code: public static String generateRandomAlphanumericString(Random rnd, int length) {
  		checkNotNull(rnd);
  		checkArgument(length >= 0);
  
  		StringBuilder buffer = new StringBuilder(length);
  		for (int i = 0; i < length; i++) {
  			buffer.append(nextAlphanumericChar(rnd));
  		}
  		return buffer.toString();
  	}
docs: Creates a random alphanumeric string of given length.
  
  @param rnd The random number generator to use.
  @param length The number of alphanumeric characters to append.