1、stream summingInt

List<Update> allItem = service.listById(a, b).stream()
            .filter(t -> t.getId() != null && t.getId() > 0)
            .collect(groupingBy(DO::getGId,
                summingInt(DO::getNumber)))
            .entrySet().stream().map(Detail -> {
                Update DTO = new DTO();
                DTO.setCode("A");
                DTO.setNum(Detail.getValue());
                DTO.setGId(Long.valueOf(Detail.getKey()));
                DTO.setSId(Joiner.on("_").join(Id, Detail.getKey()));
                return DTO;
            }).collect(Collectors.toList());