Java 8 Streams - Get distinct integers from object list as an array

user_8748538 0
Viewed 3169

I'm struggling to work out how to get the distinct integers from an object list as an array using Streams.

Consider I have the below class:

public class Person{

    private int age;
    private double height;

    public Person(int age, double height) {
        this.age= age;
        this.height = height;
    }

    public int getAge() {
        return age;
    }

    public double getHeight() {
        return height;
    }
}

Then consider that have a populated list of these objects e.g. List<Person> people.

My question is, how do I use Java streams to extract the distinct ages into a Int array?

What I have so far is:

List<Integer> distinctAge = people.stream().distinct().mapToInt().collect(Collectors.groupingBy(Person::getAge));

Obviously that doesn't compile but I'm hitting a wall as to where to go next.

Once I have this array, I'm planning to grab all the objects from the list which contain the distinct value - I assume this is also possible using Streams?

java-stream java-8 arrays
3 Answers

You can collect all ages using map and the call distinct for distinct values

Integer[] ages = persons.stream()
            .map(Person::getAge)
            .distinct()
            .toArray(Integer[]::new);

If you want to collect into int[]

int[] ages = persons.stream()
            .mapToInt(Person::getAge)
            .distinct()
            .toArray();
user_99591520

Better to use Set instead of using distinct as it improve performance.

Set<Integer> ages = persons.stream().map(Person::getAge).collect(Collectors.toSet());
user_105308050

My question is, how do I use Java streams to extract the distinct ages into a Int array?

Once I have this array, I'm planning to grab all the objects from the list which contain the distinct value - I assume this is also possible using Streams?

Paraphrasing this problem,

You need a Map<Integer,List> where key of the map is the age and the value is a List of Person of that particular age.

Map<Integer, List<Person>> agePersonMap =
        people.stream()
            .collect(Collectors.groupingBy(Person::getAge));
user_61403140
Related