public static void main(String[] args) {
        List<GroupDetailDTO> list = new ArrayList<>();
        GroupDetailDTO dto1 = new GroupDetailDTO();
        dto1.setHeadsetId(1);
        dto1.setTime("2020-01-03");
        dto1.setActConcreteTime("a");
        dto1.setPlayed(1);
        list.add(dto1);

        GroupDetailDTO dto2 = new GroupDetailDTO();
        dto2.setHeadsetId(1);
        dto2.setTime("2020-01-01");
        dto2.setActConcreteTime("b");
        dto2.setPlayed(1);
        list.add(dto2);

        GroupDetailDTO dto3 = new GroupDetailDTO();
        dto3.setHeadsetId(1);
        dto3.setTime("2020-01-02");
        dto3.setActConcreteTime("c");
        dto3.setPlayed(1);
        list.add(dto3);

        GroupDetailDTO dto4 = new GroupDetailDTO();
        dto4.setHeadsetId(2);
        dto4.setTime("2020-01-01");
        dto4.setActConcreteTime("d");
        dto4.setPlayed(4);
        list.add(dto4);

        Map<GroupDetailDTO, Integer> collect = list.stream().collect(Collectors.groupingBy(o -> new GroupDetailDTO(o.getHeadsetId(), o.getTime()), TreeMap::new, Collectors.summingInt(o -> o.getPlayed())));
        collect.forEach((k, v) -> {
            System.out.println(k + "@@@" + v);
        });
    }
 

@Data
public class GroupDetailDTO implements Comparable<GroupDetailDTO> {
    private Integer headsetId;
    private String actConcreteTime;
    private String time;
    private Integer played;

    public GroupDetailDTO() {
    }

    public GroupDetailDTO(Integer headsetId, String time) {
        this.headsetId = headsetId;
        this.time = time;
    }

    @Override
    public int compareTo(GroupDetailDTO o) {
        String a = this.headsetId + "|" + this.getTime();
        String b = o.getHeadsetId() + "|" + o.getTime();
        return a.compareTo(b);
//        return this.getTime().compareTo(o.gtTime());
    }
}
 

結果：

上述程式碼實現對物件GroupDetailDTO按裝置和時間分組求played的和，並且按時間排序了。當初想著既然是按時間排序那麼compareTo方法直接使用
return this.getTime().compareTo(o.getTime());
但是結果不正確

分析：分組求和時如果使用了排序，那麼分組時對相同屬性的合併將直接根據compareTo返回的結果合併物件，所以如果用上述方法將產生相同的時間直接合並，而不是new GroupDetailDTO(o.getHeadsetId(), o.getTime())裡面的兩個屬性都相同才合併