Question

如何使用收集器将DAO列表转换为Map<String, List<Pojo>>

daoList看起来像这样：

[0] : id = "34234", team = "gools", name = "bob", type = "old"
[1] : id = "23423", team = "fool" , name = "sam", type = "new"
[2] : id = "34342", team = "gools" , name = "dan", type = "new"

我想通过“团队”属性进行分组，并为每个团队提供一个列表，如下所示：

"gools":
       ["id": 34234, "name": "bob", "type": "old"],
       ["id": 34342, "name": "dan", "type": "new"]

"fool":
       ["id": 23423, "name": "sam", "type": "new"]

Pojo看起来像这样：

@Data
@NoArgsConstructor
@AllArgsConstructor(access = AccessLevel.PUBLIC)
public class Pojo{

    private String id;
    private String name;
    private String type;
}

这就是我要这样做的方式，显然是错误的方式：

public Team groupedByTeams(List<? extends GenericDAO> daoList)
    {

        Map<String, List<Pojo>> teamMap= daoList.stream()
            .collect(Collectors.groupingBy(GenericDAO::getTeam))
    }

Answer 1

您的当前收集器-.collect(Collectors.groupingBy(GenericDAO::getTeam))-正在生成Map<String,List<? extends GenericDAO>>。

为了生成Map<String, List<Pojo>>，必须通过将GenericDAO收集器链接到Pojo收集器来将Collectors.mapping()实例转换为Collectors.groupingBy()实例：

Map<String, List<Pojo>> teamMap = 
    daoList.stream()
           .collect(Collectors.groupingBy(GenericDAO::getTeam,
                                          Collectors.mapping (dao -> new Pojo(...),
                                                              Collectors.toList())));

这是假设您有一些Pojo构造函数，该构造函数接收GenericDAO实例或相关的GenericDAO属性。

Answer 2

将mapping用作：

public Map<String, List<Team>> groupedByTeams(List<? extends GenericDAO> daoList) {
    Map<String, List<Team>> teamMap = daoList.stream()
            .collect(Collectors.groupingBy(GenericDAO::getTeam,
                    Collectors.mapping(this::convertGenericDaoToTeam, Collectors.toList())));
    return teamMap;
}

诸如convertGenericDaoToTeam之类的转换可能类似于：

Team convertGenericDaoToTeam(GenericDAO genericDAO) {
    return new Team(genericDAO.getId(), genericDAO.getName(), genericDAO.getType());
}

使用收集器将列表转换为对象映射-Java

2 个答案: