Why is Stream<T> not an Iterable<T> ?

Sam Pullara spullara at gmail.com
Tue Mar 26 20:05:21 PDT 2013

Iterable<T> can be iterated over and over because each time you call .iterator() you get a new instance starting from the beginning. Stream is more closely related to Iterator<T>. It would be very hard to insure that existing APIs that take Iterable would work with Streams because of this property. Even wrapping it would be a bit painful as you would have to buffer the full Stream to implement the Iterable contract.


On Mar 26, 2013, at 7:56 PM, Oliver Doepner <odoepner at gmail.com> wrote:

> Hello,
> I recently joined this mailing list. I am quite excited about the
> lambda functionality coming in Java 8.
> I also had the pleasure to attend Brian's "Road to Lambda" talk at JavaOne.
> Thanks for the great work.
> Today I looked at the Javadoc at
> http://lambdadoc.net/api/java/util/stream/Stream.html
> http://lambdadoc.net/api/java/lang/Iterable.html
> I hope that site has recent information. I had some problems finding
> the source itself on hg.
> I probably misunderstood the (not so obvious?) links at
> http://hg.openjdk.java.net/lambda/lambda/.
> Anyhow, I noticed that both of the methods of Iterable (iterator,
> forEach) are also present on Stream.
> Now I wonder why Stream<T> does not extend Iterable<T>.
> Then I searched the lambda-dev archives for threads related to Iterables.
> I did not find a discussion directly related to this. I hope I didn't
> miss anything.
> I think that it would be nice if Stream instances could be passed to
> existing APIs that take Iterable.
> Naively speaking: Why have the same methods with the same signatures
> and semantics / Javadoc on two core Java interfaces and not make one a
> subtype of the other? That feels like a pair of quack-like-a-ducks
> calling for subtyping. :^)
> Thanks
> Oliver
> -- 
> Oliver Doepner
> http://doepner.net/

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