public void incrementCounter() {
firebase.runTransaction(new Transaction.Handler() {
    @Override
    public Transaction.Result doTransaction(final MutableData currentData) {
        if (currentData.getValue() == null) {
            currentData.setValue(1);
        } else {
            currentData.setValue((Long) currentData.getValue() + 1);
        }

        return Transaction.success(currentData);
    }

    @Override
    public void onComplete(FirebaseError firebaseError, boolean committed, DataSnapshot currentData) {
        if (firebaseError != null) {
            Log.d("Firebase counter increment failed.");
        } else {
            Log.d("Firebase counter increment succeeded.");
        }
    }
});