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 08:31:10 UTC 2019

On 3/15/19 9:24 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...

Ok, I confess, there is a way to do it without marker interface, but is 

Iterator<T> iterator1 = iterable.iterator();
Iterator<T> iterator2;
try {
    iterator2 = iterable.iterator();
} catch (IllegalStateException e) {
     // we have one-shot iterable
    iterator2 = null;

if (iterator2 == null) {
    // buffer elements of iterator1 while iterating for 2nd and 
subsequent iterations
} else {
     // consume iterator1, then iterator2, then create more iterators 
for subsequent iterations


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