Java-Stream - 如何在使用 Collector groupingBy 時應用排序

Question

我正在使用Collectors.groupingBy將可能的對象分組到列表中。 舉例如下：

public class Test {
    public static void main(String[] args) {
        Student student1 = new Student(98,"man","CHEN",20);
        Student student2 = new Student(100,"man","PETER",20);
        Student student3 = new Student(100,"man","TMAC",21);
        ArrayList<Student> list = Lists.newArrayList(student1, student2, student3);

        Map<Integer, List<StudentDTO>> map = list.stream()
            .collect(Collectors.groupingBy(Student::getAge,
                Collectors.mapping(e -> new StudentDTO(e.getScore(), e.getName()),
                    Collectors.toList())
            ));

        System.out.println(map);
    }

    @Data
    @AllArgsConstructor
    @NoArgsConstructor
    public static class Student {
        private Integer score;
        private String sex;
        private String name;
        private Integer age;
    }

    @Data
    @AllArgsConstructor
    @NoArgsConstructor
    public static class StudentDTO {
        private Integer score;
        private String name;
    }

}

Output：

{20=[Test.StudentDTO(score=98, name=CHEN),
     Test.StudentDTO(score=100, name=PETER)],
 21=[Test.StudentDTO(score=100, name=TMAC)]}

但我的目標是使用Comparator.comparing與流按score對StudentDTO進行排序。 我怎樣才能做到這一點？

所需的 output：

{20=[Test.StudentDTO(score=100, name=PETER),
     Test.StudentDTO(score=98, name=CHEN)],
 21=[Test.StudentDTO(score=100, name=TMAC)]}

Answer 1

要按score降序對StudentDTO的每個列表進行排序，您可以使用以下比較器：

Comparator.comparingInt(StudentDTO::getScore).reversed()

並且要在收集結果時應用排序，您可以使用收集器collectingAndThen()作為groupingBy()的下游

List<Student> students = List.of(student1, student2, student3);

Map<Integer, List<StudentDTO>> map = students.stream()
    .collect(Collectors.groupingBy(
        Student::getAge,
        Collectors.collectingAndThen(
            Collectors.mapping(e -> new StudentDTO(e.getScore(), e.getName()),
                Collectors.toList()),
            list -> { list.sort(Comparator.comparingInt(StudentDTO::getScore).reversed()); return list; }
        )
    ));

注意：針對具體實現編寫代碼會使其變得不靈活，您可能想了解使用抽象的好處。 請參閱“編程到接口”是什么意思？

Answer 2

嘗試這個。 （我添加了import static java.util.stream.Collectors.*; ）

Map<Integer, List<StudentDTO>> map = list.stream()
    .collect(groupingBy(Student::getAge,
        mapping(e -> new StudentDTO(e.getScore(), e.getName()),
            collectingAndThen(toList(),
                x -> x.stream()
                    .sorted(Comparator.comparing(StudentDTO::getScore).reversed())
                    .collect(toList())))));