code: public static String toLowerCase(String str) {
  	int len = str.length();
  	char c;
  	for (int i = 0; i < len; i++) {
  	    c = str.charAt(i);
  	    if (!((c >= 'a' && c <= 'z') || (c >= '0' && c <= '9'))) {
  		return str.toLowerCase();
  	    }
  	}
  
  	return str;
      }
docs: cast a string a lower case String, is faster than the String.toLowerCase, if all Character are
  already Low Case
  
  @param str
  @return lower case value