I'm tryin to use Sharedpreferences to save an arraylist of objects, but toJson and fromJson are causing fatal exceptions

Question

My object contains a String, long, and int. Object is called Tracker and I have an arrayList of Trackers. When I try to use toJson i get an error. The same with fromJson.

Answer 1

It looks like you're trying to convert a list to json as an object instead of a list. You could just create a wrapper class to hold your list and serialize/deserialize that.

Create a simple class that acts as a wrapper for your List.

class TrackWrapper {
    List<Tracker> trackList;

    public TrackWrapper(List<Tracker> tracks){
        this.trackList = tracks;
    }
}

Add items to the list

Tracker trax = new Tracker(description, timeToCount, imageID);
track.add(trax);

Create an instance of your wrapper class and pass your list in

TrackWrapper wrapper = new TrackWrapper(trax);

Serialise the wrapper

Gson gson = new Gson();
String json = gson.toJson(wrapper);

Answer 2

You can take a look at my SharedPreferences helper

// Save list object to another SharedPreference
SharedPreferencesManager.getInstance(AnotherPreferenceName).putValue("guns", guns);
List<Gun> datas = SharedPreferencesManager.getInstance(AnotherPreferenceName).getValues("guns", Gun[].class); // get list object