groupingBy to Collection Java8

Question

My method looks like this:

 public static <T, E>  Map<E, List<T>> groupBy(Collection<T> collection, Function<T, E> function){
    return collection.stream().collect(Collectors.groupingBy(function::apply));
}

And it works, but i want my method to look like this:

public static <T, E>  Map<E, Collection<T>> groupBy(Collection<T> collection, Function<T, E> function){
    return collection.stream().collect(Collectors.groupingBy(function::apply));
}

And then it doesnt work, because groupingBy returns Map < E, List< T>>, not Map < E, Collection< T>>.

Of course i can do it like this:

   public static <T, E>  Map<E, Collection<T>> groupBy(Collection<T> collection, Function<T, E> function){
    return (Map<E, Collection<T>>) (Map) collection.stream().collect(Collectors.groupingBy(function::apply));
}

but I don't like this solution. Also method declaration:

    public static <T, E>  Map<E, Collection<T>> groupBy(Collection<T> collection, Function<T, E> function)

HAS TO stay that way (that was part of the task). So is there any better solutions than casting? I know there is second version of groupingBy which as a second paramether it takes Collector which can change type of the map which groupingBy returns. However I don't know how to use it. There is Collectors.toCollection() , but I think it will still be returning some implementation of Collection, not Collection itself.

There is also possibility of writing my custom Collector, following this link: http://www.nurkiewicz.com/2014/07/introduction-to-writing-custom.html

but i don't know how to use it with Collection.

I would really apprieciate your help, thanks :)

Answer 1

Something like this should work:

public static <T, E>  Map<E, Collection<T>> groupBy(Collection<T> collection, Function<T, E> function){
    return collection.stream()
                     .collect(Collectors.groupingBy(function,
                                                    Collectors.toCollection(ArrayList::new)));
}

This would produce an ArrayList instance for each value in the Map . You can choose other collections if you wish (just change ArrayList::new ).

Answer 2

You can provide the type of collector and also the type of Map:

public static <T, E> Map<E, Collection<T>> groupBy(Collection<T> collection, Function<T, E> function){
    return collection.stream().collect(Collectors.groupingBy(function::apply, HashMap::new,Collectors.toCollection(ArrayList::new)));
}