Stream ordered/unordered problems

Question

I have the following code:

Set<Integer> l = new TreeSet<>();
l.add(1);
l.add(10);
l.add(3);
l.add(-3);
l.add(-4);

and I want to unorder the collection with:

l.stream().unordered().forEach(System.out::println);

but the forEach returns always the collection ordered!

Then I have also another doubt about the below sentence from here:

For sequential streams, the presence or absence of an encounter order does not affect performance, only determinism. If a stream is ordered, repeated execution of identical stream pipelines on an identical source will produce an identical result; if it is not ordered, repeated execution might produce different results.

In fact if I try this code on an unordered stream the results is always the same and never produce different results:

Arrays.stream( new int[]{8, -1, 3}).forEach(System.out::println);
Arrays.stream( new int[]{8, -1, 3}).forEach(System.out::println);

I really don't understand this API part...

Answer 1

The unordered() operation doesn't do any actions to explicitly unorder the stream. What it does is that it removes the constraint on the stream that it must remain ordered, thereby allowing subsequent operations to use optimizations that don't have to take ordering into consideration.

You can read about this in the Java 8 docs:

For sequential streams, the presence or absence of an encounter order does not affect performance, only determinism. If a stream is ordered, repeated execution of identical stream pipelines on an identical source will produce an identical result; if it is not ordered, repeated execution might produce different results.
For parallel streams, relaxing the ordering constraint can sometimes enable more efficient execution.
...

In cases where the stream has an encounter order, but the user does not particularly care about that encounter order, explicitly de-ordering the stream with unordered() may improve parallel performance for some stateful or terminal operations.