Library Management System in Java

import java.util.ArrayList;
import java.util.List;

class Book {
    private String title;
    private String author;
    private String isbn;
    private boolean isAvailable;

    public Book(String title, String author, String isbn) {
        this.title = title;
        this.author = author;
        this.isbn = isbn;
        this.isAvailable = true;
    }

    public boolean isAvailable() {
        return isAvailable;
    }

    public void setAvailable(boolean available) {
        isAvailable = available;
    }

    public String getTitle() {
        return title;
    }
}

class User {
    private String userId;
    private String name;
    private List<Book> borrowedBooks;

    public User(String userId, String name) {
        this.userId = userId;
        this.name = name;
        this.borrowedBooks = new ArrayList<>();
    }

    public void borrowBook(Book book) {
        borrowedBooks.add(book);
    }

    public void returnBook(Book book) {
        borrowedBooks.remove(book);
    }

    public List<Book> getBorrowedBooks() {
        return borrowedBooks;
    }
}

class Library {
    private List<Book> books;
    private List<User> users;

    public Library() {
        books = new ArrayList<>();
        users = new ArrayList<>();
    }

    public void addBook(Book book) {
        books.add(book);
    }

    public void addUser(User user) {
        users.add(user);
    }

    public boolean borrowBook(String isbn, String userId) {
        Book book = findBookByIsbn(isbn);
        User user = findUserById(userId);
        if (book != null && user != null && book.isAvailable()) {
            book.setAvailable(false);
            user.borrowBook(book);
            return true;
        }
        return false;
    }

    public boolean returnBook(String isbn, String userId) {
        Book book = findBookByIsbn(isbn);
        User user = findUserById(userId);
        if (book != null && user != null && user.getBorrowedBooks().contains(book)) {
            book.setAvailable(true);
            user.returnBook(book);
            return true;
        }
        return false;
    }

    private Book findBookByIsbn(String isbn) {
        for (Book book : books) {
            if (book.getTitle().equals(isbn)) {
                return book;
            }
        }
        return null;
    }

    private User findUserById(String userId) {
        for (User user : users) {
            if (user.userId.equals(userId)) {
                return user;
            }
        }
        return null;
    }
}

public class Main {
    public static void main(String[] args) {
        Library library = new Library();
        Book book1 = new Book("Effective Java", "Joshua Bloch", "12345");
        User user1 = new User("U001", "Alice");

        library.addBook(book1);
        library.addUser(user1);

        boolean borrowed = library.borrowBook("12345", "U001");
        System.out.println("Borrowed: " + borrowed); // Expected: true

        boolean returned = library.returnBook("12345", "U001");
        System.out.println("Returned: " + returned); // Expected: true
    }
}