技術標籤：兩個list求和

    public static void main(String[] args) {        List 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 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);        });    }
 

@Datapublic class GroupDetailDTO implements Comparable {    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。