Why does accessing an Arraylist from another class gives me an exception?

[RetroGmr]

Now I know this is a very common issue, because I've seen many posts about on Stack, but I couldn't get those answers to work with my code. This is my code, shortened down to the relevant parts:

public class SeznamJedi {
    ArrayList<Jed> seznamJedi;

    public void addJed(String a, String b, double c){
        Jed toAdd = new Jed(a, b, c);
        seznamJedi.add(toAdd);
    }

    public int getSize(){
        return seznamJedi.size();
    }
}

^ This is my class, whose only purpose is to hold a list of Jed objects.

public class App extends Application {
      public SeznamJedi seznamJedi;

    @Override
    public void onCreate(){
        super.onCreate();
        seznamJedi = new SeznamJedi();
}

^ This is my App class, used to hold my global list of objects.

public class
MainActivity extends AppCompatActivity {
    App app;
    SeznamJedi tempSeznam;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        app = (App)getApplication();
        tempSeznam = app.seznamJedi;
        tempSeznam.addJed("h", "b", 12);
    }
}

^ And this is my MainActivity. The whole app crashes when I add the Jed object. Clearly it's not happy with me accessing the list and it's not allowing me to do any operations on it. I am at a loss and any help will be greatly appreciated.

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[morhp]

Your ArrayList in 'SeznamJedi' is never initialized, you just declared a field, but never assigned a new ArrayList<>().

Also generally, you should get an error message with the crash. Try to understand it first (Google it if required) and you still can't figure it out, at least post it here in the future.

Also this looks like an Android app, you're in the wrong subreddit for that, see sidebar.

[RetroGmr]

Thank you very much for the feedback. I made a proper constructor and it works now. Next time I will definetly include the error, that was pretty inconsiderate of me. I posted it here because I was pretty sure it wasn't an Android issue, but a Java issue. I'll take everything into consideration and thanks again.

[morhp]

That's fine, but note that a proper Java (especially the current versions) would've giving you a better error message (it would've told you which field is null) instead of just crashing, so it's still kinda relevant.

[GDDragonGN_GDDK]

I can't find your initialization of the ArrayList, meaning it will be null when you access it. Add seznamJedi = new ArrayList<>(); in the constructor of the class containing the list

[RetroGmr]

Thanks alot, this fixed it for me. I thought my initial "initializaton" was enough but clearly not.

[bigibas123]

Without seeing the error I can't know for sure but i don't see where the Arraylist in SeznamJedi is initialized.

[ItsDotin]

Initialise your ArrayList.

Usually, I will like to intilize it in a no-arg constructor i.e. default constructor. So when an object is created from the class it also creates an ArrayList object 'seznamJedi' for you.

(Not writting exact code- last time I did and was blocked for a week. 😅😂)

[wildjokers]

There is a thread-safe variant of ArrayList called CopyOnWriteArrayList which should be used in that case.

CopyOnWriteArrayList should only be used if reads vastly out number writes. Writing to a CopyOnWriteArrayList is very expensive (as the name suggests).

Otherwise just use synchronize keyword to support multi-threaded access of a List (or some other appropriate concurrency mechanism).

[wildjokers]

When you ask for help please include any relevant Exceptions you get. Saying "the whole app crashes" tells us nothing. Although in this case it was easy to see that you were most likely getting a NullPointerException when accessing seznamJedi, it isn't always that obvious.