在学习JavaEE的过程中慢慢了解到Java中编程思想的优秀设计，以下将是一个多态-向下转型的示例，该实例是在Servlet-过滤器中的登陆验证的示例，代码中运用向下转型解决问题

import java.io.IOException;import javax.servlet.Filter;import javax.servlet.FilterConfig;import javax.servlet.FilterChain;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletReponse;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpSession;public class LoginFilter implements Filter{	public void init(FilterCOnfig config) throws ServletException{		}	public void doFilter(ServletRequest request, ServletResponse response, Filter chain) throws IOException, ServletException{		HttpServletRequest req = (HttpServletRequest) request;               //向下转型		HttpSession ses = req.getSession();                                  //取得session		if(ses.getAttribute("useid") != null){                               //判断是否登录			chain.doFilter(request, response);                           //传递请求		}else{
    			request.getRequestDispatcher("login.jsp").forward(request, response);  //转到登陆页			}		}		public void destory(){                                                     //销毁过滤			}	}

　　session本身是术语HTTP协议的范畴，但是doFilter()方法中定义的事ServletRequest类型的对象，那么要想取得session，则必须进行向下转型，将ServletRequest变为HttpServletRequest接口对象，才能通过getSession()方法取得session对象。