Stream <Install <path> to install <path>

Question

Here is the Java 8 code using threads:

 Set<String> getFields( Path xml ) { final Set<String> fields = new HashSet<>(); for( ... ) { ... fields.add( ... ); ... } return fields; } void scan() { final SortedSet<Path> files = new TreeSet<>(); final Path root = new File( "....." ).toPath(); final BiPredicate<Path, BasicFileAttributes> pred = (p,a) -> p.toString().toLowerCase().endsWith( ".xml" ); Files.find( root, 1, pred ).forEach( files::add ); final SortedSet<String> fields = new TreeSet<>(); files .stream() .parallel() .map( this::getFields ) .forEach( s -> fields.addAll( s )); // Do something with fields... } 

I want to combine the output of map( this::getFields ) , i.e. a Stream<Set<Path>> to Set<Path> , and I'm not sure if forEach used correctly.

EDIT after John Skeet's answer to summarize comments and compile code

 Stream<String> getFields( Path xml ) { final Set<String> fields = new HashSet<>(); for( ... ) { ... fields.add( ... ); ... } return fields.stream(); // returns a stream to ease integration } void scan() { final Path root = new File( "....." ).toPath(); final BiPredicate<Path, BasicFileAttributes> pred = (p,a) -> p.toString().toLowerCase().endsWith( ".xml" ); final SortedSet<Path> files = Files .find( root, 1, pred ) .collect( Collectors.toCollection( TreeSet::new )); final SortedSet<String> fields = files .stream() .parallel() .flatMap( this::getFields ) .collect( Collectors.toCollection( TreeSet::new )); // Do something with fields... } 

Two streams can be merged into one, but files reused later.

Answer 1

I suspect you want flatMap instead of map , and then use Collectors.toCollection to create a sorted set:

 final SortedSet<String> fields = files .stream() .parallel() .flatMap(x -> getFields(x).stream()) .collect(Collectors.toCollection(() -> new TreeSet<String>()); 

(I have not tried this, so the syntax may be a bit off, but I think this is about what you want.)

In general, I would suggest trying an approach that creates a collection in a streaming operation, rather than using forEach at the end to add everything - you can do the same with files .