Re: Questions about Stream/Iterable/Files - and possibly the compiler

[Phil Race]

For streams I recommend core-libs-...@openjdk.java.net. That
is probably the best place for your question.
For compiler and language questions you might try 
compiler-...@openjdk.java.net


-phil.

On 11/05/2015 05:53 AM, Fabrizio Giudici wrote:

On Thu, 05 Nov 2015 14:28:50 +0100, Kevin Rushforth
 wrote:

I guess I misunderstood your question. I thought you had a question 
about a JavaFX API in JDK 8. This isn't the place to discuss other 
Java APIs.


Oh well I see, my fault! I trusted the auto-completion of my 
email client... and I probably have to convince myself I can't use 
small fonts as I've been doing for years.


Apologies to everyone.

Re: Questions about Stream/Iterable/Files - and possibly the compiler

[Kevin Rushforth]

I guess I misunderstood your question. I thought you had a question 
about a JavaFX API in JDK 8. This isn't the place to discuss other Java 
APIs.


-- Kevin


Fabrizio Giudici wrote:

Hello.

My question is for the sake of curiosity, not being related to a real 
problem - or, better, the problem - which is tiny - can be fixed with 
a simple work around. But I'd like to blog a short post about it and 
I'd like to check I have all the context. It stemmed from a class 
about Java 8 that I recently taught and one of the participants asked 
about that.




Everything starts from this code chunk that doensnt' compile:

1.

Stream s = IntStream.rangeClosed(1, 10) // just as an 
example to quickly create a Stream

.mapToObj(n -> "String #" + n);

Files.write(Paths.get("/tmp/pippo.txt"), s);

error: no suitable method found for write(Path,Stream)
Files.write(Paths.get("/tmp/pippo.txt"), s);
method Files.write(Path,byte[],OpenOption...) is not applicable
  (argument mismatch; Stream cannot be converted to byte[])
method Files.write(Path,IterableCharSequence>,Charset,OpenOption...) is not applicable
  (argument mismatch; Stream cannot be converted to 
Iterable)
method Files.write(Path,IterableCharSequence>,OpenOption...) is not applicable
  (argument mismatch; Stream cannot be converted to 
Iterable)


2. Variation.

Files.write(Paths.get("/tmp/pippo.txt"), (Iterable)s);

This gives:

Exception in thread "main" java.lang.ClassCastException: 
java.util.stream.IntPipeline$4 cannot be cast to java.lang.Iterable

at StreamIteratorExample.main(StreamIteratorExample.java:13)

Ok, so far it's the fact described here

http://stackoverflow.com/questions/20129762/why-does-streamt-not-implement-iterablet 



on why Stream doesn't implement Iterable.

Question A: Is the answer "because iterator() is usually supposed to 
be callable multiple times, while in a Stream it can't" correct?



3. This is the known trick around the problem:

final Iterable i = s::iterator;
Files.write(Paths.get("/tmp/pippo.txt"), i);

It works and I think I understand why (Iterable has the same 
functional descriptor of Supplier, which is s::iterator, so 
they are compatible in assignment - right?).



4. But at this point putting it into the same line gives compilation 
error:


Files.write(Paths.get("/tmp/pippo.txt"), s::iterator);

error: no suitable method found for write(Path,s::iterator)
Files.write(Paths.get("/tmp/pippo.txt"), s::iterator);
method Files.write(Path,byte[],OpenOption...) is not applicable
  (argument mismatch; Array is not a functional interface)
method Files.write(Path,IterableCharSequence>,Charset,OpenOption...) is not applicable

  (argument mismatch; bad return type in method reference
  Iterator cannot be converted to Iterator)
method Files.write(Path,IterableCharSequence>,OpenOption...) is not applicable

  (argument mismatch; bad return type in method reference
  Iterator cannot be converted to Iterator)

5. This at last works:

Files.write(Paths.get("/tmp/pippo.txt"), 
(Iterable)s::iterator);



Question B: Why doesn't the compiler autonomously infer that 
s::iterator is compatible with Iterable and the cast is needed?


At last, question C: Given all those premises, is there a specific 
reason for which Files.write() hasn't been overloaded with a version 
capable of accepting a Stream? It would have been the perfect 
complement of Files.lines()



Thanks.

Re: Questions about Stream/Iterable/Files - and possibly the compiler

[Fabrizio Giudici]

On Thu, 05 Nov 2015 14:28:50 +0100, Kevin Rushforth
 wrote:

I guess I misunderstood your question. I thought you had a question  
about a JavaFX API in JDK 8. This isn't the place to discuss other Java  
APIs.


Oh well I see, my fault! I trusted the auto-completion of my email  
client... and I probably have to convince myself I can't use small fonts  
as I've been doing for years.


Apologies to everyone.


--
Fabrizio Giudici - Java Architect @ Tidalwave s.a.s.
"We make Java work. Everywhere."
http://tidalwave.it/fabrizio/blog - fabrizio.giud...@tidalwave.it