ArrayList data sharing between java class

Question

I've fetched data from Firebase to the Dashboard (Fragment) Main class , and now I need to access that same ArrayList to another Fragment class. How do i share arraylists between two classes.

My Code from the Dashboard Class:

private void GetCategory() {


    DatabaseReference db = FirebaseDatabase.getInstance().getReference("Category");
    db.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            for (DataSnapshot userSnapshot : dataSnapshot.getChildren()) {
                Catergory_List cl = userSnapshot.getValue(Catergory_List.class);
                CatList.add(cl);

            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            throw databaseError.toException();
        }
    });
}

My Second Class

private Dashboard db;
            TExpenses= db.TrasactionExpenses();
        System.out.println(TExpenses.size());

And i tried this code and its not working

  public List<Catergory_List> TrasactionExpenses() {
        return CatList;
    }

Error Log

 java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List Dashboard.TrasactionExpenses()' on a null object reference

Any help would be appreciated..i'm unable to get the Values from Dashboard Class ArrayList to Class B

Where are you setting your `Dashboard` instance in your second class? All your code shows is you declaring the `db` variable without assigning it a value and then getting a null-pointer exception for calling a method on it. — Tim Hunter, Aug 13 '21 at 14:01
private List CatList; is under Dashboard class and Under onCreate viewCatList = new ArrayList<>(); — Android God, Aug 13 '21 at 14:06
Yes, but it is the `Dashboard` variable that is `null`, not `CatList`. — Maarten Bodewes, Aug 13 '21 at 14:07
Under Dashboard CatList am able to get the data but when i go the class b its not working and I tried do under onCreateView Dashboard db=new Dashboard(); still not — Android God, Aug 13 '21 at 14:09
You should use Java standards for your code, particularly in naming types, variables, and methods. — chrylis -cautiouslyoptimistic-, Aug 13 '21 at 14:13

score 1 · Answer 1 · answered Aug 13 '21 at 14:05

1

It's normal, you did not initialize your Dashboard !

answered Aug 13 '21 at 14:05

Yanis02015

65
6

score 1 · Answer 2 · answered Aug 13 '21 at 14:33

I'm not sure what the constructor for Dashboard looks like, but you'll need to initialize the Dashboard object. You've only declared declared the dashboard db. (See the first answer for initialization vs. instantiation vs. declaration Difference between initializing a class and instantiating an object?)

You'll need to do something like:

Dashboard db = new Dashboard();

in your second class.

ArrayList data sharing between java class

2 Answers2