Java streams: flatMap returns Stream of Objects

Question

I've this code:

List<Application> applications = this.applicationDao.findAll();
Collection<Pair<Application, FileObject>> files = applications.stream()
    .flatMap(app -> streamOfFiles.map(file -> Pair.of(app, file)));

where streamOfFiles is an Stream<FileObject>

Currently, I'm getting this compilation message:

Type mismatch: cannot convert from Stream<Object> to Collection<Pair<Application,FileObject>>

Any ideas?

Answer 1

You seem to be missing the collect there :

Collection<Pair<Application, FileObject>> files = applications.stream()
        .flatMap(app -> files.stream().map(file -> Pair.of(app, file)))
        .collect(Collectors.toList()); // any collection you want

Edit: Since, streamOfFiles is getting consumed at a single flatMap operation, you should prefer to use a <collection>.stream() instead there to create the stream afresh for each app.

Answer 2

You can't reuse streamOfFiles Stream while iterating over applications!

You need to transform streamOfFiles from a Stream<FileObject> to List<FileObject> and then:

List<FileObject> listOfFiles = streamOfFiles.collect(Collectors.toList());
List<Application> applications = this.applicationDao.findAll();
Collection<Pair<Application, FileObject>> files =  applications.stream()
            .flatMap(app -> listOfFiles.stream().map(file -> Pair.of(app, file)))
            .collect(Collectors.toList());

A simple example to prove that:

Stream<Integer> s = IntStream.range(0, 100).boxed();
IntStream.range(0, 100).boxed()
         .flatMap(i -> s.map(j -> i * j))
         .collect(Collectors.toList());

It throws this Exception:

java.lang.IllegalStateException: stream has already been operated upon or closed