问题

在我的Spring Boot应用中，通常我使用投影来返回连接表的结果。然而，我尝试使用Java Stream来映射和返回嵌套实体到相应的DTO。以下是我通常遇到的一个例子：

**注意：** 我使用Hibernate设置实体关系那部分完全没问题。因此，为简洁起见，我省略了相关的代码。只需专注于将实体映射到嵌套DTO。

国家有许多州，州有许多城镇...

***Country:***

```java
public class Country {

    private Long id;

    private String name;

    @OneToMany(...)
    private Set<State> states = new HashSet<>();
}

State:

public class State {

    private Long id;

    private Long population;

    @ManyToOne(...)
    private Country country;

    @OneToMany(...)
    private Set<Town> towns = new HashSet<>();
}

Town:

public class Town {

    private Long id;

    private String name;

    @ManyToOne()
    private State state;
}

我想要获取国家列表，其中包括国家名称、属于该国家的州人口总和以及属于该国家的城镇列表。

为此，我创建了以下DTO，但不确定是否正确，以及如何一次性地将必要的数据映射到这个DTO（我不想访问数据库3次，而是希望将国家列表映射到相应的字段（由于我使用Hibernate正确构建了关系，列表中包含所有这些数据））。

@Data
public class CountryResponse {

    private Long id;

    private String name;

    private Set<StateResponse> states;

    private Long population;

    private Set<TownResponse> towns;

    public CountryResponse(Country country) {
        this.id = country.getId();
        this.name = country.getName();

        // ???
        this.states = country.getStates().stream().map(StateResponse::new)
            .collect(Collectors.toSet());
        this.towns = this.states.stream().flatMap(stateResponse -> stateResponse.getTowns().stream())
            .collect(Collectors.toSet());
    }
}

我该如何做呢？如果对于这种情况使用MapStruct是否更好？

<details>
<summary>英文:</summary>

In my Spring Boot apps, I generally use projection in order to return joined table results. However, I am trying to use Java Stream to map and return nested entities to the corresponding DTOs. Here is an example that I generally encountered:

**Note:** I setup entity relationship using Hibernate and that part is completely ok. For this reason, I omitted the related code for brevity. Just concentrate on mapping entities to the nested DTOs.

Country has many States, States has many Towns...


***Country:***

public class Country {

private Long id;

private String name;

@OneToMany(...)
private Set<State> states = new HashSet<>();

}

<br/>

***State:***

public class State {

private Long id;

private Long population;

@ManyToOne(...)
private Country country;

@OneToMany(...)
private Set<Town> towns = new HashSet<>();

}

<br/>


***Town:***

public class Town {

private Long id;

private String name;

@ManyToOne()
private State state;

}

I want to get Country list with Country Name, Sum of state population belonging to this country and Town list belonging to the country. 

For this purpose, I created the following DTOs, but not sure if it is ok and how can I map the necessary data to this DTO in one step (I don't want to go to database 3 times, instead, I just want to map the country list to the corresponding fields (the list has all of these data as I built the relations properly using Hibernate).

@Data
public class CountryResponse {

private Long id;

private String name;

private Set<StateResponse> states;

private Long population;

private Set<TownResponse> towns;

public CountryResponse(Country country) {
    this.id = country.getId();
    this.name = country.getName();

    // ???
    this.states = country.getStates.stream().map(StateResponse::new)
        .collect(Collectors.toSet());
    this.towns = this.states.stream().map(TownResponse::new)
        .collect(Collectors.toSet());
}

}

How can I do this? I would also consider using MapStruct if it is better for this scenario? 

</details>


# 答案1
**得分**: 1

获取各州人口总和的代码：

```java
country.getStates().stream().map(State::getPopulation).sum();

获取城镇列表的代码：

country.getStates().stream().map(State::getTowns).flatMap(Set::stream).collect(Collectors.toList());

country.getStates().stream().map(State::getPopulation).sum();

country.getStates().stream().map(State::getTowns).flatMap(Set::stream).collect(Collectors.toList());