Best way to list files in Java, sorted by Date Modified?

Question

I want to get a list of files in a directory, but I want to sort it such that the oldest files are first. My solution was to call File.listFiles and just resort the list ba

Answer 1

If the files you are sorting can be modified or updated at the same time the sort is being performed:

---

Java 8+

private static List listFilesOldestFirst(final String directoryPath) throws IOException {
    try (final Stream fileStream = Files.list(Paths.get(directoryPath))) {
        return fileStream
            .map(Path::toFile)
            .collect(Collectors.toMap(Function.identity(), File::lastModified))
            .entrySet()
            .stream()
            .sorted(Map.Entry.comparingByValue())
//            .sorted(Collections.reverseOrder(Map.Entry.comparingByValue()))  // replace the previous line with this line if you would prefer files listed newest first
            .map(Map.Entry::getKey)
            .map(File::toPath)  // remove this line if you would rather work with a List instead of List
            .collect(Collectors.toList());
    }
}

Java 7

private static List listFilesOldestFirst(final String directoryPath) throws IOException {
    final List files = Arrays.asList(new File(directoryPath).listFiles());
    final Map constantLastModifiedTimes = new HashMap();
    for (final File f : files) {
        constantLastModifiedTimes.put(f, f.lastModified());
    }
    Collections.sort(files, new Comparator() {
        @Override
        public int compare(final File f1, final File f2) {
            return constantLastModifiedTimes.get(f1).compareTo(constantLastModifiedTimes.get(f2));
        }
    });
    return files;
}

Both of these solutions create a temporary map data structure to save off a constant last modified time for each file in the directory. The reason we need to do this is that if your files are being updated or modified while your sort is being performed then your comparator will be violating the transitivity requirement of the comparator interface's general contract because the last modified times may be changing during the comparison.

If, on the other hand, you know the files will not be updated or modified during your sort, you can get away with pretty much any other answer submitted to this question, of which I'm partial to:

Java 8+ (No concurrent modifications during sort)

private static List listFilesOldestFirst(final String directoryPath) throws IOException {
    try (final Stream fileStream = Files.list(Paths.get(directoryPath))) {
        return fileStream
            .map(Path::toFile)
            .sorted(Comparator.comparing(File::lastModified))
            .map(File::toPath)  // remove this line if you would rather work with a List instead of List
            .collect(Collectors.toList());
    }
}

Note: I know you can avoid the translation to and from File objects in the above example by using Files::getLastModifiedTime api in the sorted stream operation, however, then you need to deal with checked IO exceptions inside your lambda which is always a pain. I'd say if performance is critical enough that the translation is unacceptable then I'd either deal with the checked IOException in the lambda by propagating it as an UncheckedIOException or I'd forego the Files api altogether and deal only with File objects:

final List sorted = Arrays.asList(new File(directoryPathString).listFiles());
sorted.sort(Comparator.comparing(File::lastModified));