Saving to database in stream pipeline

Question

As per the documentation on Oracle's website:

Side-effects in behavioral parameters to stream operations are, in general, discouraged, as they can often lead to unwitting violations of the statelessness requirement, as well as other thread-safety hazards.

Does this include saving elements of the stream to a database?

Imagine the following (pseudo) code:

public SavedCar saveCar(Car car) {
  SavedCar savedCar = this.getDb().save(car);
  return savedCar;
}

public List<SavedCars> saveCars(List<Car> cars) {
  return cars.stream()
           .map(this::saveCar)
           .collect(Collectors.toList());
}

What are the unwanted effects opposed to this implementation:

public SavedCar saveCar(Car car) {
  SavedCar savedCar = this.getDb().save(car);
  return savedCar;
}

public List<SavedCars> saveCars(List<Car> cars) {
  List<SavedCars> savedCars = new ArrayList<>();
  for (Cat car : cars) {
    savedCars.add(this.saveCar(car));
  }
  return savedCars.
}

Answer 1

The absolute easiest example is:

cars.stream()
    .map(this:saveCar)
    .count()

In this case, from java-9 and up, map will not be executed; since you do not need it to know the count, at all.

There are other multiple cases where side effects would cause you lots of pain; under certain conditions.