Stream14.java

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

System.out.println(highestDeviceCostCustomerPaid);
// Output: {Bob=Optional[300], Satti=Optional[300], Sri=Optional[200], Malli=Optional[120]}