It's Magic...................

import javax.swing.JOptionPane;

public class MagicOf9 {
    public static void main(String[] args) {
        String msg = "Think and write somewhere n digit number, \n" +
            "note that all digits must be unique \nfor e.g. for three" +
            " digit number I am assuming 123, \nwhere 1, 2, and 3 are " +
            "different and unique digits. \nNow add all the digits in " +
            "the number \nfor e.g. in our case it is 1 + 2 + 3. \nThe sum " +
            "is 6. \nNow subtract this resultant (i.e. 6 in this example) \n" +
            "from the supposed number. \nEnter your any (n - 1) digits like \n" +
            "in my case the result is 117 \nand I am giving 1 and 7. \nThe missing " +
            "digit will be given by me. \nNow press OK and give me digits.";
           
        JOptionPane.showMessageDialog(null, msg);
        long digits = Long.parseLong(JOptionPane.showInputDialog(null, "Enter your resultant n - 1 digits"));
       
        long sum = 0;
        while (digits > 0) {
            sum += digits % 10;
            digits /= 10;
        }
       
        if (sum <= 9)
            JOptionPane.showMessageDialog(null, "Missing digit is " + (9 - sum));
        else if (sum > 9 && sum <= 18)
            JOptionPane.showMessageDialog(null, "Missing digit is " + (18 - sum));
        else if (sum > 18 && sum <= 27)   
            JOptionPane.showMessageDialog(null, "Missing digit is " + (27 - sum));
        else if (sum > 27 && sum <= 36)   
            JOptionPane.showMessageDialog(null, "Missing digit is " + (36 - sum));   
        else if (sum > 36 && sum <= 45)   
            JOptionPane.showMessageDialog(null, "Missing digit is " + (45 - sum));       
    }
}