List分组

1、需求描述：

        将List根据实体类属性分组

        现在存在queryProjectMemberList 如下

List queryProjectMemberList = projectMemberMapper.queryProjectMemberByProjectId(dataIds);

         ProjectMember中有属性projectId

        将queryProjectMemberList 根据projectId分组

        最后形式为 Map>

Map> projectMemberListMap = new HashMap<>();

2、解决方法：

        （1）java8新特性使用stream()分组

//分组后的map，一个projectId对应一个List
Map> projectMemberListMap = new HashMap<>();
//等待分组的集合
List queryProjectMemberList = projectMemberMapper.queryProjectMemberByProjectId(dataIds);
//分组操作
projectMemberListMap= queryProjectMemberList.stream().collect(Collectors.groupingBy(ProjectMember::getProjectId));

        （2）java8之前的for循环分组

Map> projectMemberListMap = new HashMap<>();
List queryProjectMemberList = projectMemberMapper.queryProjectMemberByProjectId(dataIds);
//分组操作
for (ProjectMember projectMember : queryProjectMemberList) {
   Integer projectId = projectMember.getProjectId();
   if (projectMemberListMap.containsKey(projectId)) {
      projectMemberListMap.get(projectId).add(projectMember);
   } else {
      List projectMemberList = new ArrayList<>();
      projectMemberList.add(projectMember);
      projectMemberListMap.put(projectId, projectMemberList);
   }
}