List 存放 Map 的get的前面元素 被覆盖

List listMainBoard =(List) mapboard.get(new Integer(0));
Board mainBoard=new Board();
                        for(int i=0;i<listMainBoard.size();i++)
                        {
                            //循环取得主版块
                            mainBoard=((Board)listMainBoard.get(i));

                    %>
                    <tr>
                        <td colspan="4"><%=mainBoard.getBoardName() %></td>
                    </tr>
                    <%
                        //子版块
                **List listSonBoard =(List)mapboard.get(new Integer(mainBoard.getBoardId()));**
                    //out.print(mainBoard.getBoardId()); 返回主版块ID，即parentId 为 0
                    if(listSonBoard!=null){
                for(int j=0;j<listSonBoard.size();j++)
                        {
                            //循环取得子版块
                            Board sonBoard=(Board)listSonBoard.get(j);
                            //最后发表的主题
                            Topic topic =new Topic();
                            //最后发表的作者
                            User user=new User();
                            int boardId =sonBoard.getBoardId();
                            //取得该版块的主题列表
                            List listTopic=new TopicDAO().findListTopic(1, boardId);

                            if(listTopic!=null&&listTopic.size()>0)
                            {
                                //取的最后发表的帖子
                                topic=(Topic)listTopic.get(0);
                                user=new UserDAO().findById(topic.getuId());
                                out.print("用户:"+user.getuName());
                            }
                    %>

List listSonBoard =(List)mapboard.get(new Integer(mainBoard.getBoardId())); 这边只保存了map的最后一组元素，前面的被覆盖了，怎么回事啊，求解答，谢谢