Java 8 stream groupingBy object field with object as a key

user3529850 :

I have the following structure of classes (given example is meaningless, I've made it just for the question purposes):

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
    }
}

In the given scenario, groupingBy returns pretty much everything I want, but I have a problem with the key. In getCarManufacturerToCarsMap() method I would like to have a manufacturer object as a key but I am not even sure if that is possible using groupingBy. What can I do, to return a map of Map<Manufacturer, List<CarDto>> ?

Ousmane D. :

If you actually want to avoid having the map key as a String i.e. Map<String, List<CarDto>> and instead have a Map<Manufacturer, List<CarDto>> you can group by as follows:

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

Note that this would require you to override equals and hashcode method in your Manufacturer class defining that two given objects are equal if they have the same name.