Creating a Map using Java8 streams on a nested Data Structure

Question

I am searching for an elegant equivalent of this piece of code using Java 8's streams:

Collection<X> xs = ...;
Map<B, A> map = new SomeMap<>();

for (X x : xs) {
    A a = x.getA();
    Collection<B> bs = x.getBs();

    for (B b : bs)
        map.put(b, a);
}

That one is a bit too tricky for me as I can’t think of a combination using flatMap and Collectors.toMap which would implement the desired functionality.

Compilable example:

import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;

public class Application {

    public static class A {}

    public static class B {}

    public static class X {
        private A a;
        private Collection<B> bs;

        public X(A a, Collection<B> bs) {
            this.a = a;
            this.bs = bs;
        }

        public A getA() {
            return a;
        }

        public Collection<B> getBs() {
            return bs;
        }
    }

    public static void main(String[] args) {
        X x1 = new X(new A(), Arrays.asList(new B(), new B()));
        X x2 = new X(new A(), Arrays.asList(new B()));

        Collection<X> xs = Arrays.asList(x1, x2);

        Map<B, A> map = new HashMap<>();

        for (X x : xs) {
            A a = x.getA();
            Collection<B> bs = x.getBs();

            for (B b : bs)
                map.put(b, a);
        }
    }

}

you declare a `Map` but you use `map.put(b, a)` - which did you mean? — assylias, Nov 10 '14 at 16:48
You should post a simple but complete and compilable example instead of pseudo code. — assylias, Nov 10 '14 at 17:02
You should also fix your non-working example or remove it because it doesn't make sense. Leaving it there is just confusing. — Radiodef, Nov 10 '14 at 17:27
You can always nest `forEach` like `xs.forEach(x -> x.getBs().forEach(b -> map.put(b, x.getA())));` but this would require creating `map` earlier. — Pshemo, Nov 10 '14 at 17:40
@Radiodef: The first code chunk was meant to display the essential part of the code while the purpose of the second one was to be compilable. What do you exactly mean by fix it? It's pseudocodish, but essentially the same to the second code block. — good_weather, Nov 10 '14 at 19:01
`for( ... : xs) { ... = xs.getA(); ... = xs.getBs(); }` It's not just pseudocode, it actually doesn't make sense. I stared at it for a good minute before I realized that it was meaningless. — Radiodef, Nov 10 '14 at 19:06
@Radiodef: in your extract you missed the inner for loop, which fills the map. It's about the resulting map. — good_weather, Nov 10 '14 at 19:12
You are missing my point that the snippet doesn't serve to illustrate your question and only serves to make it ambiguous. If you just copy and paste the loop from the working snippet it's fine. — Radiodef, Nov 10 '14 at 19:14
@Radiodef: Okay, now I saw that I wrote 'xs' instead of 'x'. That wasn't that easy to spot, sorry. But thank you for pointing it out :D — good_weather, Nov 10 '14 at 19:38

score 5 · Accepted Answer · answered Nov 10 '14 at 17:42

5

As you thought you can do it with a mixture of flatMap and toMap:

Map<B, A> map = xs.stream()
                  .flatMap(x -> x.getBs().stream()
                                 .map(b -> new SimpleEntry<> (b, x.getA())))
                  .collect(toMap(Entry::getKey, Entry::getValue));

Note that this code differs from your original if there are duplicate Bs: your code will keep overwriting the corresponding value whereas this code will throw an exception.

answered Nov 10 '14 at 17:42

assylias

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The only way I get this to work is by casting the inner stream to Stream>. Otherwise I would get a type error at collect. – good_weather Nov 10 '14 at 18:56
1

With your example the code I've given works fine with no cast involved. You haven't forgotten the `<>` in `new SimpleEntry<> (...)` by any chance? – assylias Nov 10 '14 at 18:57
Might be the Java version. I tried it on a different machine where the same code worked while it didn't on the other machine. – good_weather Nov 10 '14 at 19:53

Creating a Map using Java8 streams on a nested Data Structure

1 Answers1