What is the idiomatic way to map a Collection onto a MultiMap?

Question

What is the idiomatic way to map a Collection<Foo> onto a MultiMap<A, B>? Were

public class Foo {
    public A key;
    public B val;
}

(This is simplified, of course.)

The way I can think to do it is to to use Collection::stream() to invoke MultiMap::put on each element, but I feel there might be a much more elegant way to do this.

And can a new MultiMap be returned with those mappings, all in one line?

Which MultiMap class are you talking about? From which library? — JB Nizet, Feb 27 '18 at 22:20
Maybe this https://google.github.io/guava/releases/19.0/api/docs/com/google/common/collect/Multimap.html — Victor M Perez, Feb 27 '18 at 22:21
the short answer is, yes, you can always define your own collector. https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collector.html — Jason Hu, Feb 27 '18 at 22:21
if you're using Guava or some other library then please tag it otherwise please provide more context. — Ousmane D., Feb 27 '18 at 22:28
Assuming `MultiMap` has similar API as `Map` you can do this: `MultiMap mm = list.stream().collect( MultiMap::new, (m, f) -> m.put( f.getKey(), f.getVal() ), MultiMap::putAll );` — tsolakp, Feb 27 '18 at 22:38
Yes, I'm using Guava; sorry, being new to Java, I thought it was a core Java thing. And yes, collectors were what I needed! @tsolakp - You answer almost worked for me, but I am getting a "MultiMap is abstract; can't be instantiated" at the `new`. I'll try to figure it out. — Andrew Cheong, Feb 28 '18 at 14:15

score 2 · Accepted Answer · answered Feb 28 '18 at 18:07

Since version 21.0 onwards, the most idiomatic way to do this is to use one of the collectors already provided by Guava, in this case the one provided by the Multimaps.toMultimap utility method:

Multimap<A, B> result = foos.stream()
    .collect(Multimaps.toMultimap(Foo::getA, Foo::getB, ArrayListMultimap::create));

This creates an ArrayListMultimap, but of course you can create the concrete Multimap you want, either by using a factory method or one of Guava's specific builders.

What is the idiomatic way to map a Collection onto a MultiMap?

1 Answers1