Android org.json.JSONException: Value of type java.lang.String cannot be converted to JSONObject

try {
        File yourFile = new File(Environment.getExternalStorageDirectory(), "textarabics.txt");
        FileInputStream stream = new FileInputStream(yourFile);
        String jsonStr = null;
        try {
            FileChannel fc = stream.getChannel();
            MappedByteBuffer bb = fc.map(FileChannel.MapMode.READ_ONLY, 0, fc.size());

            jsonStr = Charset.defaultCharset().decode(bb).toString();
            Log.d("Noga Store", "jString = " + jsonStr);
          }
          finally {
            stream.close();
          }

        Log.d("Noga Store", "jString = " + jsonStr);
             JSONObject jsonObj = new JSONObject(jsonStr);

            // Getting data JSON Array nodes
            JSONArray data  = jsonObj.getJSONArray("data");

            // looping through All nodes
            for (int i = 0; i < data.length(); i++) {
                JSONObject c = data.getJSONObject(i);

                String id = c.getString("id");
                String title = c.getString("title");
                String duration = c.getString("duration");

                // tmp hashmap for single node
                HashMap<String, String> parsedData = new HashMap<String, String>();

                // adding each child node to HashMap key => value
                parsedData.put("id", id);
                parsedData.put("title", title);
                parsedData.put("duration", duration);


                // do what do you want on your interface
              }


       } catch (Exception e) {
       e.printStackTrace();
      }

at that point i am getting crash:

JSONObject jsonObj = new JSONObject(jsonStr);

and this is my json file into my sd card:

{
"data": [
    {
        "id": "1",
        "title": "Farhan Shah",
        "duration": 10,
    },
    {
        "id": "2",
        "title": "Noman Shah",
        "duration": 10,
    },
    {
        "id": "3",
        "title": "Ahmad Shah",
        "duration": 10,
    },
    {
        "id": "4",
        "title": "Mohsin Shah",
        "duration": 10,
    },
    {
        "id": "5",
        "title": "Haris Shah",
        "duration": 10,
    }
]

}