Java 8 Stream “collect and group by” objects that map to multiple keys [duplicate]

Answer 1

Such scenarios are well supported by my free StreamEx library which enhances standard Stream API:

Map> map = StreamEx.of(list) // create an enhanced stream of Item
                // create a stream of Entry
                .cross(item -> item.getOwners().stream().map(Person::getManager))
                // swap keys and values to get stream of Entry
                .invert()
                .grouping();

Internally it's similar to @Holger solution.