Does every stateful intermediate Stream API operation guarantee a new source collection?

The API documentation makes no such guarantee “that subsequent operations no longer operate on the backing collection”, hence, you should never rely on such a behavior of a particular implementation.

Your example happens to do the desired thing by accident; there’s not even a guarantee that the List created by collect(Collectors.toList()) supports the remove operation.

To show a counter-example

Set<Integer> set = IntStream.range(0, 10).boxed()
    .collect(Collectors.toCollection(TreeSet::new));
set.stream()
    .filter(i -> i > 5)
    .sorted()
    .forEach(set::remove);

throws a ConcurrentModificationException. The reason is that the implementation optimizes this scenario, as the source is already sorted. In principle, it could do the same optimization to your original example, as forEach is explicitly performing the action in no specified order, hence, the sorting is unnecessary.

There are other optimizations imaginable, e.g. sorted().findFirst() could get converted to a “find the minimum” operation, without the need to copy the element into a new storage for sorting.

So the bottom line is, when relying on unspecified behavior, what may happen to work today, may break tomorrow, when new optimizations are added.