How can I read all files line by line from a directory by using Streams?

Question

I have a directory called Files and it has many files.I want to read these Files line by line and store them an List<List<String>> .

./Files
 ../1.txt
 ../2.txt
 ../3.txt
 ..
 ..

it goes like that.

private List<List<String>> records = new ArrayList<>();

List<Path> filesInFolder = Files.list(Paths.get("input"))
                .filter(Files::isRegularFile)
                .collect(Collectors.toList());

records = Files.lines(Paths.get("input/1.txt"))
                .map(row -> Arrays.asList(row.split(space)))
                .collect(Collectors.toList());

Answer 1

The logic basically is like

List<List<String>> records = Files.list(Paths.get("input"))
    .filter(Files::isRegularFile)
    .flatMap(path -> Files.lines(path)
        .map(row -> Arrays.asList(row.split(" "))))
    .collect(Collectors.toList());

But you are required to catch the IOException potentially thrown by Files.lines . Further, the stream returned by Files.list should be closed to release the associated resources as soon as possible.

List<List<String>> records; // don't pre-initialize
try(Stream<Path> files = Files.list(Paths.get("input"))) {
    records = files.filter(Files::isRegularFile)
        .flatMap(path -> {
            try {
                return Files.lines(path)
                    .map(row -> Arrays.asList(row.split(" ")));
            } catch (IOException ex) { throw new UncheckedIOException(ex); }
        })
        .collect(Collectors.toList());
}
catch(IOException|UncheckedIOException ex) {
    // log the error

    // and if you want a fall-back:
    records = Collections.emptyList();
}

Note that the streams returned by Files.lines used with flatMap are correctly closed automatically, as documented :

Each mapped stream is closed after its contents have been placed into this stream.

---

It's also possible to move the map step from the inner stream to the outer:

List<List<String>> records; // don't pre-initialize
try(Stream<Path> files = Files.list(Paths.get("E:\\projects\\nbMJ\\src\\sub"))) {
    records = files.filter(Files::isRegularFile)
        .flatMap(path -> {
            try { return Files.lines(path); }
            catch (IOException ex) { throw new UncheckedIOException(ex); }
        })
        .map(row -> Arrays.asList(row.split(" ")))
        .collect(Collectors.toList());
}
catch(IOException|UncheckedIOException ex) {
    // log the error

    // and if you want a fall-back:
    records = Collections.emptyList();
}