Question

我具有以下类结构（给出的例子是没有意义的，我只是出于提问的目的而做了）：

public class ManufacturerDto {
    private String name;
    private Boolean withConfirmation;

    //getters and setters omitted for brevity
}

public class CarDto {

    private ManufacturerDto manufacturer;

    //getters and setters omitted for brevity
}

public abstract class VehicleDto {

    private list<CarDto> cars = new ArrayList<>();

    //getters and setters omitted for brevity
}

public class EuropeanMarketCarDto extends VehicleDto {

    public Map<String, List<CarDto>> getCarManufacturerToCarsMap() {

       return this.getCarDtos()
                .stream()
                .collect(Collectors.groupingBy(e -> e.manufacturer().getName()));

        //getters and setters omitted for brevity
    }
}

在给定的情况下，groupingBy返回了我想要的几乎所有内容，但是密钥有问题。在getCarManufacturerToCarsMap()方法中，我想将manufacturer对象作为键，但是我什至不确定使用groupingBy是否可以实现。我该怎么做才能返回Map<Manufacturer, List<CarDto>>的地图？

Answer 1

如果您实际上想避免将地图密钥设为String即Map<String, List<CarDto>>，而改为拥有Map<Manufacturer, List<CarDto>>，则可以按以下方式进行分组：

this.getCarDtos()
      .stream()
      .collect(Collectors.groupingBy(e -> e.getManufacturer()));

请注意，这将要求您重写equals类中的hashcode和Manufacturer方法，以定义两个给定对象具有相同名称时是相等的。

Answer 2

您可以尝试这样的事情：

Map<Manufacturer, List<Car>> listing = cars.stream().collect(Collectors.groupingBy(Car::getManufacturer));

因此，这将按制造商对所有汽车进行分组，输出将类似于：

TOYOTA = [AURIS的AURIS参数，AVIA的AVIA参数]，MERCEDES = [S500的S500参数 S500 ...]

编辑2

public class Main {

    List<Car> listOfCars = new ArrayList<>();


    public static void main(String[] args) {
    List<Car> listOfCars = new ArrayList<>();
    listOfCars.add(new Car(new Manufacturer("Toyota"), "Toyota A"));
    listOfCars.add(new Car(new Manufacturer("Toyota"), "Toyota B"));
    listOfCars.add(new Car(new Manufacturer("Jeep"), "Jeep A"));
    listOfCars.add(new Car(new Manufacturer("Mercedes"), "Mercedes C"));
    listOfCars.add(new Car(new Manufacturer("Toyota"), "Toyota C"));
    listOfCars.add(new Car(new Manufacturer("Audi"), "Audi A"));
    listOfCars.add(new Car(new Manufacturer("Toyota"), "Toyota C"));



 Map<String,List<Car>> listOfCarsByManufacturers = listOfCars.stream().collect(Collectors.groupingBy(d->d.getManufacturer().getName()));

    System.out.println(listOfCarsByManufacturers);
    }




}

    public class Car {

    private Manufacturer manufacturer;
    private String nameOfTheCar;

    public Car() {
    }

    public Car(Manufacturer manufacturer, String nameOfTheCar) {
        this.manufacturer = manufacturer;
        this.nameOfTheCar = nameOfTheCar;
    }

    public Manufacturer getManufacturer() {
        return manufacturer;
    }

    public String getNameOfTheCar() {
        return nameOfTheCar;
    }

    @Override
    public String toString() {
        return "Manufacturer: " + getManufacturer() + ", name: " + getNameOfTheCar();
    }

}

public class Manufacturer {

    private String name;

    public Manufacturer() {
    }

    public Manufacturer(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return getName();
    }


}

编辑3：

Map<String,List<Car>> listOfCarsByManufacturers = listOfCars.stream().collect(Collectors.groupingBy(d->d.getManufacturer().getName()));
System.out.println(listOfCarsByManufacturers);

Java 8流分组通过以对象为键的对象字段

2 个答案: