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.
I think that there's something that possibly involves the compiler - I'll
eventually post the relevant part of the questions to the proper mailing
list.
1. Everything starts from this code chunk that doesnt' compile:
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 SO 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);
I assume that the compiler can't autonomously infer that s::iterator is
compatible with Iterable<String> so the cast is needed. I have a question
about that (feature or bug?), but this is related to the compiler, so I
think is OT here.
At last, question B: 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.
--
Fabrizio Giudici - Java Architect @ Tidalwave s.a.s.
"We make Java work. Everywhere."
http://tidalwave.it/fabrizio/blog - fabrizio.giud...@tidalwave.it
