CheckDigit

/** 
 * Calculate the check digit of HKID
 * - Zhang Haoran, 20070912
 * */

import java.util.Scanner;

public class CheckDigit {

  // Define aliases for ASCII character code
  private static final int ASC_0=48, ASC_A=65;

  // Return the checkDigit produced from hkid
  // hkid is a string that looks like A123456
  private static char check( String hkid ){

    // Step 1: Map the leading alphabet to number: A=1, B=2, ... Z=26
    // e.g. A is 65 in ASCII, so 65-65+1 gives 1
    int s=((int)hkid.charAt(0)-ASC_A+1)*8;

    // Step 2: Add the other digits like A*8 + 1*7 + 2*6 + ... + 6*2
    for(int i=1; i<7; i++){ s += ((int)hkid.charAt(i)-ASC_0) * (8-i); }

    // Step 3: s = 11 - (s mod 11)
    s = 11-s%11; // ans can be 1, 2, 3, ..., 10, 11.

    // Step 4: If s is 10, return A. If 11, return 0. Else return s.
    s = s%11; return (s==10)?'A':(char)(s+ASC_0);
  }

  // Main method begins execution of Java application
  public static void main( String [] args )
  {
    System.out.print( "Enter the first 7 digits of your HKID (e.g. A123456): " );
    Scanner input = new Scanner( System.in );
    String hkid = input.nextLine(); // read in a whole line, assuming correct format
    System.out.printf( "Your ID number is %s(%c).\n", hkid, CheckDigit.check(hkid));
  } //end method main

} // end class ID