code: public static void checkValidRadius(String radiusString, String errorMsg) {
  		checkEmptyString(radiusString, errorMsg);
  		check(isNumeric(radiusString), errorMsg);
  	}
docs: Check that a string is a valid radius value ('radius')
  as shown in <a href="http://instagram.com/developer/realtime/">Instagram Developer real time section</a>
  
  @param radiusString any string that is supposed to be a radius
  @param errorMsg error message