import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

class Test {
    public static void main(String[] args) {
        System.out.println(Arrays.asList(1, 2, 3, 4, 5, 6)
                .parallelStream()
                .reduce(0, (a, b) -> (a - b))); // NOT AN ASSOCIATIVE ACCUMULATOR
    }
}