Find max size of an element among list of objects

Question

@Data
class Person {

    private String fname;
    private String lname;

    private List<String> friends;
    private List<BigDecimal> ratings;

    ...
}

@Data
class People {

    private List<Person> people;

    ...
}

suppose i have the above classes, and need to figure out what is the most number of friends any one person has. I know the pre-streams way of doing it... by looping over every element in the list and checking if friends.size() is greater than the currently stored number of longest list. is there a way to streamline the code with streams? something like this answer?

Stream people, map to `person.getFriends()` which will convert to `IntStream` then you can find `max`. — Sam Orozco, Jul 24 '18 at 17:52
`people.stream().mapToInt(p -> p.getFriends().size()).max()` — Holger, Jul 24 '18 at 17:57
Do you want max no of friends or the person with max no of friends? — Amit Bera, Jul 24 '18 at 17:58

davidxxx · Accepted Answer · 2018-07-24T18:09:20.047

7

Compute the max Person contained in People according to a comparator that relies on the size of Person.getFriends() :

Optional<Person> p = people.getPeople()
                           .stream()
                           .max(Comparator.comparingInt(p -> p.getFriends()
                                                           .size()));

Note that you could also retrieve only the max of friends without the Person associated to :

OptionalInt maxFriends = people.getPeople()
                               .stream()
                               .mapToInt(p -> p.getFriends()
                                               .size())
                               .max();

edited Jul 24 '18 at 18:09

answered Jul 24 '18 at 18:00

davidxxx

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1) favour `Comparator.comparingInt` instead of `Comparator.comparing` when comparing ints. 2) the last one can be significantly improved to `OptionalInt max = people.getPeople().stream().mapToInt(p -> p.getFriends().size()).max();` – Ousmane D. Jul 24 '18 at 18:06

NiVeR · Answer 2 · 2018-07-24T18:45:26.747

1

You can declare the following method in Person class:

public int getNumberOfFriends(){
   return friends.size();
}

and then use it in a stream like this:

Optional <Person> personWithMostFriends = people.stream().max(Comparator.comparingInt(Person::getNumberOfFriends));

With this approach you will get the Person object with the most friends, not only the maximum number of friends as someone suggested.

edited Jul 24 '18 at 18:45

answered Jul 24 '18 at 18:01

NiVeR

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favour `Comparator.comparingInt` instead of `Comparator.comparing` when comparing ints – Ousmane D. Jul 24 '18 at 18:10
1

Also, avoid calling `get()` straight from an optional result unless you're sure the source is _definitely_ not empty. leverage the `orElse`, `ifPresent` et al. – Ousmane D. Jul 24 '18 at 18:11
@Aominè Yeah, I should've mentioned that. Thank you. – NiVeR Jul 24 '18 at 18:12
I understand up-voting / accepting another correct answer, but down-voting another correct answer ..why? – NiVeR Jul 24 '18 at 19:40

GolamMazid Sajib · Answer 3 · 2018-07-24T18:55:50.500

1

Your question already answered. but i add this for one thing. if your list of person size large and if you have multi-core pc and you want to used this efficiently then use parallelstream().

To get person:

Person person = people.getPeople()
.parallelStream()
.max(Comparator.comparingInt(p-> p.getFriends().size()))
.orElse(null);

To get size:

int size = people.getPeople()
.parallelStream()
.mapToInt(p -> p.getFriends().size())
.max().orElse(0);

edited Jul 24 '18 at 18:55

answered Jul 24 '18 at 18:17

GolamMazid Sajib

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`.orElse(null)` defies the purpose of `Optional`. At least use `orElse(0)` since you're working with `int`s. – Luiggi Mendoza Jul 24 '18 at 18:27

Find max size of an element among list of objects

3 Answers3