写一个图书馆管理系统，要求有类的封装，和借用归还功能

设计一个简单的图书馆管理系统，我们可以考虑以下几个关键类：

1. **图书类(Book)**: 封装每本书的基本信息，如书名、作者、ISBN号等，并包含一个借阅状态属性。

public class Book {
    private String title;
    private String author;
    private String isbn;
    private boolean isBorrowed;

    public Book(String title, String author, String isbn) {
        this.title = title;
        this.author = author;
        this.isbn = isbn;
        this.isBorrowed = false;
    }

    // ... getters and setters for properties
    public void borrow() {
        if (!isBorrowed) {
            isBorrowed = true;
            System.out.println("Book '" + title + "' has been borrowed.");
        } else {
            System.out.println("Book is already borrowed.");
        }
    }

    public void returnBook() {
        if (isBorrowed) {
            isBorrowed = false;
            System.out.println("Book '" + title + "' has been returned.");
        } else {
            System.out.println("Book was not borrowed to begin with.");
        }
    }
}

2. **读者类(Reader)**: 代表一个用户，可以管理他们的借阅记录。

public class Reader {
    private String name;
    private List<Book> borrowedBooks = new ArrayList<>();

    public Reader(String name) {
        this.name = name;
    }

    // ... methods for managing borrowed books
    public void borrowBook(Book book) {
        book.borrow();
        borrowedBooks.add(book);
    }

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

// 图书馆类(Library): 管理书籍和读者
public class Library {
    private Map<String, Book> books = new HashMap<>();
    
    public void addBook(Book book) {
        books.put(book.getIsbn(), book);
    }

    // ... methods for handling reader's borrowing and returning
}

在这个系统中，`Library`类负责添加书籍，而`Reader`类则负责借阅和归还书籍。每个`Book`对象都有其自身的借阅状态，这体现了类的封装。通过这样的设计，我们实现了基本的借用和归还功能。