Why is CompletableFuture join/get faster in separate streams than using one stream

Question

For the following program I am trying to figure out why using 2 different streams parallelizes the task and using the same stream and calling join/get on the Completable fut

Answer 1

The stream framework does not define the order in which map operations are executed on stream elements, because it is not intended for use cases in which that might be a relevant issue. As a result, the particular way your second version is executing is equivalent, essentially, to

List results = new ArrayList<>();
for (Integer sleepTime : sleepTimes) {
  results.add(CompletableFuture
     .supplyAsync(() -> sleepTask(sleepTime), executorService2)
     .exceptionally(ex -> { ex.printStackTrace(); return -1; }))
     .join());
}

...which is itself essentially equivalent to

List results = new ArrayList<>()
for (Integer sleepTime : sleepTimes) {
  results.add(sleepTask(sleepTime));
}