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<String> s = IntStream.rangeClosed(1, 10) // just as an 
> example to quickly create a Stream<String>
>                 .mapToObj(n -> "String #" + n);
>
>         Files.write(Paths.get("/tmp/pippo.txt"), s);
>
> error: no suitable method found for write(Path,Stream<String>)
>         Files.write(Paths.get("/tmp/pippo.txt"), s);
>     method Files.write(Path,byte[],OpenOption...) is not applicable
>       (argument mismatch; Stream<String> cannot be converted to byte[])
>     method Files.write(Path,Iterable<? extends 
> CharSequence>,Charset,OpenOption...) is not applicable
>       (argument mismatch; Stream<String> cannot be converted to 
> Iterable<? extends CharSequence>)
>     method Files.write(Path,Iterable<? extends 
> CharSequence>,OpenOption...) is not applicable
>       (argument mismatch; Stream<String> cannot be converted to 
> Iterable<? extends CharSequence>)
>
> 2. Variation.
>
>         Files.write(Paths.get("/tmp/pippo.txt"), (Iterable<String>)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<String> 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<Iterator>, 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,Iterable<? extends 
> CharSequence>,Charset,OpenOption...) is not applicable
>       (argument mismatch; bad return type in method reference
>           Iterator<String> cannot be converted to Iterator<CharSequence>)
>     method Files.write(Path,Iterable<? extends 
> CharSequence>,OpenOption...) is not applicable
>       (argument mismatch; bad return type in method reference
>           Iterator<String> cannot be converted to Iterator<CharSequence>)
>
> 5. This at last works:
>
>         Files.write(Paths.get("/tmp/pippo.txt"), 
> (Iterable<String>)s::iterator);
>
>
> Question B: Why doesn't the compiler autonomously infer that 
> s::iterator is compatible with Iterable<String> 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<String>? It would have been the perfect 
> complement of Files.lines()
>
>
> Thanks.
>
>
>