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I have a list that is being used by two separate classes. I need this list to be defined in one of my onDataChange methods however, and this is giving me some issues. At the top of my class I have my list defined as this:

public List<String> openGames;

I have that there so my RecyclerView class can use it. I then initialize my list in an onDataChange method like this

 public void updateUI(final ListCallback callBack) { FCGames.addListenerForSingleValueEvent(new ValueEventListener() { @Override public void onDataChange(@NonNull DataSnapshot dataSnapshot) { numGames = (int) dataSnapshot.getChildrenCount(); openGames = new ArrayList<>(); . . .

Those dots are there just to say there is more code, but I don't think it is necessary for this problem.

In MyRecyclerViewer class, this list is being used and when the Activity opens it tries to get the size of the list like this:

@Override public int getItemCount() { return mData.size(); }

But since the list is not defined until some things in my app occur, like the onDataChange method being called upon, this throws me an error:

Attempt to invoke interface method 'int java.util.List.size()' on a null object reference

I know the cause of this is because the list is not defined before the size of the list is taken.

So what I have done to try to get around this is adding a null list size check when it takes the list size, and that does get rid of the error, but it makes my entire RecyclerViewer not function at all

Are there any work around for this?

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  • Why not initialize it in declaration? Commented Nov 3, 2018 at 5:04
  • If I initialize it in the declaration then the List doesn’t update in real-time like I would like to Commented Nov 3, 2018 at 5:13

2 Answers 2

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You should do public List<String> openGames = new ArrayList<>(); initialization in the field.

And setup the RecyclerView adapter outside of the Firebase call as well.

Within the Firebase Callback, do not make a new list. Rather use openGames.clear() to empty it out before adding new data, which will preserve the list instance that the RecyclerView is using.

With that, you must still call notify methods of the adapter let the UI know data has changed, which is what i assume you are missing if the RecyclerView "doesn't work"

If you want to receive data changes as they happen in Firebase, you're using the wrong method

... the addListenerForSingleValueEvent() method ... triggers once and then does not trigger again.

You'll need to use addValueEventListener

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6 Comments

Thanks for the response. My RecyclerView does work, it just doesn’t update in real time at the moment. Which field do you suggest I initialize the list? I will try to clear the data before adding it and let you know how that works, but can you clarify which field I should try initializing the list?
The public one, within the class definition
I'm not sure what you want by "real time", but by using ForSingleValueEvent method, I don't think that'll be repeatedly called, and so you're only reading from the database once
I am using the ForSingleValueEvent to call the updateUI method so it works out. I changed it up like you said so it clears the list before adding anything new, and I also re-declared my list elsewhere and it solved my problem. Thank you!
It'll work once, sure, but not again without manually calling your updateUI method. If you did the other value event listener method, it'll get called automatically in the background for you
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You have to validate if your List is not null in order to get the size of it.

@Override public int getItemCount() { return mData != null ? mData.size() : 0; }
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2 Comments

If you read the end of the question, sounds like that was already tried
Yeah that poses some more issues

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