java - 使用 Java 8 Streams 将模型列表转换为具有内部映射的映射

Question

我有一个 List<Model> :

[
  {parent: "parent", child: "child1", sensor: "wohoo"}, 
  {parent: "parent", child: "child1", sensor: "bla"}, 
  {parent: "parent", child: "child2", sensor: "wohoo2"}
]

我想将其转换为 map <String, Map<String, List<String>>> .

 {
   parent: {
    child1: ["wohoo", "bla"],
    child2: ["wohoo2"]  
   },
 }

我已经试过了:

 Map<String, Map<String, List<String>>> test = currentlyReportingAgents
      .stream()
      .collect(Collectors.groupingBy(
        Model::getParent,
        Collectors.groupingBy(Model::getChild, Collectors.toList())));

但是有一些有线编译错误..我错过了什么？

编辑:添加了错误的屏幕截图:

Answer 1

您的解决方案返回不同的类型:

Map<String, Map<String, List<Model>>> = ...;

要实现你想要的，你需要转一个List<Model>进入List<String>通过 mapping(mapper, downstream) :

Map<String, Map<String, List<String>>> r = currentlyReportingAgents.stream()
    .collect(groupingBy(Model::getParent,
                        groupingBy(Model::getChild, mapping(Model::getChild, toList()))));