); } catch (Exception e) { throw new RuntimeException(e); } } } public int BookID() { return BID; } public int MemberID() { return MID; } private int nextBID() { return BID++; } private int nextMID() { return MID++; } private int nextLID() { return LID++; } public List<member> Members() { return new ArrayList<member>(members.values()); } public List<book> Books() { return new ArrayList<book>(catalog.values()); } public List<loan> CurrentLoans() { return new ArrayList<loan>(currentLoans.values()); } public member Add_mem(String lastName, String firstName, String email, int phoneNo) { member member = new member(lastName, firstName, email, phoneNo, nextMID()); members.put(member.getId(), member); return member; } public book Add_book(String a, String t, String c) { book b = new book(a, t, c, nextBID()); catalog.put(b.ID(), b); return b; } public member getMember(int memberId) { if (members.containsKey(memberId)) return members.get(memberId); return null; } public book Book(int bookId) { if (catalog.containsKey(bookId)) return catalog.get(bookId); return null; } public int loanLimit() { return LOAN_LIMIT; } public boolean memberCanBorrow(member member) { if (member.getNumberOfCurrentLoans() == LOAN_LIMIT ) return false; if (member.getFinesOwed() >= MAX_FINES_OWED) return false; for (loan loan : member.getLoans()) if (loan.isOverDue()) return false; return true; } public int loansRemainingForMember(member member) { return LOAN_LIMIT - member.getNumberOfCurrentLoans(); } public loan issueLoan(book book, member member) { Date dueDate = Calendar.getInstance().getDueDate(LOAN_PERIOD); loan loan = new loan(nextLID(), book, member, dueDate); member.takeOutLoan(loan); book.Borrow(); loans.put(loan.getId(), loan); currentLoans.put(book.ID(), loan); return loan; } public loan getLoanByBookId(int bookId) { if (currentLoans.containsKey(bookId)) { return currentLoans.get(bookId); } return null; } public double calculateOverDueFine(loan loan) { if (loan.isOverDue()) { long daysOverDue = Calendar.getInstance().getDaysDifference(loan.getDueDate()); double fine = daysOverDue * FINE_PER_DAY; return fine; } return 0.0; } public void dischargeLoan(loan currentLoan, boolean isDamaged) { member member = currentLoan.Member(); book book = currentLoan.Book(); double overDueFine = calculateOverDueFine(currentLoan); member.addFine(overDueFine); member.dischargeLoan(currentLoan); book.Return(isDamaged); if (isDamaged) { member.addFine(DAMAGE_FEE); damagedBooks.put(book.ID(), book); } currentLoan.Loan(); currentLoans.remove(book.ID()); } public void checkCurrentLoans() { for (loan loan : currentLoans.values()) { loan.checkOverDue(); } } public void repairBook(book currentBook) { if (damagedBooks.containsKey(currentBook.ID())) { currentBook.Repair(); damagedBooks.remove(currentBook.ID()); } else { throw new RuntimeException("Library: repairBook: book is not damaged"); } } }