Get a list of map keys from a deque in Java

Question

I have a deque

Deque<Map<int, String> cars = new LinkedList();

I want to use Java stream to collect all the keys from the deque map into a

List<int>. 

Is there any way to do this?

I tried something like

cars.stream().map(car -> car.keySet()).collect(Collectors.toList()

This question is different from the previous question. I want to collect all the keys, as opposed to removing a map from the deque.

Answer 1

Use flatMap() :

cars.stream()
    .map(Map::keySet)
    .flatMap(Set::stream)
    .collect(Collectors.toList())

Answer 2

You could flatMap the maps' keys:

List<Integer> keys =
    cars.stream().flatMap(car -> car.keySet().stream()).collect(Collectors.toList());