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I want to calculate the average of each dish by their ratings and display in a bar graph, therefore I need the values of each dish with each thier rating

Example 3 people rated milk as {4,5,3} by each user from the user-specific node linkToJSONTree

I need an ArrayList which can store multiple lists inside each index of its own listing:

private void letMeCalculate() 

{



    databaseReference = FirebaseDatabase.getInstance().getReference("/Feedback/"+mdate+"/"+mealtime+"/");



    databaseReference.addValueEventListener(new ValueEventListener() {

        @Override

        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {



            for (DataSnapshot i :  dataSnapshot.getChildren())

            {

                stringArrayList.add(i.getKey());

            }



        }



        @Override

        public void onCancelled(@NonNull DatabaseError databaseError) {

        }

    });



    for (String s : stringArrayList)

    {

        databaseReference = FirebaseDatabase.getInstance().getReference("/Feedback/"+mdate+"/"+mealtime+"/"+s+"/");



        databaseReference.addValueEventListener(new ValueEventListener() {

            @Override

            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {



               for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren())

               {

                   NewDishRating newDishRating = dataSnapshot1.getValue(NewDishRating.class);

                   newDishRatingList.add(newDishRating);



               }



            }



            @Override

            public void onCancelled(@NonNull DatabaseError databaseError) {

            }

        });

    }// end of for loop

}// end of function

When using the stringArrayList outside the callback, it will always be empty due the asynchronous behaviour of the onDataChange() method. If you want to use the stringArrayList, you should use it only inside the callback. So to solve this, please move the second for loop inside the onDataChange() method right after the first for loop ends.

If you need to use that list outside the callback, I recommend you see the last part of my anwser from this post in which I have explained how it can be done using a custom callback. You can also take a look at this video for a better understanding.

I got the solution instead of using Array list of Array List I can create genericTypeIndicator follow this link for the solution

https://stackoverflow.com/a/41503903/5867698