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

Tagir Valeev amaembo at
Wed Mar 6 10:10:41 UTC 2019


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> 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:
> Bug report:
> Webrev:
> 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

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