One Scanner object not detecting anything in a file written to within the program

Question

I wrote a program to keep a user out unless they know the username and password. It's just a program I wrote for fun, and it doesn't really have a purpose. However, while testing the code I noticed an issue when trying to read from a file that was recently written to. For whatever reason the scanner object I created for the username.txt file, userOut, doesn't detect their is anything in the file (i.e. .hasNextLine() returns false), but only if it was written to within the program. If I write some text to the file, then run the program, it works just fine. Please note that the password.txt file works completely fine. Why is this happening?

Here's my code:

import java.io.File;
import java.io.FileOutputStream;
import java.io.PrintWriter;
import java.util.Scanner;

public class Challenge5 {

public static void main(String[] args) {
    try {
        File usernameFile = new File("username.txt");
        File passwordFile = new File("password.txt");

        PrintWriter userOut = new PrintWriter(new FileOutputStream(usernameFile, true));
        PrintWriter passOut =  new PrintWriter(new FileOutputStream(passwordFile, true));

        Scanner in = new Scanner(System.in);

        Scanner userIn = new Scanner(usernameFile, "UTF-8");
        Scanner passIn = new Scanner(passwordFile, "UTF-8");

        System.out.println("Welcome to Luke's secure program.");
        if (!userIn.hasNextLine() || !userIn.hasNextLine()) {
            System.out.println("New users must set up their account: ");
            if (!userIn.hasNextLine()) {
                System.out.print("Enter a username: ");
                String username = in.nextLine().trim();
                userOut.print(username);
                userOut.flush();

            }
            if (!userIn.hasNextLine()) {
                System.out.print("Enter a password: ");
                String password = in.nextLine().trim();
                passOut.print(password);
                passOut.flush();

            }
            System.out.println("Your account has been created. You may now log in:");
        }

        System.out.print("Please enter username: ");
        String username = in.nextLine().trim();

        System.out.print("Please enter password: ");
        String password = in.nextLine().trim();

        String fileUsername = "";           
        while (userIn.hasNextLine()) {
            fileUsername = userIn.nextLine();
        }
        System.out.println(fileUsername);

        String filePassword = "";
        while (passIn.hasNextLine()) {
            filePassword = passIn.nextLine();
        }
        System.out.println(filePassword);

        if (username.equals(fileUsername) && password.equals(filePassword)) {
            System.out.println("Welcome!");


            while (true) {
                System.out.println("What would you like to do? ");
                System.out.println("1. Change username\n2. Change password\n3. Exit");
                System.out.print("> ");

                int choice;
                while (!in.hasNextInt()) {
                    System.out.println("What would you like to do? ");
                    System.out.println("1. Change username\n2. Change password\n3. Exit");
                    System.out.print("> ");
                    in.next();
                }
                choice = in.nextInt();
                in.nextLine();

                if (choice == 1) {
                    userOut = new PrintWriter(new FileOutputStream(usernameFile, false));
                    System.out.print("Enter new username: ");
                    String newUsername = in.nextLine().trim();
                    userOut.print(newUsername);
                    userOut.flush();
                } else if (choice == 2) {
                    passOut = new PrintWriter(new FileOutputStream(passwordFile, false));;
                    System.out.print("Enter new password: ");
                    String newPassword = in.nextLine().trim();
                    passOut.print(newPassword);
                    passOut.flush();
                } else if (choice == 3) {
                    System.out.println("Goodbye!");
                    break;
                } else {
                    System.out.println("Please try again.");
                }
            }
        } else {
            System.out.println("INVALID USERNAME OR PASSWORD, EXITING PROGRAM...");
        }

        userOut.close();
        passOut.close();
        in.close();
        userIn.close();
        passIn.close();

    } catch (Exception ex) {
        ex.printStackTrace();
    }
}
}

I recognize my code isn't perfect, but I think it should at the very least work. Any help would be appreciated.

Answer 1

You need to reinitialize your Scanner objects to open input stream to the updated files content.

System.out.print("Please enter password: ");
String password = in.nextLine().trim();

Just after above two lines, you need the following:

userIn = new Scanner(usernameFile, "UTF-8");
passIn = new Scanner(passwordFile, "UTF-8");

The reason you fail to read nothing is because you open input stream connection to file username and password using userIn and passIn when there is nothing in file. Adding the above two lines open new input stream connection to the updated file hence, you will be able to read the content.

You seem to be having a bit of issue with your conditions below:

//if (!userIn.hasNextLine() || !userIn.hasNextLine()) {
  if (!userIn.hasNextLine() || !passIn.hasNextLine()) {
            System.out.println("New users must set up their account: ");
            if (!userIn.hasNextLine()) {
                System.out.print("Enter a username: ");
                String username = in.nextLine().trim();
                userOut.print(username);
                userOut.flush();

            }
            //why again !userIn.hasNextLine(), you should check passIn
            //if (!userIn.hasNextLine()) {
              if (!passIn.hasNextLine()) {
                System.out.print("Enter a password: ");
                String password = in.nextLine().trim();
                passOut.print(password);
                passOut.flush();

            }
            System.out.println("Your account has been created. You may now log in:");
        }

I have fixed the two conditions and commented out your code bit.