FirebaseDatabaseHelper

public class FirebaseDatabaseHelper {
    private FirebaseDatabase mDatabase;
    private DatabaseReference mReferenceCoins;
    private List<Coin> coins = new ArrayList<>();

    public interface DataStatus{
        void DataIsLoaded(List<Coin> coins, List<String> keys);
        void DataIsInserted();
        void DataIsUpdated();
        void DataIsDeleted();
    }

    public FirebaseDatabaseHelper() {
        mDatabase = FirebaseDatabase.getInstance();
        mReferenceCoins = mDatabase.getReference("coins");
    }

    public void readCoins( final DataStatus dataStatus) {
        mReferenceCoins.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                coins.clear();
                List<String> keys = new ArrayList<>();
                for(DataSnapshot keyNode : dataSnapshot.getChildren()) {
                    keys.add(keyNode.getKey());
                    Coin coin = keyNode.getValue(Coin.class);
                    coins.add(coin);
                }
                dataStatus.DataIsLoaded(coins, keys);
            }

            @Override
            public void onCancelled(@NonNull @NotNull DatabaseError error) {

            }
        });
    }
}