如何使用Java 8 Collectors groupingBy获取带有集合Map的Map？

Question

想象一下这些课程

class Subject {
   private int id;
   private Type type;
   private String origin;
   private String name;

   Subject(int id, Type type, String origin, String name) {
      this.id = id;
      this.type = type;
      this.origin = origin;
      this.name = name;
   }

   // Getters and Setters
}

enum Type {
   TYPE1,
   TYPE2
}

我有一个这些主题类的列表

List<Subject> subjects = Arrays.asList(
    new Subject(1, Type.TYPE1, "South", "Oscar"),
    new Subject(2, Type.TYPE2, "South", "Robert"),
    new Subject(3, Type.TYPE2, "North", "Dan"),
    new Subject(4, Type.TYPE2, "South", "Gary"));

我希望得到的结果是首先使用Collectors.groupingBy()Map分组Subject对象Subject.origin然后进行分组Subject.type

得到一个像这样的对象 Map<String, Map<Type, List<Subject>>>

Answer 1

groupingBy接受下游收集器,也可以是groupingBy:

subjects.stream()
        .collect(groupingBy(
                Subject::getOrigin, 
                groupingBy(Subject::getType)    
        ));