JDK 1.8 List 多次 分组

public static void main(String[] args){

//无视对象，在此记录，以防遗忘

//两次 groupBy    List<RStudentExam> list = new ArrayList<>();    RStudentExam rse1 = new RStudentExam();    rse1.setId(1L);    rse1.setSchoolId(1L);    rse1.setStudentId(3L);    list.add(rse1);    RStudentExam rse2 = new RStudentExam();    rse2.setId(1L);    rse2.setSchoolId(2L);    rse2.setStudentId(3L);    list.add(rse2);    RStudentExam rse3 = new RStudentExam();    rse3.setId(2L);    rse3.setSchoolId(1L);    rse3.setStudentId(3L);    list.add(rse3);    RStudentExam rse4 = new RStudentExam();    rse4.setId(2L);    rse4.setSchoolId(2L);    rse4.setStudentId(3L);    list.add(rse4);    RStudentExam rse5 = new RStudentExam();    rse5.setId(2L);    rse5.setSchoolId(2L);    rse5.setStudentId(3L);    list.add(rse5);    Map<Long,Map<Long,List<RStudentExam>>> map = list            .stream()            .collect(Collectors.groupingBy(RStudentExam::getId,Collectors.groupingBy(RStudentExam::getSchoolId)));}