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

This is my code so far that only works (displays the welcome message) when both a matching username and password have been entered.

I am unsure how to integrate a while loop that stops the user from progressing to the password stage if the usernameInput string does not match the username string, and loops over and over until the correct username has been entered. Then after I would like the same thing for the passwordInput string until it matches the password string. The idea is to have the program be able to loop over and over until the correct username AND password have been entered by the user.

[NOTE: I'm coming into java from a very basic knowledge of python, so everything may be a little disordered and inefficient]

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();
    }

}

Thanks for any help!

Answer 1

Can you please check if are you looking for something like this?

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();
            }
        }

    }
}

Output:

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

You can use a while loop:

while (someBooleanCondition) {
    statement(s);
}

When both conditions are true (password and username) then the boolean condition breaks the while loop and tells the user they have successfully logged in.

Here is a link that may be useful

Answer 3

You can use assignment operator when checking the condition inside while : while (!username.equals(usernameInput = scan.nextLine())) . Then you won't need any extra boolean variables and/or break / continue statements to implement necessary flow.

Aside comments related to the use of Scanner :

1. there's no need to create a separate instance to input username and password.
2. It's good to see that you cared for closing both Scanner instances. However, using try-with-resources construction guarantees that the resource implementing AutoCloseable interface will be closed automatically.

That being said, the code may be rewritten as follows:

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!");
        }
    }
}