需要帮助在我的 java 程序中集成一个 while 循环。希望程序更好地处理错误输入

Question

到目前为止，这是我的代码，只有在输入了匹配的用户名和密码时才有效（显示欢迎消息）。

我不确定如何集成一个 while 循环，如果 usernameInput 字符串与用户名字符串不匹配，该循环会阻止用户进入密码阶段，并反复循环直到输入正确的用户名。然后在我想要密码输入字符串相同的东西之后，直到它与密码字符串匹配。这个想法是让程序能够一遍又一遍地循环，直到用户输入了正确的用户名和密码。

[注意：我是从非常基础的python知识进入java的，所以一切可能有点混乱和低效]

import java.util.Scanner;

public class Login {

    public static void main(String[] args) {
        
        String username,usernameInput, password, passwordInput;
        username= "KenBoneL0ver";
        password= "KenBone4Life";
        
        Scanner myObj1= new Scanner(System.in);
        
        System.out.println("Please enter your username:");
        usernameInput= myObj1.nextLine();
        
        if (!usernameInput.equals(username)) {    
            System.out.println("Incorrect username. Please try again:");
        }
        else {
            System.out.println("Now please enter the correct password:");
        }
        
        Scanner myObj2= new Scanner(System.in);
        
        passwordInput= myObj2.nextLine();
        
        if (!passwordInput.equals(password)) {
            System.out.println("Incorrect password. Please try again:");
        }
        else {
            System.out.println("You have successfully logged in!"); 
        }
        
        myObj1.close();
        myObj2.close();
    }

}

感谢您的帮助！

Answer 1

请问您是否正在寻找类似的东西？

import java.util.Scanner;

public class Login {

    public static void main(String[] args) {
        Scanner myObj1 = null;
        try {
            String username, usernameInput, password, passwordInput;
            username = "KenBoneL0ver";
            password = "KenBone4Life";

            myObj1 = new Scanner(System.in);

            while (true) {
                System.out.println("Please enter your username:");
                usernameInput = myObj1.nextLine();
                if (!usernameInput.equals(username)) {
                    System.out.println("Incorrect username. Please try again:");
                    continue;
                }
                break;
            }

            while (true) {
                // Scanner myObj2 = new Scanner(System.in);
                System.out.println("Now please enter the correct password:");
                passwordInput = myObj1.nextLine();

                if (!passwordInput.equals(password)) {
                    System.out.println("Incorrect password. Please try again:");
                    continue;
                } else {
                    System.out.println("You have successfully logged in!");
                    break;
                }
            }

            // myObj2.close();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (myObj1 != null) {
                myObj1.close();
            }
        }

    }
}

输出：

Please enter your username:
test
Incorrect username. Please try again:
Please enter your username:
KenBoneL0ver
Now please enter the correct password:
test
Incorrect password. Please try again:
Now please enter the correct password:
KenBone4Life
You have successfully logged in!

Answer 2

你可以使用while循环：

while (someBooleanCondition) {
    statement(s);
}

当两个条件都为真（密码和用户名）时，布尔条件会中断 while 循环并告诉用户他们已成功登录。

Here is a link that may be useful

Answer 3

检查while内的条件时可以使用赋值运算符： while (!username.equals(usernameInput = scan.nextLine()))。那么您将不需要任何额外的布尔变量和/或break/continue 语句来实现必要的流程。

除了与Scanner的使用相关的cmets：

1. 无需创建单独的实例来输入用户名和密码。
2. 很高兴看到您关心关闭两个Scanner 实例。但是使用try-with-resources构造保证实现AutoCloseable接口的资源会自动关闭。

话虽如此，代码可以改写如下：

import java.util.Scanner;

public class Login {

    public static void main(String[] args) {
        
        String 
            username = "user",
            password = "pswd",
            usernameInput, 
            passwordInput;
        
        try (Scanner scan = new Scanner(System.in)) {
            System.out.println("Please enter your username:");
            while(!username.equals(usernameInput = scan.nextLine())) {
                System.out.println("Incorrect username. Please try again:");
            }
            
            System.out.println("Now please enter the correct password:");
            while(!password.equals(passwordInput = scan.nextLine())) {
                System.out.println("Incorrect password. Please try again:");
            }
            System.out.println("You have successfully logged in!");
        }
    }
}