Five Steps for downloading a file using AsyncTask in android

1. Declare the reference of ProgressDialog class in your Activity globally. Ex:-

   ProgressDialog mProgressDialog;

2. Instantiate the above reference in onCreate() method of activity. Ex:-

   mProgressDialog = new ProgressDialog(this); // here this is the object of your activity mProgressDialog.setMessage("Please wait");

3. Create a class MyAsyncTask in same activity.

    private class MyAsyncTask extends AsyncTask {

    private Context context;

    public MyAsyncTask(Context context) {
        this.context = context;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        mProgressDialog.show();
    }

    @Override
    protected String doInBackground(String... sUrl) {
        // take CPU lock to prevent CPU from going off if the user 
        // presses the power button during download
        PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
        PowerManager.WakeLock wl = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, getClass().getName());
        wl.acquire();

        try {
            InputStream input = null;
            OutputStream output = null;
            HttpURLConnection connection = null;
            try {
                URL url = new URL(sUrl[0]);
                connection = (HttpURLConnection) url.openConnection();
                connection.connect();

                // expect HTTP 200 OK, so we don't mistakenly save error report 
                // instead of the file
                if (connection.getResponseCode() != HttpURLConnection.HTTP_OK)
                     return "Server returned HTTP " + connection.getResponseCode() 
                         + " " + connection.getResponseMessage();

                // this will be useful to display download percentage
                // might be -1: server did not report the length
                int fileLength = connection.getContentLength();

                // download the file
                input = connection.getInputStream();
                output = new FileOutputStream("/sdcard/file_name.extension");

                byte data[] = new byte[4096];
                long total = 0;
                int count;
                while ((count = input.read(data)) != -1) {
                    // allow canceling with back button
                    if (isCancelled())
                        return null;
                    total += count;
                    // publishing the progress....
                    if (fileLength > 0) // only if total length is known
                        publishProgress((int) (total * 100 / fileLength));
                    output.write(data, 0, count);
                }
            } catch (Exception e) {
                return e.toString();
            } finally {
                try {
                    if (output != null)
                        output.close();
                    if (input != null)
                        input.close();
                } 
                catch (IOException ignored) { }

                if (connection != null)
                    connection.disconnect();
            }
        } finally {
            wl.release();
        }
        return null;
    }

    @Override
    protected void onProgressUpdate(Integer... progress) {
        super.onProgressUpdate(progress);
        // if we get here, length is known, now set indeterminate to false
        mProgressDialog.setIndeterminate(false);
        mProgressDialog.setMax(100);
        mProgressDialog.setProgress(progress[0]);
    }

    @Override
    protected void onPostExecute(String result) {
        mProgressDialog.dismiss();
        if (result != null)
            Toast.makeText(context,"Download error: "+result, Toast.LENGTH_LONG).show();
        else
            Toast.makeText(context,"File downloaded", Toast.LENGTH_SHORT).show();
    }
}

4. Call MyAsyncTask class in your onCreate() Method of your Activity.

    MyAsyncTask object = new MyAsyncTask(this);
    object.execute("URL Goes here")

5. Add following permission in your manifest

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WAKE_LOCK" />