java - Group list by elements of nested list

Question

This question already has answers here: question from:https://stackoverflow.com/questions/65884493/group-list-by-elements-of-nested-list

Answer 1

Collectors#mapping requires as a second parameter a downstream Collector, not a mapping function.

public static <T,U,A,R> Collector<T,?,R> mapping(
    Function<? super T,? extends U> mapper, 
    Collector<? super U,A,R> downstream)

You want to use Collectors.toList() instead:

return cars.stream()
    .flatMap(car -> car.getComponents()
                       .stream()
                       .map(component -> new AbstractMap.SimpleEntry<>(car, component)))
    .collect(Collectors.groupingBy(
            AbstractMap.SimpleEntry::getValue,
            Collectors.mapping(AbstractMap.SimpleEntry::getKey, Collectors.toList())));

As long as you use java-10 or later, you can use simplify the whole Stream into one collector using Collectors#flatMapping as of java-9:

return cars.stream()
    .collect(Collectors.flatMapping(
             car -> car.getComponents()
                       .stream()
                       .map(component -> new AbstractMap.SimpleEntry<>(car, component)),
             Collectors.groupingBy(AbstractMap.SimpleEntry::getValue,
                     Collectors.mapping(AbstractMap.SimpleEntry::getKey, 
                             Collectors.toList()))));