新手求指点~该怎么解决

新手求指点~~

Java code

public List<User> list(){        List<User> users=new ArrayList<User>();        Connection conn=this.getConnection();        ResultSet rs=null;        String sql="select id,username,password,age,address from user";        PreparedStatement ps=this.prepare(conn, sql);        User user=new User();        try{            rs=ps.executeQuery();            while(rs.next()){                user.setId(rs.getInt(1));                user.setUsername(rs.getString(2));                user.setPassword(rs.getString(3));                user.setAddress(rs.getString(4));                users.add(user);            }        }catch(Exception e){            e.printStackTrace();        }        return users;    }

Java code

public static void main(String[] args) {        Main main=new Main();        List<User> allUser=main.list();        Iterator<User> iterator=allUser.iterator();        while(iterator.hasNext()){            User user=iterator.next();            System.out.println(user.getUsername());        }            }

Java code

            while(rs.next()){                user.setId(rs.getInt(1));                user.setUsername(rs.getString(2));                user.setPassword(rs.getString(3));                user.setAddress(rs.getString(4));                users.add(user);            }