Stream9.java

Map<Type, Set<String>> allUsersByDeviceType = consumerList
        .stream()
        .flatMap(consumer -> consumer.getThings()
                .stream()
                .map(thing -> new AbstractMap.SimpleImmutableEntry<>(thing.getType(), consumer.getName()))
        ).collect(Collectors.groupingBy(AbstractMap.SimpleImmutableEntry::getKey,
                Collectors.mapping(entry -> entry.getValue(), Collectors.toSet()))
        );

System.out.println(allUsersByDeviceType);
//Output: {HOME=[Satti, Bob, Sri, Malli], HEALTH=[Bob, Sri], FIELD=[Satti, Bob]}
// In the above code, interesting thing I like is Collectors.mapping, which is helping to collect map values as set.