Java 8 | Finding Map Entry with highest size of value

sandeep pandey :

I have model Person[city, name]. I have collected them in Map And Grouped them by city. I need to trace the city that has most no of person staying there and return only that entry as part of Map. I'v tried and also it is working but i was wondering is there any better way of doing.

Comparator<Entry<String, List<Person>>> compareByCityPopulation =
        Comparator.comparing(Entry<String, List<Person>>::getValue, (s1, s2) -> {
            return s1.size() - s2.size();
        });

HashMap mapOfMostPopulatedCity = persons.stream()
        .collect(Collectors.collectingAndThen(Collectors.groupingBy(Person::getCity), m -> {

            Entry<String, List<Person>> found = m.entrySet().stream().max(compareByCityPopulation).get();

            HashMap<String, List<Person>> hMap = new HashMap<>();
            hMap.put(found.getKey(), found.getValue());

            return hMap;
        }));

System.out.println("*City with Most no of people*");
mapOfMostPopulatedCity.forEach((place, peopleDetail) -> System.out.println("Places " + place + "-people detail-" + peopleDetail));

Please Suggest how can we write better in java 8.

Ravindra Ranwala :

After getting the max map entry, you have to convert that into a map which has a single entry. For that you can use Collections.singletonMap()

Map<String, List<Person>> mapOfMostPopulatedCity = persons.stream()
    .collect(Collectors.groupingBy(Person::getCity)).entrySet().stream()
    .max(Comparator.comparingInt(e -> e.getValue().size()))
    .map(e -> Collections.singletonMap(e.getKey(), e.getValue()))
    .orElseThrow(IllegalArgumentException::new);

With Java9 you can use Map.of(e.getKey(), e.getValue()) to build the map with a single entry.