I’m trying to understand why there is a different when I change from y.addAll(x) to x.addAll(y) in code snippet below:

        List<Integer> result = List.of(1, 2)
                .parallelStream() 
                .collect(ArrayList::new,
                        (x, y) -> x.add(y),
                        (x, y) -> y.addAll(x)
                );
        System.out.println(result);

I’m new with paralell stream java. I know, when I use parallelStream(), there are more than one thread run at a time, collect() method has three parameter, the first two parameter I can understand.

So, with the third parameter, I know x, y are substream and they are ArrayList, but I can’t understand why the result variable are different each case. I think they are same result.

• (x, y) -> y.addAll(x) // output: [1]

• (x, y) -> x.addAll(y) // output: [1, 2]

Please be patient with me. Thanks