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

Question

想象一下这些类

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
}

我有这些学科类别的列表

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"));

我想得到使用 Collectors.groupingBy() 的结果首先将 Subject 分组的 map 对象 Subject.origin然后按 Subject.type 分组

得到像这样的对象 Map<String, Map<Type, List<Subject>>>

Answer 1

groupingBy 接受下游收集器，它也可以是 groupingBy:

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