The FilenameFilter functional interface can be used to filter files from a folder as well. First, we need to define a filter (for example, the following is a filter for files of the PDF type):

String[] files = path.toFile().list(new FilenameFilter() {

  @Override
  public boolean accept(File folder, String fileName) {
    return fileName.endsWith(".pdf");
  }
});

We can do the same in functional-style:

FilenameFilter filter = (File folder, String fileName) 
  -> fileName.endsWith(".pdf");

Let's make this more concise:

FilenameFilter filter = (f, n) -> n.endsWith(".pdf");

In order to use this filter, we need to pass it to the overloaded File.list​(FilenameFilter filter) or File.listFiles​(FilenameFilter filter) method:

String[] files = path.toFile().list(filter);

The files array will only contain the names of the PDF files.