Retrive ArrayList from SharedPrefernce

I have function for save and get arraylist in my application like below

public void saveArrayList(ArrayList<String> list, String key){

        Gson gson = new Gson();
        String json = gson.toJson(list);
        editor.putString(key, json);
        editor.apply();     // This line is IMPORTANT !!!
    }

    public ArrayList<String> getArrayList(String key){
        Gson gson = new Gson();
        String json = pref.getString(key, "");
        Type type = new TypeToken<ArrayList<String>>() {}.getType();
        return gson.fromJson(json, type);
    }

I am trying to retrive it in my one of fragment like below

ArrayList<String> UserVideoLang;

and on create its like below

if (getActivity() != null) {
            customDialogBuilder = new CustomDialogBuilder(getActivity());
            parentViewModel = ViewModelProviders.of(getActivity()).get(HomeViewModel.class);
            sessionManager = new SessionManager(getActivity());
            UserVideoLang= sessionManager.getArrayList("UserVideoLang");
            if(UserVideoLang.isEmpty()){
                UserVideoLang.add("all");
            }
        }

when I run I am getting NullPointerException on below line

if(UserVideoLang.isEmpty()){

and full error is like below

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.isEmpty()' on a null object reference
        at com.vedagames.wowtok.view.home.ForUFragment.onActivityCreated

Let me know what I am missing and how I should fix it, Thanks!

Replace your getArrayList method with this

public ArrayList<String> getArrayList(String key){
        Gson gson = new Gson();
        String json = pref.getString(key, "[]");
        Type type = new TypeToken<ArrayList<String>>() {}.getType();
        return gson.fromJson(json, type);
    }

passing default value [] means empty JSON array. so if no value in shared preference empty array list will be returned.

Sharedpref is used to same small data for application. I don't know the size of the arraylist and single or multiple arraylist. If your data is large, you should choose Room Database for saving the data. You can check Room on this link.

OR,
For null check, first check

UserVideoLang != null

then check

UserVideoLang.isEmpty()

If there is problem with parsing Gson to Class object, you can use a easier technique for this. Like...

UserVideoLangContainer container = new Gson().fromJson(userInfo, UserVideoLangContainer.class);

Where UserVideoLangContainer.class is like

public class UserVideoLangContainer implements Serializable {
    public List<String> list;
}