The method setBook(int) is undefined for the type Lender上面的代码

谢谢指出！抱歉忘记了 Lender 类没有实现 `setBook(int)` 方法。可以在 `User` 类中添加一个 `protected` 访问修饰符的 `setBook(int)` 方法，这样 `Lender` 类就可以使用了。修改后的代码如下：

public class User implements IUser {
    private String name;
    private int book;

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

    @Override
    public String getName() {
        return name;
    }

    @Override
    public int getBook() {
        return book;
    }

    @Override
    public void moreBook(int number) {}

    protected void setBook(int book) {
        this.book = book;
    }
}

public class Lender extends User {
    
    public Lender(String name, int booksLent) {
        super(name, -booksLent); // lending books is the same as borrowing negative number of books
    }
    
    @Override
    public void moreBook(int number) {
        int currentBooksLent = -getBook(); // get the current number of books lent (which is stored as negative in User class)
        currentBooksLent += number; // increase the number of books lent by the given number
        setBook(-currentBooksLent); // set the book instance variable of User class to negative of current number of books lent
    }
}