Stream of a reverse list

[Brian Goetz]

All lists have a reverse order because they are finite.  Not all streams have a reverse order.  

More generally, we probably want more "view" combinators on collections, for things like concat, reverse, or merge without paying the price of a full copy.  That's a story for another day. 


Sent from my iPad

On Jul 16, 2013, at 3:59 PM, Dan Smith <daniel.smith at oracle.com> wrote:

> On Jul 15, 2013, at 1:01 PM, David M. Lloyd <david.lloyd at redhat.com> wrote:
> 
>> On 07/15/2013 01:19 PM, Dan Smith wrote:
>>> On Jul 15, 2013, at 7:53 AM, Remi Forax <forax at univ-mlv.fr> wrote:
>>> 
>>>> On 07/15/2013 03:36 PM, Brian Goetz wrote:
>>>>> We did consider such a stream op and triaged it away as bring too niche. It also requires a full barrier to get the first element.  And for infinite streams obviously blows up.
>>>>> 
>>>>> Given that it always requires a full barrier, toArray seems the best way to go.
>>>>> 
>>>>> Sent from my iPad
>>>> 
>>>> We reject having a method reverse() on Stream but not the fact that one can create a Stream that will iterate over the list in a backward way.
>>>> But perhaps, it's better to have a method of List named reverseList() that return a reverse view of the list
>>>> and calls stream() on it.
>>> 
>>> Has to be prioritized, of course, but I think 'List.reverseStream' is in principle a good suggestion.  Looks a lot like the idea of having different methods on CharSequence to get char-based and int-based views of the same data.  In this case, we're getting front-to-back and back-to-front views of the List.
>>> 
>>> Collection is _not_ a good place to put a method like this, because Collections are not designed to support reverse-order traversal.  Lists are (see ListIterator).  As are Deques (see Deque.desendingIterator).  (I was surprised, actually, to not find a similar List.reverseIterator method -- I guess the intended idiom is to call 'list.iterator(list.size())' and then iterate with 'ListIterator.previous'.)
>> 
>> If that is directed at me... I did not suggest Collection, I suggested Collections, in lieu of adding a default method to List (though that's an option too).
> 
> Sort of a conglomeration of you mentioning Collections and Brian talking about a Stream method.  But, yeah, I did realize when reading carefully that you were talking about a static method operating on Lists.
> 
> The idea is it's fairly painless to "streamify" existing concepts, like reverse List and Deque iteration; much more expensive (and perhaps ill-advised) to create new concepts, like every Stream having a reverse order.
> 
> —Dan