异常处理。自定义异常类。定义一个异常类，当set方法的3个整形参数不能构成一个合法的日期时候，抛出异常，Date2类中的其他方法将捕获并处理该异常。-Exception handling. Custom exception class. Define a custom exception class, when the set of three integer parameter method can not constitute a valid date when an exception is thrown, Date2 class other methods to catch and handle the exception.