java.lang.NumberFormatException How do I fix this one in my code?

Question

I have spent a few hours writing a code for my final programmation project, and after researching and looking my code a lot of times I cant figure out where the problem is. The application simulates a bank account where I have a name and a bank account number, the problem comes when I try to check that the bank account number is correct, it uses a weird algorithm that bank uses here in my country to check if a bank account number is correct and I have made a method to check it myself, here it is:

private static boolean esCorrecto(String numero) {

    if(numero.length() != 20){
    return false;
}else{
        String digCtrl = numero.substring(8, 9);
        int digCtrlV = (Integer.parseInt(numero.substring(0,0)) * 4) + (Integer.parseInt(numero.substring(1,1)) * 8) +
                (Integer.parseInt(numero.substring(2,2)) * 5) + (Integer.parseInt(numero.substring(3,3)) * 10) +
                (Integer.parseInt(numero.substring(4,4)) * 9) + (Integer.parseInt(numero.substring(5,5)) * 7) +
                (Integer.parseInt(numero.substring(6,6)) * 3) + (Integer.parseInt(numero.substring(7,7)) * 6);
        digCtrlV = digCtrlV%11;
        digCtrlV = 11 - digCtrlV;
        if(digCtrlV == 10){
            digCtrlV = 1;
        }
        int digCtrlV1 = (Integer.parseInt(numero.substring(10,10)) * 4) + (Integer.parseInt(numero.substring(11,11)) * 4) +
                (Integer.parseInt(numero.substring(12,12)) * 4) + (Integer.parseInt(numero.substring(13,13)) * 8) +
                (Integer.parseInt(numero.substring(14,14)) * 5) + (Integer.parseInt(numero.substring(15,15)) * 10) +
                (Integer.parseInt(numero.substring(16,16)) * 9) + (Integer.parseInt(numero.substring(17,17)) * 7) +
                (Integer.parseInt(numero.substring(18,18)) * 3) + (Integer.parseInt(numero.substring(19,19)) * 6);
        digCtrlV1 = digCtrlV1%11;
        digCtrlV1 = 11 - digCtrlV1;
        if(digCtrlV1 == 10){
            digCtrlV1 = 1;
        }
        String digCtrlVerdaderos = Integer.toString(digCtrlV) + Integer.toString(digCtrlV1);
        if(digCtrl.equals(digCtrlVerdaderos)){
            return true;
        }else
            return false;
    }

First it checks if the length of the number is correct (20 digit) and then it checks if the control numbers (digit 9 and 10) are correct by doing some operations. Main method has several lines but Im getting error in the following part:

 public static void main(String[] args) throws Ex {
       int control = -1;
       Scanner teclado = new Scanner(System.in);
       System.out.println("Introduce nombre del titular y un numero de cuenta (pulsar enter despues de cada introduccion) ");
       CuentaBancaria cta = new CuentaBancaria(teclado.next(), teclado.next(), 0);
       if(esCorrecto(cta.getNumeroCuenta())){

Finally the error Im getting is this one:

Exception in thread "main" java.lang.NumberFormatException: For input string: "" at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65) at java.lang.Integer.parseInt(Integer.java:592) at java.lang.Integer.parseInt(Integer.java:615) at proyectofinal.ProyectoFinal.esCorrecto(ProyectoFinal.java:82) at proyectofinal.ProyectoFinal.main(ProyectoFinal.java:21) Java Result: 1

Answer 1

numero.substring(n, n) is a zero-length string. To extract a one-character string you want numero.substring(n, n+1). See String.substring doc.

Instead of making substrings, you could also use Character.digit; although that returns -1 for invalid input rather than throwing an exception, so to catch errors (and for neatness) it would be better to wrap it into a method:

private static int digit(String s, int pos) {
    int digit = Character.digit(s.charAt(pos), 10);
    if (digit == -1) throw new NumberFormatException(s);
    return digit;
}

Answer 2

You keep using substring with equal start and end, and substring extracts and empty string "". Use substring(i, i+1) in order to get a digit on the position i.

substring documentation