在Java中 - 如何将resultSet映射到复杂对象？ [英] In Java - How to map resultSet into a complex object?

问题描述

如何将结果集从几个表映射到复杂对象？
让我详细说明：

How can I map a resultset from a few tables into a complex object? let me elaborate:

假设我有这两个类：

public class User {
    private int user_id;
    private String fname;
    private String lname;
    //getters setters...
}

public class Country{
    private String country;
    private ArrayList<User> users;
    }

sql 查询：

SELECT * FROM users U，Country C WHERE c.user_id = U.id

SELECT * FROM users U, Country C WHERE c.user_id = U.id

给了我以下输出 -

gives me the following output -

id | fname | lname| country 
1  | Jack  | Levi |  USA
1  | Jack  | Levi |  UK
2  | Mike  | Brown|  Germany

如何将这些结果映射到Country类？
我需要从它得到一个 Json 字符串来得到这样的东西：

How can I map these results into the Country class? I need make from it a Json string to get something like this:

{
id : 1,
fname: "jack",
lname" "levi",
countries : {"USA", "UK"}
}

非常感谢!!

推荐答案

创建一个HashMap，其中user id是键，user是值。然后在处理结果集时，执行map.get（resultset.get（userid） ），如果它为null，则创建一个新用户，但如果不是，则执行逻辑将用户附加到该国家。

Create a HashMap of where user id is the key, and user is the value. Then when you process the result set, do map.get(resultset.get(userid)), if it's null, create a new user, but if it isn't then do the logic to append a user to the country.

HashMap<Integer, User> users;
HashMap<String, Country> countries;
//inside the result set processing
User u = users.get(resultSet.get("id"));
    if(null != u) {
      //populate user here
    } else {
       Country c = countries.get(resultSet.get(country);
       c.users.add(u);
    }

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