Firebase: update an integer value

So im coding an inventory management app and I'm using a barcode scanner to scan my products. My idea is that every time that a product gets scanned I need to remove one quantity of it on my database. Like this: Scan product-->quantity=quantity-1

And this is my code

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mFirestore = FirebaseFirestore.getInstance();
        scannerView = new ZXingScannerView(this);
        setContentView(scannerView);
       

    private void updateData() {

   //myresult is the item barcode
        final DatabaseReference mDatabase = FirebaseDatabase.getInstance().getReference("Items").child(myresult).child("itemquantity");
        mDatabase.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                long qty =(long) dataSnapshot.getValue();
                mDatabase.setValue(qty - 1);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

    @Override
    public void onDestroy() {
        super.onDestroy();
        scannerView.stopCamera();
    }

   
    private void showMessageOKCancel(String message, DialogInterface.OnClickListener okListener) {
        new AlertDialog.Builder(addquantityactivity.this)
                .setMessage(message)
                .setPositiveButton("OK", okListener)
                .setNegativeButton("Cancel", null)
                .create()
                .show();
    }

    @Override
    public void handleResult(Result result) {
         myResult = result.getText();
        Log.d("QRCodeScanner", result.getText());
        Log.d("QRCodeScanner", result.getBarcodeFormat().toString());

        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Scan Result");
        builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                updateData();
                scannerView.resumeCameraPreview(addquantityactivity.this);
                
            }
        });
        builder.setNeutralButton("Visit", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                Intent browserIntent = new Intent(Intent.ACTION_VIEW, Uri.parse(myResult));
                startActivity(browserIntent);
            }
        });
        builder.setMessage(result.getText());
        AlertDialog alert1 = builder.create();
        alert1.show();

    }
}

i call updateData() in my handleResult function.
The error i get is :

> java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long

That's not the right way to decrement the value of a property in Firebase Realtime Database. Because we are creating apps that will be used in a multi-user environment, a more appropriate approach should be the use of transactions as explained in my answer from the following post:

> https://stackoverflow.com/questions/48307610/how-to-save-users-score-in-firebase-and-retrieve-it-in-real-time-in-android-stud

Or, in a more simpler way:

mDatabase.setValue(ServerValue.increment(-1));

Try this :

long qty = dataSnapshot.getValue(Long.class);