Java string index out of bound , CharAt problem

my code is throwing an error "String index out of bound : 1" , here is the code :

     package inlamningsuppgift2;
import java.util.Random;
import java.util.Scanner;
import java.lang.Math;
public class inlamningsuppgift2del2 {
public static void main(String[] args) {
System.out.println("Guess a number between 01-99");
randomNumber();
}
public static void randomNumber () {
Scanner scanner = new Scanner(System.in);
Random random = new Random();
int value = Integer.valueOf(scanner.nextLine());
String str1 = Integer.toString(value);
String sa = String.format("%02d", value);
int randomNum = 10 * random.nextInt(9);
int randomNum2 = 1 * random.nextInt(9) +1;
int wholeNum = randomNum + randomNum2;
String str2 = Integer.toString(randomNum + randomNum2);
String s = String.format("%02d", randomNum + randomNum2);
System.out.println("You guessed :" + sa);
System.out.println("The correct number is : " + s);
if (str2.charAt(0) == str1.charAt(0) && str2.charAt(1) == str1.charAt(1)) {
System.out.println("Congratulations, you guessed the right number in the right order and won 1000$!");
}
else if (str1.charAt(0) == str2.charAt(1) && str1.charAt(1) == str2.charAt(0) ) {
System.out.println("Congratulations, you guessed the right number but in the wrong order and won 500$ !");
}
else if (str2.charAt(0) == str1.charAt(0) && str2.charAt(1) != str1.charAt(1)
|| str2.charAt(0) != str1.charAt(0) && str2.charAt(1) == str1.charAt(1)
|| str1.charAt(1) == str2.charAt(0) && str1.charAt(0) != str2.charAt(1)
|| str1.charAt(0) == str2.charAt(1) && str1.charAt(1) != str2.charAt(0)) {
System.out.println("Congratulations, you guessed one of the numbers right!");
}
else {
System.out.println("Sorry you guessed wrong, try again");
}
}
}

The assignment instructions are : "A lottery program generates a double digit number between 01-99. If the user guesses the number right in the right order, they win 1000$. If the user guesses the right number in the wrong order, example 16 instead of 61, they win 500$. If the user guesses one of the numbers right, they win 100$."

The code is working fine when user input and/or randomNum are 10 and above. However ,when it is below 10 , the error occurs because the value is read as 1 char instead of 2, example : 09 is read as 9 , so charAt(1) does not exist.

How can I fix this? Is there a way to make 0 count as charAt(0) so that charAt(1) counts the digits after the 0, or do I need to change something else?

Thanks!

As I mentioned in the comments: You have String sa = String.format("%02d", value); but you use str1 in the main code. Use sa instead which will have 2 chars even if the number is less than 10.

Another option is to not use strings. Here is a math only version.

public static void randomNumber () {
{
System.out.println("Guess a number between 01-99");
Scanner scanner = new Scanner(System.in);
Random random = new Random();
int value = Integer.valueOf(scanner.nextLine());
// TODO: add a check 1 <= value <= 99
int randomNum = random.nextInt(98) + 1;
System.out.printf("You guessed :%02d%n", value);
System.out.printf("The correct number is : %02d%n", randomNum);
// Check for exact match
if (value == randomNum) {
System.out.println("Congratulations, you guessed the right number in the right order and won 1000$!");
return;
}
// Get the digits of the random number (works for single digit numbers, too)
int randOnes = randomNum % 10;
int randTens = randomNum / 10;
// Check for reverse match
if (value == (randOnes * 10 + randTens)) {
System.out.println("Congratulations, you guessed the right number but in the wrong order and won 500$ !");
return;
}
// Get user's digits
int userOnes = value % 10;
int userTens = value / 10;
// Check for single digit match
if (userOnes == randOnes || userOnes == randTens || userTens == randOnes || userTens == randTens) {
System.out.println("Congratulations, you guessed one of the numbers right!");
return;
}
System.out.println("Sorry you guessed wrong, try again");
}