从 List 到 List 的 Java 流

通过使用groupingBy()和mapping()采集器将数据按modelId分组，然后按serviceId分组，可以创建嵌套的中间映射.

然后在条目集上创建一个流.并根据modelId和serviceId的每个组合展平每个内部贴图，创建new MyDTO.

Map<String, Map<String, List<String>>> vehicleIdByModelIdAndServiceId =
    listOfObjectArrays.stream()
        .collect(Collectors.groupingBy(objects -> (String) objects[1],
            Collectors.groupingBy(objects -> (String) objects[2],
                Collectors.mapping(objects -> (String) objects[0],
                    Collectors.toList()))));
        
List<MyDTO> result = vehicleIdByModelIdAndServiceId.entrySet().stream()
        .flatMap(baseEntry -> baseEntry.getValue().entrySet().stream()
                    .map(entry -> new MyDTO(baseEntry.getKey(), entry.getKey(), entry.getValue())))
        .collect(Collectors.toList());

另一个选项是在中间映射中使用Map.Entry作为键，值将是vehicleId的列表.

List<MyDTO> result = listOfObjectArrays.stream()
        .collect(Collectors.groupingBy(objects -> new AbstractMap.SimpleEntry<>((String) objects[1], (String) objects[2]),
                Collectors.mapping(objects -> (String) objects[0],
                    Collectors.toList())))
        .entrySet().stream()
        .map(entry -> new MyDTO(entry.getKey().getKey(),
                                entry.getKey().getValue(),
                                entry.getValue()))
        .collect(Collectors.toList());