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The Laws of Cryptography with Java Code.

// GCD: Greatest Common Divisor
public class GCD {

   public static long gcd1(long x, long y) {
      if (y == 0) return x;
      return gcd1(y, x % y);
   }

   public static long gcd2(long x, long y) {
      while (y != 0) {
         long r = x % y;
         x = y; y = r;
      }
      return x;
   }

   public static void main(String[] args) {
      long x = Long.parseLong(args[0]);
      long y = Long.parseLong(args[1]);
      long z = GCD.gcd1(x, y);
      System.out.println("Method 1: gcd(" + x + ", " + y + ") = " + z);
           z = GCD.gcd2(x, y);
      System.out.println("Method 2: gcd(" + x + ", " + y + ") = " + z);
   }
}

Here are several runs of this program:

% java GCD 819 462
Method 1: gcd(819, 462) = 21
Method 2: gcd(819, 462) = 21
% java GCD 40902 24140
Method 1: gcd(40902, 24140) = 34
Method 2: gcd(40902, 24140) = 34