Joe Bowbeer joe.bowbeer at
Thu Feb 21 08:14:14 PST 2013

When this question was raised 2 weeks ago, you asked:

Can we make our best attempt to specify better than
Iterable.iterator() was?

I haven't worked out how to say this yet, but the idea is:

- If at all possible to ensure that each call to stream() returns an actual
working and independent stream, you really really should do that.
- If that's just not possible, the second call to stream() really really
should throw ISE.

Is this something we should address?  There was no discussion about this
last time.
On Feb 21, 2013 8:07 AM, "Kevin Bourrillion" <kevinb at> wrote:

> 1. Yes please.
> 2. And this time I won't hijack the thread.
> On Thu, Feb 21, 2013 at 7:44 AM, Brian Goetz <brian.goetz at>wrote:
>> Currently we define stream() and parallelStream() on Collection, with
>> defaults like:
>>     default Stream<E> stream() {
>>         return
>>            () -> Streams.spliterator(iterator()**, size(),
>>                                      Spliterator.SIZED),
>>            Spliterator.SIZED);
>>     }
>> In other words, if a Collection does not override stream(), it gets the
>> stream based on the iterator.
>> It has been suggested that we could move stream/parallelStream() up to
>> Iterable.  They could use an almost identical default, except that they
>> don't know the SIZED flag.  (The default in Collection would stay, so
>> existing inheritors of the Collection default wouldn't see any difference.
>>  (This is why default methods are virtual.))
>> Several people have asked why not move these to Iterable, since some APIs
>> return "Iterable" as a least-common-denominator aggregate type, and this
>> would allow those APIs to participate in the stream fun.  There are also a
>> handful of other types that implement Iterable, such as Path
>> (Iterable<Path>) and DirectoryStream (where we'd added an entries() method,
>> but that would just then become stream()).
>> The sole downside is that it creates (yet another) external dependency
>> from java.lang.Iterable -- now to
>> Thoughts?
> --
> Kevin Bourrillion | Java Librarian | Google, Inc. | kevinb at

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