Functional Fibonacci, from Java to Kotlin

Fibonacci.java

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

class NextInSeries {
    public final int current;
    public final int next;

    NextInSeries(int current, int next) {
        this.current = current;
        this.next = next;
    }

    public NextInSeries nextValue() {
        return new NextInSeries(next, current + next);
    }
}

class Fibonacci {

    public static int numberAtPosition(int position) {

        NextInSeries initial = new NextInSeries(0, 1);

        return IntStream.range(0, position)
                .mapToObj(ignore -> new NextInSeries(0, 0))
                .reduce(initial, (result, ignore) -> result.nextValue())
                .next;
    }

    public static List<Integer> seriesUntilPosition(int position) {
        return IntStream.rangeClosed(0, position)
                .mapToObj(index -> numberAtPosition(index))
                .collect(Collectors.toList());

    }

}

FibonacciK.kt

class FibonacciK {

    companion object {

        @JvmStatic
        fun numberAtPosition(position: Int): Int {
            return (0..position)
                    .map { Pair(0, 1) }
                    .reduce { (first, second), next -> Pair(second, first + second) }
                    .second
        }

        @JvmStatic
        fun seriesUntilPosition(position: Int): List<Int> {
            return (1..position)
                    .map { numberAtPosition(it) }
        }

    }

}

FibonacciTest.java

import org.junit.jupiter.api.Test;

import java.util.Arrays;

import static org.junit.jupiter.api.Assertions.assertAll;
import static org.junit.jupiter.api.Assertions.assertEquals;

public class FibonacciTest {
    @Test
    void numberAtPosition() {
        assertAll(
                () -> assertEquals(1, FibonacciK.numberAtPosition(0))
                , () -> assertEquals(1, FibonacciK.numberAtPosition(1))
                , () -> assertEquals(2, FibonacciK.numberAtPosition(2))
                , () -> assertEquals(3, FibonacciK.numberAtPosition(3))
                , () -> assertEquals(5, FibonacciK.numberAtPosition(4))
                , () -> assertEquals(8, FibonacciK.numberAtPosition(5))
                , () -> assertEquals(89, FibonacciK.numberAtPosition(10))
        );
    }

    @Test
    void series() {
        assertAll(
                () -> assertEquals(Arrays.asList(1), FibonacciK.seriesUntilPosition(0))
                , () -> assertEquals(Arrays.asList(1, 1), FibonacciK.seriesUntilPosition(1))
                , () -> assertEquals(Arrays.asList(1, 1, 2), FibonacciK.seriesUntilPosition(2))
                , () -> assertEquals(Arrays.asList(1, 1, 2, 3), FibonacciK.seriesUntilPosition(3))
                , () -> assertEquals(Arrays.asList(1, 1, 2, 3, 5), FibonacciK.seriesUntilPosition(4))
                , () -> assertEquals(Arrays.asList(1, 1, 2, 3, 5, 8), FibonacciK.seriesUntilPosition(5))
                , () -> assertEquals(Arrays.asList(1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89), FibonacciK.seriesUntilPosition(10))
        );
    }
}

Bello Kotlin!