如何使用 Stream.flatmap 将被展平的对象与展平的对象结合起来

Question

I have a stream of the following class:

class A {
    Stream<B> bs;
}

and I want to convert it to a stream of the following class:

class AB {
    A a;
    B b;
    AB(A a, B b) {
        this.a = a;
        this.b = b;
    }
}

I can see that flatMap will give me a stream of B:

astream.flatMap(a -> a.bs);

but I can't figure out how to pass the a object through to a map like:

astream.flatMap(a -> a.bs).map(b -> new AB(a, b));

If I try this I get an error:

error: cannot find symbol

which I sort of expected.

Answer 1

You have to map the B s of the bs Stream to AB s using map :

List<AB> abs = astream.flatMap(a -> a.bs.map(b->new AB(a,b)))
                      .collect(Collectors.toList());