Gson & SharedPreferences lead to a NullPointerException

Question

I'm trying to put an ArrayList of self-created objects in SharedPreferences. While the saving seems to work, I get the following error when getSharedPreferences() is called:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object com.google.gson.Gson.fromJson(java.lang.String, java.lang.reflect.Type)' on a null object reference

The code where I put it into SharedPreferences:

// FYI: gson = new Gson(); is set earlier on
// same goes for: ArrayList<MovieModel> movieModelList = new ArrayList<>();
// and: Type listType;

movieModelList.add(result);

String jsonMovieModelList = gson.toJson(movieModelList);

SharedPreferences prefs = getSharedPreferences("settings",
   MODE_PRIVATE);
SharedPreferences.Editor editor = prefs.edit();
editor.putString("jsonMovieModelList", jsonMovieModelList);
editor.apply();

And the SharedPreferences class:

public void getSharedPreferences() {
    SharedPreferences prefs = getSharedPreferences("settings",
            MODE_PRIVATE);

    String jsonMovieModelList = prefs.getString("jsonMovieModelList", null);
    Log.d("TAG", "Message" + jsonMovieModelList); // output is as expected

    // if not filled in, no action necessary
    if (jsonMovieModelList != null) {

        listType = new TypeToken<ArrayList<MovieModel>>(){}.getType();
        ArrayList<MovieModel> movieModelListSaved = gson.fromJson(jsonMovieModelList, listType);

        MovieModel lastMovie = movieModelListSaved.get(movieModelListSaved.size() - 1);

        showTitleSecond = (TextView)findViewById(R.id.show_title_2);
        showTitleSecond.setText(lastMovie.getTitle());
    }
}

The gson code is based on: https://google.github.io/gson/apidocs/com/google/gson/Gson.html

To be clear: I do understand what a NullPointerException is, but I don't understand why I receive one in this situation since I check before executing that the String jsonMovieModelList is at least not null. Why is gson 'losing' the reference?

I look at this question: Android ArrayList of custom objects - Save to SharedPreferences - Serializable? and the only difference I see is that I have an ArrayList

score 1 · Accepted Answer · answered Sep 24 '16 at 18:07

1

Seems like you've not initialized variable gson before its usage in this method. Just add Gson gson = new GsonBuilder().create(); before the first usage, or in this method you're calling and that should fix it.

answered Sep 24 '16 at 18:07

Vucko

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Thanks for looking, but, as I state in a commented line I initialize gson earlier on with gson = new Gson(); Any other suggestion? – CorneeldH Sep 24 '16 at 18:18
Uhm, show the whole code, maybe the variable scope has something to do with this... – Vucko Sep 24 '16 at 21:13
Reading your comment and re-reading your answer, you were right! In the getSharedPreferences I didn't initialize gson and I load this method before anything else when the app is restarted. So, apologies from my side and thanks for answer + comment :) – CorneeldH Sep 25 '16 at 08:04
No problem, I assumed it was something like that, happened to me bunch of times, that's why I knew :D – Vucko Sep 25 '16 at 16:18

Gson & SharedPreferences lead to a NullPointerException

1 Answers1