private static void carry(int[] ca, int index) {
   if (ca[index] == (CHARS.length() - 1)) {
     ca[index] = 0;
     carry(ca, --index);
   } else {
     ca[index] = ca[index] + 1;
   }
 }
  /**
   * Get a unique string
   *
   * @return the unique string
   */
  private static String getUniqueString() {
    char[] buf = new char[NUM_CHARS];

    carry(lastString, buf.length - 1);
    for (int i = 0; i < buf.length; i++) {
      buf[i] = CHARS.charAt(lastString[i]);
    }
    return new String(buf);
  }