java how to turn list to map of sublists if elements

user_13598637 0
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I'm trying to learn more about the streams in java and I was wondering how can I use the toMap to change a List of items into a map that has a property of those items as key and a sublist of the same List as value. For example if I have an class:

public class Car {
     private String color;
     private String model;

     // getters and setters
}

if I have: List<Car> carList;, how can I get a Map<String, List<Car>> where the entries will be something like:

"BLUE" : {car1, car3, car7},
"RED" : {car2, car5, car6, car8},
"WHITE" : {car4, car9}

I understand that in order to get it I can run something like the following statement however I don't understand what the second parameter should be:

Map<String, List<Car>> carsMap = list.stream()
        .collect(Collectors.toMap(Car::getColor, ?));

would someone please help me get this correct?

java-stream java
2 Answers

Sounds more of groupingBy job than a toMap:

list.stream().collect(Collectors.groupingBy(Car::getColor));
user_58620710

toMap() normally used to collect into a Map that contains a single value by key.

You can use groupingBy for group by Color, groupingBy is better choice to collect into a Map that contains multiple values by key.

Map<String, List<Car>> carsMap = list.stream()
        .collect(Collectors.groupingBy(Car::getColor));
user_42073060
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