public class GetThumbs extends AsyncTask<WeakHashMap<VideoFile, ImageView>, Object, Void> {

    @Override
    protected Void doInBackground(WeakHashMap<VideoFile, ImageView>[] weakHashMaps) {
        WeakHashMap<VideoFile, ImageView> weakHashMap = weakHashMaps[0];
        for (Map.Entry<VideoFile, ImageView> entry : weakHashMap.entrySet()) {
            File preview = new File(entry.getKey().thumbPath);
            if (!preview.exists()) {  //creating preview thumbnails
                Bitmap thumbnail = ThumbnailUtils.createVideoThumbnail(entry.getKey().videoPath, MediaStore.Images.Thumbnails.MINI_KIND);
                try (FileOutputStream fos = new FileOutputStream(entry.getKey().thumbPath)) {
                    thumbnail.compress(Bitmap.CompressFormat.PNG, 100, fos);
                } catch (Exception e) {
                    Log.e(TAG, "output", e);
                }
            }
            publishProgress(entry.getKey().thumbPath, entry.getValue()); //setting preview to its imageview

        }
        return null;
    }

    @Override
    protected void onProgressUpdate(Object[] objects) {
        String path = (String) objects[0];
        ImageView image = (ImageView) objects[1];
        image.setImageBitmap(BitmapFactory.decodeFile(path));
    }

}