Do I need a custom Spliterator to avoid extra .stream() call?

Question

I have this code which works fine, but I find it ugly.

@EqualsAndHashCode
public abstract class Actions {

    @Getter
    private List<ActionsBloc> blocs;

    public Actions mergeWith(@NotNull Actions other) {

        this.blocs = Stream.of(this.blocs, other.blocs)
                                    .flatMap(Collection::stream)
                                    .collect(groupingBy(ActionsBloc::getClass, reducing(ActionsBloc::mergeWith)))
                                    .values()
                                    .stream()
                                    .filter(Optional::isPresent)
                                    .map(Optional::get)
                                    .collect(toList());

        return this;
    }
}

ActionsBloc is a super type which contains a list of Action.

public interface ActionsBloc {

    <T extends Action> List<T> actions();

    default ActionsBloc mergeWith(ActionsBloc ab) {
        this.actions().addAll(ab.actions());
        return this;
    }
}

What I want to do is merge blocs of Actions together based on the Class type. So I'm grouping by ActionsBloc::getClass and then merge by calling ActionsBloc::mergeWith.

What I find ugly is calling the values().stream() after the first stream was ended on collect.

Is there a way to operate only on one stream and get rid of values().stream(), or do I have to write a custom Spliterator? In other words have only one collect in my code.

Answer 1

You can work with a reducing identity to sort that out possibly. One way could be to update the implementation of mergeWith as :

default ActionsBloc mergeWith(ActionsBloc ab) {
    this.actions().addAll(Optional.ofNullable(ab)
            .map(ActionsBloc::actions)
            .orElse(Collections.emptyList()));
    return this;
}

and then modify the grouping and reduction to:

this.blocs = new ArrayList<>(Stream.of(this.blocs, other.blocs)
        .flatMap(Collection::stream)
        .collect(groupingBy(ActionsBloc::getClass, reducing(null, ActionsBloc::mergeWith)))
        .values());

Edit: As Holger pointed out such use cases of using groupingBy and reducing further could be more appropriately implemented using toMap as :

this.blocs = new ArrayList<>(Stream.concat(this.blocs.stream(), other.blocs.stream())
        .collect(Collectors.toMap(ActionsBloc::getClass, Function.identity(), ActionsBloc::mergeWith))
        .values());