Why does Stream<T> not implement Iterable<T>?

People have already asked the same on the mailing list ☺. The main reason is Iterable also has a re-iterable semantic, while Stream is not.

I think the main reason is that Iterable implies reusability, whereas Stream is something that can only be used once — more like an Iterator.

If Stream extended Iterable then existing code might be surprised when it receives an Iterable that throws an Exception the second time they do for (element : iterable).

To convert a Stream to an Iterable, you can do

Stream<X> stream = null;
Iterable<X> iterable = stream::iterator

To pass a Stream to a method that expects Iterable,

void foo(Iterable<X> iterable)

simply

foo(stream::iterator) 

however it probably looks funny; it might be better to be a little bit more explicit

foo( (Iterable<X>)stream::iterator );

I would like to point out that StreamEx does implement Iterable (and Stream), as well as a host of other immensely awesome functionality missing from Stream.