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

[Tagir Valeev]

Hello, Remi!

It actually works thanks to auto-boxing/unboxing. E.g. this
implementation works:

static Iterable<Integer> range(int from, int to) {
  return () -> new PrimitiveIterator.OfInt() {
    int cur = from;

    @Override
    public int nextInt() {
      if (cur >= to) {
        throw new NoSuchElementException();
      }
      return cur++;
    }

    @Override
    public boolean hasNext() {
      return cur < to;
    }
  };
}

public static void main(String[] args) {
  for(int i : range(0, 100)) {
    System.out.println(i);
  }
}

It correctly compiles and prints numbers from 0 to 99. As IntStream
extends BaseStream<Integer, IntStream> and BaseStream<T, S extends
BaseStream<T, S>> defines Iterator<T> iterator(), it would be no
problem with using IntStream.range in such code pattern were
BaseStream extends IterableOnce<T>.

Of course this produces unnecessary garbage, as I said.

With best regards,
Tagir Valeev.

On Wed, Mar 6, 2019 at 7:37 PM Remi Forax <forax at univ-mlv.fr> wrote:
>
> Hi Tagir,
> try to do it now and you will see that you can't, because you can not write Iterable<int> yet.
> Once we will get generics over value types, it will be a no-brainer.
>
> Rémi
>
> ----- Mail original -----
> > De: "Tagir Valeev" <amaembo at gmail.com>
> > À: "Stuart Marks" <stuart.marks at oracle.com>
> > Cc: "core-libs-dev" <core-libs-dev at openjdk.java.net>
> > Envoyé: Mercredi 6 Mars 2019 11:10:41
> > Objet: Re: Proposal: JDK-8148917 Enhanced-For Statement Should Allow Streams
>
> > Hello!
> >
> > By the way one of the painful code patterns in modern Java is `for(int
> > i = 0; i<bound; i++)` which is very verbose, hackish, confusing for
> > newbies and prone to errors as the variable need to be repeated three
> > times. Also the variable is not effectively final, despite it never
> > changes within the loop body, so could have been considered as
> > redeclared on every loop iteration (like in for-each). With the new
> > proposal it's possible to write `for(int i : range(0, bound).boxed())`
> > (assuming import static j.u.s.IntStream.range), which looks much
> > better, though it has obvious performance drawback. Moving
> > IterableOnce to BaseStream would enable to use `for(int i : range(0,
> > bound))` which looks even better, though again we have plenty of
> > garbage (but considerably less than in previous case!). I wonder
> > whether Java could evolve to the point where such kind of code would
> > be a recommended way to iterate over subsequent integer values without
> > any performance handicap.
> >
> > 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