FileFilter is another functional interface that can be used to filter files and folders. For example, let's filter only folders:

File[] folders = path.toFile().listFiles(new FileFilter() {

  @Override
  public boolean accept(File file) {
    return file.isDirectory();
  }
});

We can do the same in functional-style:

File[] folders = path.toFile().listFiles((File file) 
  -> file.isDirectory());

Let's make this more concise:

File[] folders = path.toFile().listFiles(f -> f.isDirectory());

Finally, we can do this via member reference:

File[] folders = path.toFile().listFiles(File::isDirectory);