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

If you are open to using a third-party library, there is the method groupByEach from Eclipse Collections:

Multimap multimap = 
    Iterate.groupByEach(stationList, Station::getRadioList);

This can also be written using Java 8 Streams with the Collectors2 utility from Eclipse Collections:

Multimap multimap =
        stationList.stream().collect(
                Collectors2.groupByEach(
                        Station::getRadioList,
                        Multimaps.mutable.list::empty));

Note: I am a committer for Eclipse Collections.