Stream10.java

Map<String, Integer> consumerWithTotalAmount = consumerList
        .stream()
        .flatMap(consumer -> consumer.getThings()
                .stream()
                .map(thing -> new AbstractMap.SimpleImmutableEntry<>(consumer.getName(), thing.getCost()))
        ).collect(Collectors.groupingBy(AbstractMap.SimpleImmutableEntry::getKey,
                Collectors.mapping(entry -> entry.getValue(), Collectors.summingInt(value -> value))));

System.out.println(consumerWithTotalAmount);
// Output: {Bob=680, Satti=360, Sri=320, Malli=180}
// In the above code, interesting thing I like is Collectors.summingInt, which is summing device cost.