How can I turn a List of Lists into a List in Java 8?

Answer 1

Just as @Saravana mentioned:

flatmap is better but there are other ways to achieve the same

 listStream.reduce(new ArrayList<>(), (l1, l2) -> {
        l1.addAll(l2);
        return l1;
 });

To sum up, there are several ways to achieve the same as follows:

private  List mergeOne(Stream> listStream) {
    return listStream.flatMap(List::stream).collect(toList());
}

private  List mergeTwo(Stream> listStream) {
    List result = new ArrayList<>();
    listStream.forEach(result::addAll);
    return result;
}

private  List mergeThree(Stream> listStream) {
    return listStream.reduce(new ArrayList<>(), (l1, l2) -> {
        l1.addAll(l2);
        return l1;
    });
}

private  List mergeFour(Stream> listStream) {
    return listStream.reduce((l1, l2) -> {
        List l = new ArrayList<>(l1);
        l.addAll(l2);
        return l;
    }).orElse(new ArrayList<>());
}

private  List mergeFive(Stream> listStream) {
    return listStream.collect(ArrayList::new, List::addAll, List::addAll);
}