How to retainAll of List of Lists using stream reduce

Question

I faced following problem. I have a list of lists which i simply want to retainAll. I\'m trying to do with streams

private List> ids =         


        

Answer 1

addAll returns a boolean, not the union of the two lists. You want

List reduce = ids.stream().reduce(ids.get(0), (a, b) -> {
    a.addAll(b);
    return a;
});