Proposal: JDK-8148917 Enhanced-For Statement Should Allow Streams

[Tagir Valeev]

One more alternative which could be considered (though it requires a
language change) is to allow the inference of Iterable<T> type for
enhanced for when iteration value is a function expression and
iteration parameter type is explicitly specified as T. In this case
for(T t : stream::iterator) {} would be a valid syntax. Also people
could use enhanced for with any iterator they already have like for(T
t : () -> iterator) {}. Instead of for (; it.hasNext() ; ) { T t =
it.next(); ... }. It looks better than allowing to specify iterator
directly (like for(T t : stream.iterator())) as this could break some
programs where the same class implements Iterator and Iterable.

With best regards,
Tagir Valeev.

On Fri, Mar 1, 2019 at 9:47 AM Stuart Marks <stuart.marks at oracle.com> wrote:
>
> Hi all,
>
> Please review and comment on this proposal to allow Stream instances to be used
> in enhanced-for ("for-each") loops.
>
> Abstract
>
> Occasionally it's useful to iterate a Stream using a conventional loop. However,
> the Stream interface doesn't implement Iterable, and therefore streams cannot be
> used with the enhanced-for statement. This is a proposal to remedy that
> situation by introducing a new interface IterableOnce that is a subtype of
> Iterable, and then retrofitting the Stream interface to implement it. Other JDK
> classes will also be retrofitted to implement IterableOnce.
>
> Full Proposal:
>
>      http://cr.openjdk.java.net/~smarks/reviews/8148917/IterableOnce0.html
>
> Bug report:
>
>      https://bugs.openjdk.java.net/browse/JDK-8148917
>
> Webrev:
>
>      http://cr.openjdk.java.net/~smarks/reviews/8148917/webrev.0/
>
> Note, this changeset isn't ready to push yet. In particular, it has no tests
> yet. However, the implementation is so simple that I figured I should include
> it. Comments on the specification wording are also welcome.
>
> Thanks,
>
> s'marks