How to get List of List of Objects with Java Streams

Question

I can't get my head around this problem.

Problem

I want to writhe this with Java streams:

List<Manufacturer> manufacturers = ... // List of Manufacturer-Objects

List<List<Car>> cars = new ArrayList<>();
for (Manufacturer man : manufacturers) {
   cars.add(man.getCars()); // Let's just say getCars() returns a List of cars.
}

I think with Java Streams it should look like the following:

List<List<Car>> cars = manufacturers.stream().
   .flatMap(man -> man.getCars().stream())
   .collect(Collectors.toList());

Errors

But my IDE says the following no instance(s) of type variable(s) R exist so that List<Event> conforms to Stream<? extends R>

And the java compiler:

java: incompatible types: cannot infer type-variable(s) R
    (argument mismatch; bad return type in lambda expression
      java.util.List<elements.Car> cannot be converted to java.util.stream.Stream<? extends R>)

Any solutions for this? Thanks!

`List> cars = manufacturers.stream() .map(man -> man.getCars()) .collect(Collectors.toList());` — Hadi J, Mar 30 '20 at 14:24
Oh yes, I forgot the second "stream" but this returns still the same error. I will edit the question. — Philipp, Mar 30 '20 at 14:27
`List> result = manufacturers.stream() .map(Manufacturer::getCars) .collect(Collectors.toList());` — Ravindra Ranwala, Mar 30 '20 at 14:34
@RavindraRanwala This was the correct answer. I was one click away from marking it as the approved answer :) — Philipp, Mar 30 '20 at 14:36
Ohh I deleted since it is a trivial answer. Let me retract it — Ravindra Ranwala, Mar 30 '20 at 14:37

score 4 · Accepted Answer · answered Mar 30 '20 at 14:26

The issue here is that you are using the flatMap operator which takes all the lists and gives you a List<Car>. Instead you have to use the map operator to get the desired result. Here's how it looks.

List<List<Car>> result = manufacturers.stream()
    .map(Manufacturer::getCars)
    .collect(Collectors.toList());

How to get List of List of Objects with Java Streams

Problem

Errors

1 Answers1