Question

我是java的新手，我正在开发一个典型的记事本程序，可以让你保存，编辑，删除和创建笔记。对于应用程序，我正在尝试将身份验证添加到我的登录页面。我在UserList的初始化时遇到问题。我究竟做错了什么？很确定这是一个语法问题，我现在太过忘记了

UserList.java

import java.util.ArrayList;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class UserList {

    //Declares 2 arraylists, one for the list of usernames and one for passwords
    private ArrayList<String> validUsernames;
    private ArrayList<String> validPasswords;
    private UserList theUserList;

    //Constructor that adds usernames and passwords to the arraylists
    public UserList() {

        validUsernames = new ArrayList();
        validPasswords = new ArrayList();

        validUsernames.add("");
        validUsernames.add("");
        validUsernames.add("");
        validUsernames.add("");
        validUsernames.add("");
        validPasswords.add("");
        validPasswords.add("");
        validPasswords.add("");
        validPasswords.add("");
        validPasswords.add("");
    }

    //Returns arraylist of valid users
    public ArrayList<String> getValidUsernames() {
        return validUsernames;
    }

    //Returns arraylist of valid passwords
    public ArrayList<String> getValidPasswords() {
        return validPasswords;
    }
}

LoginCntl （验证方法）

public boolean authenticate(String username, String password){

        validUsernames = new ArrayList();
        **validUsernames = theUserList.getValidUsernames();** [line 42]

        validPasswords = new ArrayList();
        validPasswords = theUserList.getValidPasswords();
        for (int i = 0; i < validUsernames.size(); i++) {
            if (validUsernames.get(i).equals(username) && validPasswords.get(i).equals(password)) {
                return true;
            }
        }
        theLoginUI.password.setText("");
        JOptionPane.showMessageDialog(null, "alert", "alert", JOptionPane.ERROR_MESSAGE);
        return false; 
    }

Answer 1

此行有NullPointerException：

validUsernames = theUserList.getValidUsernames();

唯一能导致此行NullPointerException的是theUserList为空。确保在尝试访问之前初始化theUserList 。在这种情况下，在您调用身份验证之前。例如。您可以在声明时初始化它，如下所示：

UserList theUserList = new UserList();

或者你必须在尝试访问它之前检查null，并在那种情况下做一些适当的事情。例如

if(theUserList == null) { // do something about it } validUsernames = new ArrayList(); validUsernames = theUserList.getValidUsernames(); ...

如何向UserList添加身份验证？

1 个答案: