How to access individual objects stream inside of nested stream in Java 8?

Java is nominal, as in, it likes names of things.

You're fighting the language. Stop doing that. Go with the flow.

List<Integer> could indeed be used to represent a point, but it's real shite at it. It doesn't print nicely, and it is trivial to make an 'invalid state' object - List.of(), what coordinate is that? List.of(1, 2, 3), what's going on there?

Make a class that represents the idea of a 2D point.

public record Point(int x, int y) {}

will do that, for exmaple.

But, let's say you want to keep shooting yourself in the foot and abuse List<Integer> as a coordinate pair, you do not want flatmap.

Given, say, the origin coordinate ((0, 0)), as well as x=5 and y=10, then flatMap would let you obtain a stream that is just the sequence 0, 0, 5, 10. This is not good - streams work by letting you inspect and operate on individual items. There's no way to check x * y at that point.

So, you don't want flatmap. You'd just operate on your numbers.stream(), which is a stream of List<Integer> objects. You can then apply your logic to each of these objects, which are just a real bad way of representing 2D coordinates.

numbers.stream().anyMatch(list -> list.get(0) * list.get(1) < 0);

you can convert the inner list to stream but the flat map is used to flatten the list. it will give you one element, not a pair.

List<Integer> list = numbers.stream().flatMap(point -> point.stream()).filter(x -> x>0).collect(Collectors.toList());

Just now you asked a question. When I came back after writing the answer, your question was closed. I have to answer the question you just asked under this question.

package lambda;

import java.util.*;

import static java.util.stream.Collectors.*;

/**
 * @Author: Beer Bear
 * @Description: todo
 * @Date: 2021/6/29 10:41
 */
public class stackoverflow {
    public static void main(String[] args) {
        List<Integer> list1 = Arrays.asList(1,2);
        List<Integer> list2 = Arrays.asList(-1,2);
        List<Integer> list3 = Arrays.asList(1,100);
        List<List<Integer>> numbers = new LinkedList();
        numbers.add(list1);
        numbers.add(list2);
        numbers.add(list3);
        //System.out.println(allList);
        Map<Boolean, List<Integer>> map = numbers.stream().collect(groupingBy(list -> list.get(0) * list.get(1) > 0, mapping(list -> function(list.get(0), list.get(1)), toList())));
        System.out.println(map);
        int sumOfCase1 = map.get(true).stream().mapToInt(item -> item).sum();
        int sumOfCase2 = map.get(false).stream().mapToInt(item -> item).sum();
        System.out.println(sumOfCase1+" "+sumOfCase2);
    }
    private static int function(int a, int b){
        return a+b;
    }
}