Can a Java 8 `Stream` be parallel without you even asking for it?

Question

As I see it, the obvious code, when using Java 8 Streams, whether they be \"object\" streams or primitive streams (that is, IntStream and friends)

Answer 1

Well, answer to self...

After thinking about it a little more seriously (go figure, such things only happen after I actually ask the question), I actually came up with a reason why...

Intermediate operations may NOT be thread safe; as such, the API cannot make any assumptions, hence if the user wants a parallel stream, it has to explicitly ask for it and ensure that all intermediate operations used in the stream are thread safe.

There is however the somewhat misleading case of Collectors; since a Collector cannot know by advance whether it will be called as a terminal operation on a stream which is parallel or not, the contract makes it clear that "just to be safe", any Collector must be thread safe.