public TopicObj getById(int topicId) {
        TopicObj item = null;
        String sql = "select * from Topic where topic_id = ?";
        try {
            connect();
            PreparedStatement pre = con.prepareStatement(sql);
            pre.setInt(1, topicId);
            ResultSet rs = pre.executeQuery();
            if(rs!=null && rs.next()){
                item = new TopicObj();
                item.setTopicId(rs.getInt("topic_id"));
                item.setName(rs.getString("name"));
                item.setFileName(rs.getString("filename"));
                item.setPath(rs.getString("path"));
                item.setObjId(rs.getInt("obj_id"));
            }
            close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return item;
    }