使用Java 8流收集到Guava的ListMultiMap中

Question

I am trying to collect into a ListMultiMap using java 8 without using the forEach operation.

If I were to write the code in Java 7, it will be something like this:

ListMultimap<String, String> result = ArrayListMultimap.create();
     for(State state: states) {
      for(City city: state.getCities()) {
        result.put(state.getName(), city.getName());
      }
    }

I found online a website that talks about creating your own collectors to use in scenarios such as this one. I used this implementation for the collector. I then wrote the following code:

     ListMultimap<String, String> result = states
        .stream()
        .flatMap(state -> state.getCities().stream()
            .map(city -> {
              return new Pair(state, city);
            }))
        .map(pair -> {
          return new Pair(pair.first().getName(), pair.second().getName()));
        })
        .collect(MultiMapCollectors.listMultimap(
                    Pair::first,
                    Pair::second
                )
        );

But at the collect level, I can only pass just one parameter, and I can seem to find a way to pass two parameters. Following the example from the website, I understood that to use both, I need to store a "pair" in the multimap such as the following:

ArrayListMultimap<String, Pair> testMap = testObjectList.stream().collect(MultiMapCollectors.listMultimap((Pair p) -> p.first().getName()));

However this is not what I'm looking for, I want to collect into ListMultimap using the state's name and the city's name using java 8's collector (and no forEach).

Can someone help me with that ? Thank you!

Answer 1

ImmutableListMultimap.flatteningToImmutableListMultimap

return states.stream()
   .collect(flatteningToImmutableListMultimap(
      State::getName,
      state -> state.getCities().stream().map(City::getName)));

Answer 2

You could create a custom collector for that (note that Louis Wasserman's answer will do a forEachOrdered internally, you can't really escape a forEach either internally or externally).

 ListMultimap<String, String> list = states.collect(Collector.of(ArrayListMultimap::create, (multimap, s) -> {
        multimap.putAll(s.getName(), s.getCities().stream().map(City::getName).collect(Collectors.toList()));
    }, (multi1, multi2) -> {
        multi1.putAll(multi2);
        return multi1;
    }));