在线图书馆系统

以下是一个稍微复杂一些的Java代码，它的作用是实现一个简单的在线图书馆系统，可以添加、删除、借阅、归还和查询图书信息：

import java.util.ArrayList;
import java.util.Scanner;

public class LibrarySystem {
    private ArrayList<Book> books;
    private ArrayList<User> users;
    private Scanner scanner;

    public LibrarySystem() {
        books = new ArrayList<>();
        users = new ArrayList<>();
        scanner = new Scanner(System.in);
    }

    public void run() {
        int choice = 0;
        do {
            System.out.println("Welcome to the Library System!");
            System.out.println("1. Add book");
            System.out.println("2. Remove book");
            System.out.println("3. Borrow book");
            System.out.println("4. Return book");
            System.out.println("5. Search book");
            System.out.println("6. Register user");
            System.out.println("7. Remove user");
            System.out.println("8. Exit");
            System.out.print("Enter your choice: ");
            choice = scanner.nextInt();
            switch (choice) {
                case 1:
                    addBook();
                    break;
                case 2:
                    removeBook();
                    break;
                case 3:
                    borrowBook();
                    break;
                case 4:
                    returnBook();
                    break;
                case 5:
                    searchBook();
                    break;
                case 6:
                    registerUser();
                    break;
                case 7:
                    removeUser();
                    break;
                case 8:
                    System.out.println("Goodbye!");
                    break;
                default:
                    System.out.println("Invalid choice!");
            }
        } while (choice != 8);
    }

    private void addBook() {
        System.out.print("Enter book title: ");
        String title = scanner.next();
        System.out.print("Enter book author: ");
        String author = scanner.next();
        System.out.print("Enter book ISBN: ");
        String isbn = scanner.next();
        Book book = new Book(title, author, isbn);
        books.add(book);
        System.out.println("Book added successfully!");
    }

    private void removeBook() {
        System.out.print("Enter book ISBN: ");
        String isbn = scanner.next();
        for (int i = 0; i < books.size(); i++) {
            Book book = books.get(i);
            if (book.getIsbn().equals(isbn)) {
                books.remove(i);
                System.out.println("Book removed successfully!");
                return;
            }
        }
        System.out.println("Book not found!");
    }

    private void borrowBook() {
        System.out.print("Enter book ISBN: ");
        String isbn = scanner.next();
        System.out.print("Enter user ID: ");
        int userId = scanner.nextInt();
        for (int i = 0; i < books.size(); i++) {
            Book book = books.get(i);
            if (book.getIsbn().equals(isbn)) {
                if (book.isBorrowed()) {
                    System.out.println("Book already borrowed!");
                    return;
                } else {
                    book.setBorrowed(true);
                    book.setBorrowedBy(userId);
                    System.out.println("Book borrowed successfully!");
                    return;
                }
            }
        }
        System.out.println("Book not found!");
    }

    private void returnBook() {
        System.out.print("Enter book ISBN: ");
        String isbn = scanner.next();
        for (int i = 0; i < books.size(); i++) {
            Book book = books.get(i);
            if (book.getIsbn().equals(isbn)) {
                if (book.isBorrowed()) {
                    book.setBorrowed(false);
                    book.setBorrowedBy(-1);
                    System.out.println("Book returned successfully!");
                    return;
                } else {
                    System.out.println("Book not borrowed!");
                    return;
                }
            }
        }
        System.out.println("Book not found!");
    }

    private void searchBook() {
        System.out.print("Enter book title or author: ");
        String keyword = scanner.next();
        for (int i = 0; i < books.size(); i++) {
            Book book = books.get(i);
            if (book.getTitle().contains(keyword) || book.getAuthor().contains(keyword)) {
                System.out.println(book);
            }
        }
    }

    private void registerUser() {
        System.out.print("Enter user ID: ");
        int id = scanner.nextInt();
        System.out.print("Enter user name: ");
        String name = scanner.next();
        User user = new User(id, name);
        users.add(user);
        System.out.println("User registered successfully!");
    }

    private void removeUser() {
        System.out.print("Enter user ID: ");
        int id = scanner.nextInt();
        for (int i = 0; i < users.size(); i++) {
            User user = users.get(i);
            if (user.getId() == id) {
                users.remove(i);
                System.out.println("User removed successfully!");
                return;
            }
        }
        System.out.println("User not found!");
    }
}

这个程序实现了一个简单的在线图书馆系统，用户可以执行以下操作：

1. 添加图书
2. 删除图书
3. 借阅图书
4. 归还图书
5. 查询图书
6. 注册用户
7. 删除用户
8. 退出系统

这个程序使用了许多面向对象编程的概念，包括类、对象、继承、封装和多态。它还演示了Java中许多常用类和方法的使用，例如ArrayList、Scanner和switch语句。这个程序比之前的例子更加复杂，但它可以作为一个更高级的Java应用程序的基础。