Loop from a specific point to another point in Java?

Question

I'm working on a project in Java that is kind of like a digital time-machine, it basically saves what happened on a specific sate, archives it to a specific file, and stores it in a directory somewhere for safe keeping. Right now I'm working on a password function. I have the basics down for checking if it's right or wrong, but when I input the wrong answer, the class ends after saying "Invalid Password". Is there a way to have it loop from the end of else statement to the beginning of main so I don't have to re-execute the class everytime?

import java.util.Scanner;
import java.util.*;

public class Adam {

    public static void main(String args[]) {

        String passwrd = "password";
        String inputPassword;

        System.out.print("Please Input Password ");
        Scanner sc = new Scanner(System.in);
        inputPassword = sc.nextLine();

        if (inputPassword.equals(passwrd)) {
            System.out.println("Password Accepted, Loading Archive...");
        } else {
            System.out.println("Invalid Password");
        }
    }
}

Answer 1

You can use a do while loop, if user input is invalid then continue the loop else not.

import java.util.Scanner;
import java.util.*;

public class Adam {

    public static void main(String args[]) {
        String passwrd = "password";

        String inputPassword;
        boolean b = true;
        Scanner sc = new Scanner(System.in);
        do {

            System.out.print("Please Input Password ");

            inputPassword = sc.nextLine();

            if (inputPassword.equals(passwrd)) {

                System.out.println("Password Accepted, Loading Archive...");
                b = false;
            } else {

                System.out.println("Invalid Password");
                b = true;

            }

        } while (b);

    }
}

DEMO

Answer 2

Whenever you want to run some code, check the result and then repeat if necessary the most obvious structure is do while .

Something like the following:

String inputPassword = null;
do {
    if (inputPassword != null)
        System.out.println("Invalid password");
     System.out.println("Enter password");
     inputPassword = sc.nextLine();
} while (!inputPassword.equals(password));

Answer 3

Just put whole code into while(true) loop which you will break if password is walid

public static void main(String args[]) 
{
    String passwrd = "password";
    String inputPassword;

    while(true)
    {    
        System.out.print("Please Input Password ");
        Scanner sc = new Scanner(System.in);
        inputPassword = sc.nextLine();

        if (inputPassword.equals(passwrd)) 
        {
            System.out.println("Password Accepted, Loading Archive...");
            break; //here you are getting out of loop
        } 
        else 
        {
            System.out.println("Invalid Password");
            //you are not breaking loop so the program will return to the beggining
        }
    }
}

Answer 4

    public static void main(String args[]) {
       String password = "password";
       String inputPassword;

        while(inputPassword == null || !inputPassword.equals(password)){
           System.out.print("Please Input Password ");
           Scanner sc = new Scanner(System.in);
           inputPassword = sc.nextLine();

           if (!inputPassword.equals(password)) {
               System.out.println("Invalid Password");
           } 
       }
       System.out.println("Password Accepted, Loading Archive...");
    }

Answer 5

It's a good idea to have a limited number of tries to input the password. For example to have three tries to enter password, you'd have this code:

Scanner sc = new Scanner(System.in);
for (i = 0; i < 4; ++i) {
    System.out.print("Please Input Password ");
    inputPassword = sc.nextLine();

    if (inputPassword.equals(passwrd)) {
        System.out.println("Password Accepted, Loading Archive...");
        break;
    } 
    else {
        System.out.println("Invalid Password");
    }
}

Answer 6

I have added one while loop and one flag in your program. Please have a look:

public static void main(String[] args) {
        String passwrd = "password";

        boolean wrongPassword = true;

        while(wrongPassword){

            String inputPassword;

            System.out.print("Please Input Password ");
            Scanner sc = new Scanner(System.in);
            inputPassword = sc.nextLine();

            if (inputPassword.equals(passwrd)) {

                System.out.println("Password Accepted, Loading Archive...");
                wrongPassword = false;

            } 
            else {
                System.out.println("Invalid Password");
            }
        }

    }

Answer 7

use a do-while loop :

String passwrd = "password";
String inputPassword ="";
Scanner sc = new Scanner(System.in);

 do{
    System.out.print("Please Input Password ");
   // get the password from user
    inputPassword = sc.nextLine();

    // if password matches print success and break out of loop
    if (inputPassword.equals(passwrd)) {
       System.out.println("Password Accepted, Loading Archive...");
       break;
    } 
    else{
       System.out.println("Invalid Password");
     }

   }while(true);

Answer 8

Use a do-while loop. Also, your code has some security issues:

1. Don't use System.in for passwords. Use Console instead. (However, if you run the program from an IDE, you may not have a Console. In that case, use a GUI method instead.)

2. Hash your password straight after you input it.

3. Clear your password buffer as soon as possible after input (ie overwrite it). See Console documentation.

    Console con = System.console(); if (con == null) throw new IOException("This JVM has no console"); boolean pwdCorrect; do { char[] inputPassword = con.readPassword("Please input password: "); String hashedInputPassword = hashPassword(inputPassword); // search Internet for how to hash a password for (int i = 0; i < inputPassword.length; i++) inputPassword[i] = '\\0'; pwdCorrect = isValidPassword(hashedInputPassword); // eg by looking it up in a password file if (pwdCorrect) System.out.println("Password accepted. Loading archive..."); else System.out.println("Invalid password."); } while (!pwdCorrect);