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;
    }

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());
        }
        
    }

            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);
            }