Java 8 convert List to Lookup Map

Question

I have a list of Station, in each Station there is a list of radios. I need to create a lookup Map of radio to Station. I know how to use Java 8 stream forEach to do it:

Answer 1

I don't think that you can do it in more concise way using Collectors, as compared to mixed solution like

    stationList.stream().forEach(station -> {
        for ( Long radio : station.getRadioList() ) {
            radioToStationMap.put(radio, station);
        }
    });

or

    stationList.forEach(station -> {
        station.getRadioList().forEach(radio -> {
            radioToStationMap.put(radio, station);
        });
    });

(you can call .forEach directly on collections, don't need to go through .stream())

Shortest fully 'functional' solution I was able to come up with would be something like

 stationList.stream().flatMap(
     station -> station.getRadioList().stream().map(radio -> new Pair<>(radio, station)))
 .collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));

using any of the Pair classes available in third party libraries. Java 8 is very verbose for simple operations, compared to dialects like Xtend or Groovy.