静态属性上的NullPointerException

我在java中创建一个简单的登录程序.这是我到目前为止的代码.

import java.util.Scanner;
import java.io.*;
import java.util.Arrays;

public class PasswordProgram {

    public static String user;
    public String password;
    public static boolean part1Finish = false;
    public File file = new File("D:/file.txt");
    public FileWriter UsernameWrite;
    public char[] user1;

    public void part1() {
        System.out.println("Please create an account: ");
        Scanner input = new Scanner(System. in );
        System.out.println("Type in a username: ");
        String user = input.next();

        System.out.println("Type in a Password: ");
        String password = input.next();
        try {
            UsernameWrite = new FileWriter(file);
            UsernameWrite.write(user);
            UsernameWrite.write(password);
            System.out.println(user);
            UsernameWrite.close();
            part1Finish = true;
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public void part2() {
        Scanner scan = new Scanner(System. in );

        System.out.println("Please confirm your username: ");
        String usercheck = scan.next();
        int PassAttempts = 5;
        int UserAttempts = 5;
        user1 = user.toCharArray();
        user1 = password.toCharArray();
        char[] usernamecheck = java.util.Arrays.copyOfRange(user1, 0, user.length());
        System.out.println(usernamecheck);
        do {
            if (usercheck.equals(usernamecheck)) {
                while (PassAttempts > 0) {
                    System.out.println("Please confirm your password: ");
                    String passcheck = scan.next();
                    if (passcheck.equals(password)) {
                        System.out.println("Thank You ");
                    } else if (passcheck != password && PassAttempts > 0) {
                        PassAttempts--;
                        System.out.println("That is incorrect. Please Try Again");
                        passcheck = scan.nextLine();
                    } else {
                        System.out.println("You have run out of Password Attempts");
                        break;
                    }
                }
            } else if (usercheck != user && UserAttempts > 0) {
                UserAttempts--;
                System.out.println("That is an incorrect username. Please Try Again");
                usercheck = scan.nextLine();
            } else {
                System.out.println("You have run out of Username Attempts");
                break;
            }
        } while (UserAttempts > 0);

    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        PasswordProgram login = new PasswordProgram();
        login.part1();
        if (part1Finish == true) {
            login.part2();
        }
    }
}

我得到的问题是在方法中part2.在这里,当我尝试将保存在变量下的用户名添加user到字符数组中以将其用作范围时,我得到了错误NullPointerException.

调查后我发现当运行is part2的值时,我得到了错误.usernull

有没有办法可以通过这个FileReader方法来做到这一点,或者我如何解决我得到的当前错误？谢谢.

因为静态字段user是从未分配的part1,你得到一个NullPointerException当您尝试使用它part2.

发布的代码中还存在其他问题:

为什么涉及文件尚不清楚

你用!=用String,比如在passcheck != password

您使用之间平等String和char[]在usercheck.equals(usernamecheck)

passcheck 被约束但从未使用过

局部变量(因为它们的名称)隐藏了一些字段

UsernameWrite并且UserAttempts有非常规名称(应该是 usernameWrite和userAttempts