How to list only N files in directory using java

Question

If I have a directory contains a lot of files (about 1000 file). Some of these files named .processed and other not.

How can I list only 10 unprocessed files.

I am using this code to filter the processed file.

File[] inputFileList = inputDirectory.listFiles(new FileFilter() {
                @Override
                public boolean accept(File pathname) {
                    return !pathname.getName().endsWith(".processed");
                }
            });

But if the number of un-processed files is huge, this may lead to memory error. so I need to read a limited number of files each time the application will run.

Answer 1

Which is why you should use java.nio.file. Using Java 8:

final Path baseDir = Paths.get("path/to/dir");

final List<Path> tenFirstEntries;

final BiPredicate<Path, BasicFileAttributes> predicate = (path, attrs)
    -> attrs.isRegularFile() && path.getFileName().endsWith(".processed");

try (
    final Stream<Path> stream = Files.find(baseDir, 1, predicate);
) {
    tenFirstEntries = stream.limit(10L).collect(Collectors.toList());
}

Using Java 7:

final Path baseDir = Paths.get("path/to/dir");

final List<Path> tenFirstEntries = new ArrayList<>(10);

final DirectoryStream.Filter<Path> filter = new DirectoryStream.Filter<Path>()
{
    @Override
    public boolean accept(final Path entry)
    {
        return entry.getFileName().endsWith(".processed")
            && Files.isRegularFile(entry);
    }
};

try (
    final DirectoryStream<Path> stream 
        = Files.newDirectoryStream(baseDir, filter);
) {
    final Iterator<Path> iterator = stream.iterator();
    for (int i = 0; iterator.hasNext() && i < 10; i++)
        tenFirstEntries.add(iterator.next());
}

Unlike File.listFiles(), java.nio.file use lazily populated streams of directory entries.

One more reason to ditch File. This is 2015 after all.