Java Object Stream group by multiple field and map in map to list

java stream 을 사용하여 객체 리스트를 여러개 필드로 그룹핑 후 맵형태로 생성 후 리스트로 출력 해보도록 한다.

• 원본 리스트를 그룹핑 하여 출력되는 최종 결과는 아래 그림과 같다.

  

  List<Course> courseList = Arrays.asList(
              Course.builder()
                      .code("A0001")
                      .subject("Math")
                      .semester("First")
                      .price(10000l)
                      .build(),
              Course.builder()
                      .code("A0001")
                      .subject("Math")
                      .semester("Second")
                      .price(10000l)
                      .build(),
              Course.builder()
                      .code("A0005")
                      .subject("OperatingSystem")
                      .semester("Second")
                      .price(10000l)
                      .build(),
              Course.builder()
                      .code("A0005")
                      .subject("OperatingSystem")
                      .semester("Second")
                      .price(10000l)
                      .build(),
              Course.builder()
                      .code("A0003")
                      .subject("DataScience")
                      .semester("Second")
                      .price(10000l)
                      .build()
      );
  
      List<Course> resultList = courseList.stream()
              //groupby code and subject
              //output-> Map<String,Map<String,List<Course>>>
              .collect(groupingBy(Course::getCode, groupingBy(Course::getSubject)))
              .entrySet()
              //output -> Set<Map<K,V>.Entry<String,Map<String,List<Course>>>>
              .stream()
              //output -> Stream<Map<K,V>.Entry<String,Map<String,List<Course>>>>
              .map(h -> h.getValue()
                      //output -> Map<String,List<Course>> 
                      .entrySet()
                      //output -> Set<Map<K,V>.Entry<String,List<Course>>>
                      .stream()
                      //output -> Stream<Map<K,V>.Entry<String,List<Course>>>
                      .map(m -> m.getValue()
                              .stream()
                              .reduce((c1, c2) -> Course.builder()
                                      .code(c1.getCode())
                                      .semester(c1.getSemester())
                                      .subject(c1.getSubject())
                                      .price(c1.getPrice() + c2.getPrice())
                                      .build()))
                      //output -> Stream<Optional<Course>>
                      .map(z -> z.get())
                      //output -> Stream<Course>
                      .collect(Collectors.toList()))
              //output -> Stream<List<Course>>
              .collect(Collectors.toList())
              //output -> List<List<Course>>
              .stream()
              //output -> Stream<List<Course>>
              .flatMap(x -> x.stream())
              //output -> Stream<Course>
              .collect(Collectors.toList());