如何構造複雜map

數據庫中有一組學生
如何構造出這麼一個Map集合  Map<String,Map<String,List<Student>>>
第一個String 是性別
第二個String 是年齡就是先從數據庫裏面查詢出來

如何構造出這樣的集合

普通寫法：

Map<String, Map<String, List<Student>>> resultMap = new HashMap<>();
Map<String, List<Student>> sexMap = new HashMap<>();
            for (Student student : students) {
                List<Student> tempList = sexMap.get(student.getSex());
                if (tempList == null) {
                    tempList = new ArrayList<>();
                    tempList.add(student);

                }
                else {
                    tempList.add(student);
                }
                sexMap.put(student.getSex(), tempList);
            }
            System.out.println(sexMap);

            for (Map.Entry<String, List<Student>> entry : sexMap.entrySet()) {
                String key = entry.getKey().toString();//sex
                List<Student> value = entry.getValue();
                System.out.println("key=" + key + " value=" + value);
                Map<String, List<Student>> ageMap = new HashMap<>();
                for (Student student : value) {
                    List<Student> tempList = ageMap.get(student.getAge());
                    if (tempList == null) {
                        tempList = new ArrayList<>();
                        tempList.add(student);

                    }
                    else {
                        tempList.add(student);
                    }
                    ageMap.put(student.getAge(), tempList);
                    resultMap.put(key,ageMap);
                }

            }
            System.out.println(resultMap);

java8寫法：

Map<String, Map<String, List<Student>>> resultMap = new HashMap<>();
Map<String, List<Student>> sexMap = students.stream().collect(Collectors.groupingBy(Student::getSex));
sexMap.entrySet().forEach(entry -> resultMap.put(entry.getKey(),entry.getValue().stream().collect(Collectors.groupingBy(Student::getAge))));