ATH pin coder PIN CODIFICAR

import javax.swing.JOptionPane;

public class Ath_primer {

    public static void main(String[] args) {

        String inputString = JOptionPane.showInputDialog("Enter your PIN (4 digits)");

        inputString = inputString.trim();

        int length = inputString.length();

        if( length != 4 ) {
            System.err.println("Error en el PIN. Son solo 4 dígitos.");
            System.exit(0);
        }

        char pin1 = inputString.charAt(0);

/*      char pin2 = inputString.charAt(1);
        char pin3 = inputString.charAt(2);
        char pin4 = inputString.charAt(3);  */

        String outputString = "";

        outputString += pin1 ;  // String outputString = String.valueOf( pin1 );

/*      outputString += pin2 ;
        outputString += pin3 ;
        outputString += pin4 ; */

        System.out.println();
        System.out.println( inputString);
        System.out.println( outputString);

        final char ASCII_0 = '0';

        pin1 = (char) ( (pin1 - ASCII_0) + 'A');

/*      pin2 = (char) ( (pin2 - ASCII_0) + 'A');
        pin3 = (char) ( (pin3 - ASCII_0) + 'A');
        pin4 = (char) ( (pin4 - ASCII_0) + 'A');
*/

        outputString = "";
        outputString = outputString + pin1;

/*      outputString = outputString + pin2;
        outputString = outputString + pin3;
        outputString = outputString + pin4;
*/

        System.out.println();
        System.out.println( inputString);
        System.out.println( outputString);

        System.exit(0);

    } // main
} // class