Java MessageDigest create random String

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

public class Main {
   private static SecureRandom prng;
   static {//from   w w  w.ja v a 2 s. c o m
      try {
         // Initialize SecureRandom
         prng = SecureRandom.getInstance("SHA1PRNG");
      } catch (NoSuchAlgorithmException e) {
         System.out.println("ERROR while intantiating Secure Random :   " + prng);
      }
   }
   public static void main(String[] args) {

      System.out.println(getToken());
      System.out.println(getToken());
   }
   public static String getToken() {
      try {
         String token;
         // generate a random number
         String randomNum = Integer.toString(prng.nextInt());
         // get its digest
         MessageDigest sha = MessageDigest.getInstance("SHA-1");
         byte[] result = sha.digest(randomNum.getBytes());
         token = hexEncode(result);
         return token;
      } catch (NoSuchAlgorithmException ex) {
         return null;
      }
   }

   private static String hexEncode(byte[] aInput) {
      StringBuilder result = new StringBuilder();
      char[] digits = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
            'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z' };
      for (byte b : aInput) {
         result.append(digits[(b & 0xf0) >> 4]);
         result.append(digits[b & 0x0f]);
      }
      return result.toString();
   }
}