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How to reduce a limited stream (e.g. an ArrayList) from right in Java?

I would like to convert a list to a self-implemented linked list using reduce. I managed to do that in a reverse order:

import java.util.Arrays;
import java.util.List;

public class Reverse {
    static class Node {
        int val;
        Node next;

        Node(int v, Node n) {
            val = v;
            next = n;
        }
    }

    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
        Node result = numbers
                .stream()
                .reduce(null,
                        (node, val) -> new Node(val, node),
                        (n1, n2) -> new Node(-1, null));

        while(result != null) {
            System.out.println(result.val);
            result = result.next;
        }
    }
}

Output is

6
5
4
3
2
1

I think to obtain result in the correct order, I may have to use something like foldRight or reduceRight in other languages. Does java provide anything similar, given that the stream is guaranteed to be limited?

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Answer

As of the newest Java 16, the current Stream API doesn’t provide any operations “from the right” such as “reduceRight`.

The only possible workaround is to use Collections#reverse(List) method to reverse the order of the items in the collection right before applying reduction.

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
Collections.reverse(numbers);
Node result = numbers
    .stream()
    .reduce(null,
            (node, val) -> new Node(val, node),
            (n1, n2) -> new Node(-1, null));


Read more here: Java 8 stream reverse order

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