Without any kind of filter, we can easily loop a folder's content (one level deep) via the Files.newDirectoryStream(Path dir) method. This method returns a DirectoryStream<Path>, which is an object that we can use to iterate over the entries in a directory:

Path path = Paths.get("D:/learning/books/spring");

try (DirectoryStream<Path> ds = Files.newDirectoryStream(path)) {

  for (Path file: ds) {
    System.out.println(file.getFileName());
  }
}

If we want to enrich this snippet of code with a filter, then we have at least two solutions. One solution relies on another flavor of the newDirectoryStream() method, newDirectoryStream​(Path dir, String glob). Besides Path, this method receives a filter by using the glob syntax. For example, we can filter the D:/learning/books/spring folder for files that are of the PNG, JPG, and BMP types:

try (DirectoryStream<Path> ds =
    Files.newDirectoryStream(path, "*.{png,jpg,bmp}")) {

  for (Path file: ds) {
    System.out.println(file.getFileName());
  }
}

When glob syntax cannot help us anymore, it's time to use another flavor of newDirectoryStream() that gets a Filter, that is, newDirectoryStream​(Path dir, DirectoryStream.Filter<? super Path> filter). First, let's define a filter for files larger than 10 MB:

DirectoryStream.Filter<Path> sizeFilter 
    = new DirectoryStream.Filter<>() {

  @Override
  public boolean accept(Path path) throws IOException {
    return (Files.size(path) > 1024 * 1024 * 10);
  }
};

We can also do this in functional-style:

DirectoryStream.Filter<Path> sizeFilter 
  = p -> (Files.size(p) > 1024 * 1024 * 10);

Now, we can apply this filter like so:

try (DirectoryStream<Path> ds =
    Files.newDirectoryStream(path, sizeFilter)) {

  for (Path file: ds) {
    System.out.println(file.getFileName() + " " +
      Files.readAttributes(file, BasicFileAttributes.class).size() 
        + " bytes");
  }
}

Let's check out a few more filters that we can use with this technique:

• The following is a user-defined filter for folders:

DirectoryStream.Filter<Path> folderFilter 
    = new DirectoryStream.Filter<>() {

  @Override
  public boolean accept(Path path) throws IOException {
    return (Files.isDirectory(path, NOFOLLOW_LINKS));
  }
};

• The following is a user-defined filter for files that have been modified today:

DirectoryStream.Filter<Path> todayFilter 
    = new DirectoryStream.Filter<>() {

  @Override
  public boolean accept(Path path) throws IOException {
    FileTime lastModified = Files.readAttributes(path,
      BasicFileAttributes.class).lastModifiedTime();

    LocalDate lastModifiedDate = lastModified.toInstant()
      .atOffset(ZoneOffset.UTC).toLocalDate();
    LocalDate todayDate = Instant.now()
      .atOffset(ZoneOffset.UTC).toLocalDate();

    return lastModifiedDate.equals(todayDate);
  }
};

• The following is a user-defined filter for hidden files/folders:

DirectoryStream.Filter<Path> hiddenFilter 
    = new DirectoryStream.Filter<>() {

  @Override
  public boolean accept(Path path) throws IOException {
    return (Files.isHidden(path));
  }
};

In the following sections, we'll take a look at the different ways we can filter a file.