class DriveHelper {

    public List<com.google.api.services.drive.model.File> getFilesFromDrive(
            @Nullable String folderName,
            @Nullable String parentId) throws IOException {

        List<com.google.api.services.drive.model.File> files = new ArrayList<>();

        String mimeType = folderName != null ? FOLDER_MIME_TYPE : null;
        String requestString = generateSearchQuery(folderName, parentId, mimeType);
        String fields = "nextPageToken, files(modifiedTime, id, name, mimeType)";
        Drive.Files.List request = buildRequest(requestString, fields);

        if (request != null) {
            driveService.fetchAllFiles(request, files);
        }
        return files;
    }

      DriveHelper(@NonNull GoogleAccountCredential credential,
                @NonNull HttpTransport transport,
                @NonNull JsonFactory jsonFactory);
      DriveHelper(DriveService driveService);

}