Library Management System

// Interface for basic library operations
interface LibraryOperations {
    void addBook(Book book);
    void removeBook(String bookId);
    void lendBook(String bookId, User user);
    void returnBook(String bookId, User user);
}

// Interface for user-related operations
interface UserOperations {
    void registerUser(User user);
    void unregisterUser(String userId);
}

// Interface for book-related operations
interface BookOperations {
    String getBookDetails();
}

// Book class implementing BookOperations
class Book implements BookOperations {
    private String id;
    private String title;
    private String author;
    private boolean isAvailable;

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

    @Override
    public String getBookDetails() {
        return "ID: " + id + ", Title: " + title + ", Author: " + author + ", Available: " + isAvailable;
    }

    public String getId() {
        return id;
    }

    public boolean isAvailable() {
        return isAvailable;
    }

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

// User class implementing UserOperations
class User implements UserOperations {
    private String id;
    private String name;

    public User(String id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public void registerUser(User user) {
        // Implementation for registering user
    }

    @Override
    public void unregisterUser(String userId) {
        // Implementation for unregistering user
    }

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }
}

// Library class implementing LibraryOperations
class Library implements LibraryOperations {
    private Map<String, Book> books = new HashMap<>();
    private Map<String, User> users = new HashMap<>();

    @Override
    public void addBook(Book book) {
        books.put(book.getId(), book);
    }

    @Override
    public void removeBook(String bookId) {
        books.remove(bookId);
    }

    @Override
    public void lendBook(String bookId, User user) {
        Book book = books.get(bookId);
        if (book != null && book.isAvailable()) {
            book.setAvailable(false);
            System.out.println("Book lent to " + user.getName());
        } else {
            System.out.println("Book is not available.");
        }
    }

    @Override
    public void returnBook(String bookId, User user) {
        Book book = books.get(bookId);
        if (book != null && !book.isAvailable()) {
            book.setAvailable(true);
            System.out.println("Book returned by " + user.getName());
        } else {
            System.out.println("Book was not lent out.");
        }
    }
}

// Example usage
public class LibraryManagementSystem {
    public static void main(String[] args) {
        Library library = new Library();
        User user1 = new User("1", "Alice");
        User user2 = new User("2", "Bob");

        Book book1 = new Book("101", "Java Programming", "John Doe");
        Book book2 = new Book("102", "Design Patterns", "Jane Doe");

        library.addBook(book1);
        library.addBook(book2);

        library.lendBook("101", user1);
        library.returnBook("101", user1);
    }
}