How to get List of List of Objects with Java Streams [duplicate]

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<List<Car>> cars = manufacturers.stream() .map(man -> man.getCars()) .collect(Collectors.toList()); — Hadi
– Hadi, Commented Mar 30, 2020 at 14:24
Oh yes, I forgot the second "stream" but this returns still the same error. I will edit the question. — Philipp
– Philipp, Commented Mar 30, 2020 at 14:27
List<List<Car>> result = manufacturers.stream() .map(Manufacturer::getCars) .collect(Collectors.toList()); — Ravindra Ranwala
– Ravindra Ranwala, Commented Mar 30, 2020 at 14:34
@RavindraRanwala This was the correct answer. I was one click away from marking it as the approved answer :) — Philipp
– Philipp, Commented Mar 30, 2020 at 14:36
Ohh I deleted since it is a trivial answer. Let me retract it — Ravindra Ranwala
– Ravindra Ranwala, Commented Mar 30, 2020 at 14:37

Ravindra Ranwala · Accepted Answer · 2020-03-30 14:26:54Z

4

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());

answered Mar 30, 2020 at 14:26

Ravindra Ranwala

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How to get List of List of Objects with Java Streams [duplicate]

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