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?

flatMap() is not terminal operation. You should call any terminal operation if you want to return result. — Hatice
– Hatice, Commented Feb 13, 2019 at 14:24

Naman · Accepted Answer · 2021-04-27 04:09:41Z

6

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.

edited Apr 27, 2021 at 4:09

answered Feb 13, 2019 at 14:23

Naman

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Naman · Accepted Answer · 2019-02-13 15:07:29Z

3

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

edited Feb 13, 2019 at 15:07

Naman

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answered Feb 13, 2019 at 14:31

David Pérez Cabrera

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1 Comment

Naman Over a year ago

You're right. Its called for each Application and that is where it is getting operated upon. and hence the error. <collection>.stream should be a better approach here.

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