Java 8映射到对象列表，其中一个字段分组为一个列表 [英] Java 8 Map to a List of Objects with one field grouped into a List

问题描述

新手问题.我有一个原始bean来自数据库，行为

Newbie question. I have an original bean coming from the DB row-by-row as

public class DataBean {
   private Integer employeeId;
   private String org;
   private String comments;
   // + Constructors, getters/setters
}

我需要将其映射到具有多个Org的另一个Bean，这些组织按Employee ID分组到一个列表中. 一个雇员ID的组织只能是多个；注释字段必定是相同的.

I need to map it to a different bean with multiple Org's grouped by Employee ID into a List. Only Orgs can be multiple for an EmployeeID; the Comments field is guaranteed to be the same.

public class CustomDataBean {
   private Integer employeeId;
   private List<String> orgs;
   private String comments;
   // + Constructors, getters/setters
}

努力入门.想到了下面的groupingBy，但是返回了Map，我没有建立子列表.

Struggling to get started. Was thinking of groupingBy such as the below but that returns a Map, and I'm not building a sub-List.

Map<Integer, List<String>> temp = origData.stream().collect(
    Collectors.groupingBy(OrigBean::getEmployeeId,
    /* 2nd param? */ .. ))

我的目标是转变为List<CustomDataBean>.

推荐答案

您可以使用此功能:

List<CustomDataBean> result = origData.stream()
        .collect(Collectors.groupingBy(DataBean::getEmployeeId))
        .entrySet().stream()
        .map(e -> new CustomDataBean(
                e.getKey(),
                e.getValue().stream().map(DataBean::getOrg).collect(Collectors.toList()),
                e.getValue().get(0).getComments()))
        .collect(Collectors.toList());

这会将分组结果映射到您的CustomDataBean对象.

This maps the grouped results to your CustomDataBean object.

输入:

List<DataBean> origData = Arrays.asList(
        new DataBean(1, "a", "c"),
        new DataBean(1, "b", "c"),
        new DataBean(1, "c", "c"),
        new DataBean(2, "a", "d"),
        new DataBean(2, "c", "d")
);

结果将是这样:

CustomDataBean[employeeId=1, orgs=[a, b, c], comments='c']
CustomDataBean[employeeId=2, orgs=[a, c], comments='d']

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