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

You can do it without Streams, of course, probably making it a bit more readable.

Map LOOK_UP = new HashMap<>();
List stations = ...


stations.forEach(station -> {
    station.getRadios().forEach(radio -> {
         LOOK_UP.put(radio, station);
    });
});

This is not very different than a plain loop with:

for (Station station : stations) {
     for (Radio radio : station.getRadios()) {
          LOOK_UP.put(radio, station);
     }
}

The obvious problem here is that LOOK_UP::put will always replace the value for a certain key, hiding the fact that you ever had duplicates. For example:

[StationA = {RadioA, RadioB}]
[StationB = {RadioB}]

When you search for RadioB - what should you get as a result?

If you could have such a scenario, the obvious thing is to change the LOOK-UP definition and use Map::merge:

    Map> LOOK_UP = new HashMap<>();
    List stations = new ArrayList<>();

    stations.forEach(station -> {
        station.getRadios().forEach(radio -> {
            LOOK_UP.merge(radio,
                          Collections.singletonList(station),
                          (left, right) -> {
                              List merged = new ArrayList<>(left);
                              merged.addAll(right);
                              return merged;
                          });
        });
    });

Another possibility is to throw an Exception when there are these kid of mappings:

stations.forEach(station -> {
       station.getRadios().forEach(radio -> {
            LOOK_UP.merge(radio, station, (left, right) -> {
                 throw new RuntimeException("Duplicate Radio");
            });
       });
 });

The problem with this last snippet, is that you can't really log the radio that is to be blamed for non-uniqueness. left and right are Stationss. If you want that too, you will need to use a merger that does not rely on Map::merge internally, like in this answer.

So you can see, that it all depends on how and what exactly you need to handle.