leftPadZeroes

/**
 * Return a string padded with leading zeroes for a given total length.
 * There are other solutions around in stackoverflow etc., in particular 
 * org.apache.commons.lang.StringUtils.leftPad(), but this one is a concise
 * and cheap solution for the particular case of 0-padding, as a reasonable 
 * feature for fixed-length <strong>binary</strong> is missing in the String 
 * and Integer classes up to now.
 * 
 * @param s
 * 	Input string (in particular binary form of number) to be padded.
 * @param len
 *  Total length wanted for the padded String (input + padding).
 * @return
 *  The padded String.
 */
static public String leftPadZeroes(String s, int len){
	return String.format(
			String.format("%%0%dd", Math.max(0, len-s.length())),	0) 
			+ s;
}