Converting a list of objects in java to a map of String key to List<objects> using one of its field as the key

Question

I have a list of objects List<Car> cars

public class Car{
  String make;
  String model;
  String year;
  String price;
}

I want a succinct way of converting this list to a map Map<String, List<Car>> using make as the key. I could write a function to do that but I was wondering if Lambdas or stream api in Java 8 provides an out of the box way to achieve this.

I have modified the question for a Map<String, List<Car>>

Well, you could use How to convert List to Map or if you really want to look l334, you could use Java 8 List<V> into Map<K, V> or you could try one of the many logs you can find through Google like Java 8 – Convert List to Map — MadProgrammer
– MadProgrammer, Commented Apr 27, 2018 at 4:15
If you're using Guava: Maps.uniqueIndex(cars, Car::getMake) — shmosel
– shmosel, Commented Apr 27, 2018 at 4:23

Ousmane D. · Accepted Answer · 2018-04-27 06:37:06Z

4

Yes its very well supported.

Do like this :

Map<String, List<Car>> youCarMap = yourCarList.stream().collect(
                Collectors.groupingBy(Car::getMake,
                    Collectors.mapping(c->c, Collectors.toList())));

edited Apr 27, 2018 at 6:37

Ousmane D.

56.6k8 gold badges101 silver badges134 bronze badges

answered Apr 27, 2018 at 4:21

Shanu Gupta

3,8272 gold badges21 silver badges29 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

sophist_pt Over a year ago

Slight modification to the question. I actually also want to prepare a Map<String, List<Car>> makeToCars. How can I achieve this? @Shanu Gupta

Ousmane D. · Accepted Answer · 2018-04-27 06:35:26Z

1

Map<String, List<Car>> result = 
          myList.stream()
                .collect(Collectors.groupingBy(Car::getMake));

answered Apr 27, 2018 at 6:35

Ousmane D.

56.6k8 gold badges101 silver badges134 bronze badges

Comments

Stefan Zhelyazkov · Accepted Answer · 2018-04-27 04:23:36Z

0

You can do it like so

cars.stream()
   .collect(Collectors.toMap(
      car -> car.make,
      Function.identity()
   )

answered Apr 27, 2018 at 4:23

Stefan Zhelyazkov

3,0614 gold badges23 silver badges53 bronze badges

Collectives™ on Stack Overflow

Converting a list of objects in java to a map of String key to List<objects> using one of its field as the key

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related