简介

今天在开发中遇到一个分组的问题，因为当时数据库的用的JPA开发的，当时我不想去写原生的SQL来进行分组处理。所以就查询了jdk8中的分组。先看一个分组的例子：

@Data
public class Student {
    private String name;
    private int age;
    private int code;

    public Student(String name, int age, int code) {
        this.name = name;
        this.age = age;
        this.code = code;
    }

    public static void main(String[] args) {
        Student student = new Student("小名",20,115);
        Student student1 = new Student("小名",24,115);
        Student student2 = new Student("小名",20,115);

        List<Student> studentList = Arrays.asList(student, student1, student2);

        //按照年龄分组
        Map<Integer, List<Student>> ageMap = studentList.stream().collect(Collectors.groupingBy(Student::getAge));
        System.out.println("ageMap = " + ageMap);

        //按照年龄分组，并求出个数
        Map<Integer, Long> ageMap2 = studentList.stream().collect(Collectors.groupingBy(Student::getAge, Collectors.counting()));
        System.out.println("ageMap2 = " + ageMap2);

    }
}

开发中的代码：

    public List<AreaDeviceConfigDTO> areaList() {
        List<DeviceConfig> deviceConfigs = deviceConfigRepository.findAll();
        Map<ParkingLot, List<DeviceConfig>> listMap = deviceConfigs.stream().collect(Collectors.groupingBy(DeviceConfig::getParkingLot));
        List<AreaDeviceConfigDTO> deviceConfigDTOS = Lists.newArrayList();
        for (Map.Entry<ParkingLot, List<DeviceConfig>> entry : listMap.entrySet()) {
            AreaDeviceConfigDTO areaDeviceConfigDTO = new AreaDeviceConfigDTO();
            ParkingLot parkingLot = entry.getKey();
            areaDeviceConfigDTO.setId(parkingLot.getId());
            areaDeviceConfigDTO.setParkingLotName(parkingLot.getParkingLotName());
            areaDeviceConfigDTO.setMain(parkingLot.isMain());
            areaDeviceConfigDTO.setDeviceConfigs(entry.getValue());
            deviceConfigDTOS.add(areaDeviceConfigDTO);
        }

        return deviceConfigDTOS;
    }

ps：更多的信息请参考JDK8中的stream的用法