AsyncTask.java

   @SuppressLint("StaticFieldLeak")
    class DownloadFile extends AsyncTask<String, String, String> {
        int id = 1;
        NotificationManager mNotifyManager;
        NotificationCompat.Builder mBuilder;
        CountDownTimer cdt;

        /**
         * Before starting background thread
         * Show Progress Bar Dialog
         */
        @Override
        protected void onPreExecute() {
            mNotifyManager =
                    (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
                if (mNotifyManager  != null) {
                    mNotifyManager .createNotificationChannel(Objects.requireNonNull(createdownload(mNotifyManager )));
                }
            }
            mBuilder = new NotificationCompat.Builder(context,DOWNLOAD_CHANNEL_ID);
            mBuilder.setContentTitle("Downloading File")
                    .setContentText("name")
                    .setProgress(0, 100, false)
                    .setOngoing(true)
                    .setSmallIcon(R.mipmap.ic_launcher)
                    .setPriority(Notification.PRIORITY_LOW);
           // mBuilder.setProgress(100, 0, false);
        }


        protected String doInBackground(String... requestObjects) {

            String f_url = requestObjects[0];
            String fileName = "aa.pdf";
            int count;
            Log.d(TAG, "URL: "+ f_url);
            try {



                URL url = new URL(f_url);
                URLConnection conection = url.openConnection();
                conection.connect();
                // getting file length
                int lenghtOfFile = conection.getContentLength();

                mBuilder.setStyle(new NotificationCompat.BigTextStyle(mBuilder)
                        .setSummaryText(getStringSizeLengthFile(lenghtOfFile)));
                Log.d(TAG, "leam: "+getStringSizeLengthFile(lenghtOfFile));
                mNotifyManager.notify(id,mBuilder.build());

                // input stream to read file - with 8k buffer
                InputStream input = new BufferedInputStream(url.openStream(), 8192);

                // Output stream to write file
                //   OutputStream output = new FileOutputStream("/sdcard/"+f_url[i]);
                OutputStream output = new FileOutputStream(
                        folder.getAbsolutePath()+"/"+ fileName);

                Log.d(TAG,"out: "+folder.getAbsolutePath()+"/"+ fileName);


                byte[] data = new byte[1024];

                long total = 0;

                while ((count = input.read(data)) != -1) {
                    total += count;
                    // publishing the progress....
                    // After this onProgressUpdate will be called
                    publishProgress("" + (int) ((total * 100) / lenghtOfFile));

                    // writing data to file
                    output.write(data, 0, count);
                }


                // flushing output
                output.flush();

                // closing streams
                output.close();
                input.close();

            } catch (Exception e) {
                Log.e(TAG,"Error: ", e);
            }

            return null;
        }

        /**
         * Updating progress bar
         */

        /**
         * After completing background task
         * Dismiss the progress dialog
         **/
        @Override
        protected void onProgressUpdate(String... values) {
            // Update progress
            mBuilder.setProgress(100, Integer.parseInt(values[0]), false);
            mNotifyManager.notify(id, mBuilder.build());
            super.onProgressUpdate(values);
        }

        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            mBuilder.setContentText("Download complete");
            // Removes the progress bar
            mBuilder.setProgress(0, 0, false);
            mNotifyManager.notify(id, mBuilder.build());

        }
    }