Nested ArrayList to single dimension

Question

I have some code that looks like this.

class A {}
class B extends A {
    private String name; // assume there's a getter
}
class C extends A {
    private List<B> items = new ArrayList<>(); // assume a getter
}

In another class, I have an ArrayList (ArrayList<A>). I'm trying to map this list to get all the names.

List<A> list = new ArrayList<>();
// a while later
list.stream()
    .map(a -> {
        if (a instanceof B) {
            return ((B) a).getName();
        } else {
            C c = (C) a;
            return c.getItems().stream()
                .map(o::getName);
        }
    })
    ...

The problem here is that I end up with something like this (JSON for visual purposes).

["name", "someName", ["other name", "you get the idea"], "another name"]

How can I map this list so I end up with the following as my result?

["name", "someName", "other name", "you get the idea", "another name"]

Use a flatMap.. It would be useful if you show how you collect the result after the map step (the else returns a Stream whereas the if part returns a String). Are you collecting it into a List<Object>? — Thiyagu
– Thiyagu, Commented Jan 23, 2019 at 8:27
take a look at stackoverflow.com/questions/26684562/… it will help you understand the use of flatMap — mkjh
– mkjh, Commented Jan 23, 2019 at 8:38

Eran · Accepted Answer · 2019-01-23 08:26:50Z

4

Use flatMap:

list.stream()
    .flatMap(a -> {
        if (a instanceof B) {
            return Stream.of(((B) a).getName());
        } else {
            C c = (C) a;
            return c.getItems().stream().map(o::getName);
        }
    })
    ...

This will produce a Stream<String> of all the names, without nesting.

answered Jan 23, 2019 at 8:26

Eran

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