Stream API. Collect all the values ​of the String field of the Model class into one collection, print

//A task:
There is a Model class that has a field of type String. There are two container classes. Class A contains a list of objects of a particular type Model and a field of type B. Class B also contains a list of objects Model. Create a collection of objects A, fill in their internal state (list, object B), similarly to B. Collect all values ​​of the String field of the Model class into one collection, print.
wrote like this:
How to write it shorter?

public class Task_2 {
public static void main(String[] args) {
List<Model> modelsForB = new ArrayList<>();
modelsForB.add(new Model("B1 "));
modelsForB.add(new Model("B2 "));
B b = new B(modelsForB);
List<Model> modelsForA = new ArrayList<>();
modelsForA.add(new Model("A1 "));
modelsForA.add(new Model("A2 "));
A a = new A(modelsForA, b);
final List<String> collected = new ArrayList<>();
Stream.of(a)
.flatMap(item -> Stream.of(item.b))
.flatMap(itemB -> itemB.models.stream())
.map(model -> model.string)
.forEach(collected::add);
Stream.of(a)
.flatMap(itemA -> itemA.models.stream())
.map(model -> model.string)
.forEach(collected::add);
collected.forEach(System.out::println);
}
static class Model {
String string;
public Model(String string) {
this.string = string;
}
}
static class A {
private List<Model> models;
B b;
public A(List<Model> models, B b) {
this.models = models;
this.b = b;
}
}
static class B {
private List<Model> models;
public B(List<Model> models) {
this.models = models;
}
}

how to write it shorter?

The static concat method could be used for that task:

    List<String> collected = Stream
.concat(a.models.stream(), a.b.models.stream())
.map(model -> model.string)
.collect(Collectors.toList());

Per your question, you want to "Create a collection of objects A", so I would make sure your answer provides for this, and not just a single A.

I also threw in using Arrays.asList (for the added entries), just for brevity

public static void main(String[] args) {
List<Model> modelsForB = new ArrayList<>();
modelsForB.add(new Model("B1 "));
modelsForB.add(new Model("B2 "));
B b = new B(modelsForB);
B b1 = new B(Arrays.asList( new Model("B1.1" ), new Model("B1.2" )));
List<Model> modelsForA = new ArrayList<>();
modelsForA.add(new Model("A1 "));
modelsForA.add(new Model("A2 "));
A a = new A(modelsForA, b);
A a1 = new A(Arrays.asList( new Model("A1.1" ), new Model( "A1.2" )), b1 );
List<A> listOfA = Arrays.asList( a, a1 );
// If you need the collection, per the requirement, otherwise just use forEach to print instead of collect
final List<String> collected = listOfA.stream()
.flatMap( x -> Stream.concat( x.models.stream(), x.b.models.stream() ))
.map(m -> m.string )
.collect(Collectors.toList());
collected.forEach(System.out::println);
}