code: private static String hex4(char c) {
  		char[] r = new char[] { '0', '0', '0', '0' };
  		int i = 3;
  		while (c > 0) {
  			r[i] = HEX_DIGITS[c & 0xF];
  			c >>>= 4;
  			--i;
  		}
  		return new String(r);
  	}
docs: Converts the given character to a four-digit hexadecimal string
  @param c the character to convert
  @return the string