adduser but it is not going to be installed

The "adduser" package is typically pre-installed on many Linux systems, so it's possible that it is already installed on your system and you don't need to install it again. If you are sure that it is not installed, you can try installing it using your system's package manager.

For example, on Debian/Ubuntu systems, you can try running the following command to install the "adduser" package:

sudo apt-get install adduser

If you are using a different package manager or operating system, you can search for instructions on how to install the package using that system.

相关问题

addUser2 is not defined"

这个错误提示通常是因为你在代码中调用了一个名为 addUser2 的函数，但是这个函数并没有被定义。可能有以下几种原因导致这个问题：

1. addUser2 函数确实没有被定义。你需要检查一下代码中是否有定义这个函数，或者是否拼写错误。

2. addUser2 函数被定义在了其他文件中，并且你没有正确导入这个文件。你需要检查一下是否正确导入了包含 addUser2 函数的文件。

3. addUser2 函数被定义在了其他作用域中，而你在当前作用域中无法访问到。你需要检查一下函数定义的作用域，或者将函数定义移动到当前作用域中。

希望这些提示能够帮助你解决问题。

Second, the Library class is the class that contains all the data for the library. Therefore the Library class plays the role of the model. Therefore the Library class needs to keep an arraylist of model listeners that need to be notified every time the library (the model) changes. 1. Add to the Library class an private instance variable, an arraylist of ModelListener. When a library is created, it has an empty arraylist of listeners. 2. Also add to the Library class an addListener method that takes a ModelListener as argument and adds it to the arraylist of listeners. 3. Also add to the Library class a private notifyListeners method that takes nothing as argument and calls the update method of all the listeners of the library. 4. Then change the addUser and moreBook methods so that they call the notifyListeners every time a change is made to the library's data (only the addUser and moreBook methods change the library’s data, so only these two methods need to call the notifyListeners method; the totalBorrowedBooks and getBook methods do not change the library’s data, they only inspect the data, so they do not need to call the notifyListeners method).

Sure, here's an updated version of the Library class with the changes you described:

import java.util.ArrayList;

public class Library {
    private ArrayList<User> userList;
    private ArrayList<Book> bookList;
    private ArrayList<ModelListener> listeners;

    public Library() {
        userList = new ArrayList<User>();
        bookList = new ArrayList<Book>();
        listeners = new ArrayList<ModelListener>();
    }

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

    public void moreBook(Book book) {
        bookList.add(book);
        notifyListeners();
    }

    public int totalBorrowedBooks() {
        int count = 0;
        for (User user : userList) {
            count += user.getBorrowedBooks().size();
        }
        return count;
    }

    public Book getBook(String title) {
        for (Book book : bookList) {
            if (book.getTitle().equals(title)) {
                return book;
            }
        }
        return null;
    }

    public void addListener(ModelListener listener) {
        listeners.add(listener);
    }

    private void notifyListeners() {
        for (ModelListener listener : listeners) {
            listener.update();
        }
    }
}

As you can see, we added a `listeners` instance variable to keep track of all the model listeners, an `addListener` method to add new listeners to the list, and a `notifyListeners` method to call the `update` method of all the listeners whenever the library's data changes.

We also modified the `addUser` and `moreBook` methods to call `notifyListeners` after adding a new user or book to the corresponding list.

Note that we didn't change the `totalBorrowedBooks` and `getBook` methods since they only inspect the library's data and don't modify it.