Hi Harshana,
They have written no need to remember. But you have to understand concept what <Servlet> , GenericServlet, <ServletRequest>, <ServletResponse>, <HttpServletRequest>, <HttpServletResponse> gives you and first you remember concept it will automatically get idea.
Like this all class provide you basic concept of servlet. Like GenericServlet and HttpServlet is abstract class and they implemented basic method. Which method you can override at your servlet and which one not. Why... so chill just understand concept...