code: public static String hump2Underline(String string) {
  		if (string == null) return null;
  		if (string.trim().length() == 0) return "";
  		string = toFirstLower(string);
  		StringBuilder sb = new StringBuilder();
  		char[] arr = string.toCharArray();
  		for (char c : arr) {
  			if (c >= 65 && c <=90) {
  				c += 32;
  				sb.append("_").append(String.valueOf(c).toLowerCase());
  			} else {
  				sb.append(c);
  			}
  		}
  		return sb.toString();
  	}
docs: e.g. transform 'StrUtils' to 'str_utils', 'strUtils' to 'str_utils'
  @param string e.g. StrUtils
  @return e.g. str_utils