使用收集器将列表转换为对象映射 - Java

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groupBy 'team'属性，并且每个队都有一个列表，如下：

"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;
}

这就是我尝试这样做的方式，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

您当前的收集器 - .collect(Collectors.groupingBy(GenericDAO::getTeam)) - 正在生成一个 Map<String,List<? extends GenericDAO>>。

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

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 构造函数接收 GenericDAO 实例或相关的 GenericDAO 属性。

Answer 2

使用mapping作为：

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;
}

其中 convertGenericDaoToTeam 之类的转换可能是这样的：

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

Use Collectors to convert List to Map of Objects - Java

java

generics

java-8

java-stream

collectors