I don't understand why we need IterableOnce ? Was: Proposal: JDK-8148917 Enhanced-For Statement Should Allow Streams

Peter Levart peter.levart at gmail.com
Fri Mar 15 08:51:07 UTC 2019

On 3/15/19 9:38 AM, Ivan Gerasimov wrote:
> Hi Peter!
> On 3/15/19 1:24 AM, Peter Levart wrote:
>> On 3/15/19 9:03 AM, forax at univ-mlv.fr wrote:
>>>>       * @since 13
>>>>       */
>>>>      interface Once {}
>>>> What do you think of that?
>>> It's not clear to me if an annotation, available at runtime, is not 
>>> a better fit.
>>> Anyway, i'm not sure not sure introducing such interface/annotation 
>>> worth its maintenance cost, as you said the use case is pretty narrow.
>> It is narrow, but in a situation like that, where you want to code an 
>> optimal generic algorithm and all you have access to is an Iterable, 
>> there's no other way (short of providing additional methods, which is 
>> ugly). Just think of this situation. You have to decide upfront if 
>> you need to buffer the elements obtained from 1st iteration or not, 
>> but 1st iteration always succeeds...
> Can you please explain how the interface Once would help to solve this?
> If an Iterable does not implement Once, it does not mean it allows 
> multiple passes, right?

It does not guarantee multiple passes, that's right, but that's legacy. 
This situation is the same for IterableOnce as a subtype of Iterable, 
but marker interface has less chance to intrude into "visible" static 
types that consist of method signatures, type parameters and variable 
types and therefore does not cause confusion in that area.

"Once" is not perfect, but allows generic algorithms to work on those 
instances that do implement it at least.

Regards, Peter

> With kind regards,
> Ivan

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