I actually want to retrieve data from the firebase database in order of recent one first

Question

Here is my Database Reference:

firebaseDatabase = FirebaseDatabase.getInstance();
    myRef = (DatabaseReference) firebaseDatabase.getReference().child("Notifications").orderByChild("Date");

Here is a snapshot of my Firebase console:

Answer 1

You can retrieve recent data as follow:

List<Notifications> notificationList = new ArrayList();
firebaseDatabase = FirebaseDatabase.getInstance();  
myRef = (DatabaseReference) firebaseDatabase.getReference().child("Notifications").orderByKey();
 myRef.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                        Notifications notification = snapshot.getValue(Notifications.class);
                        notificationList.add(0,notification);
                }

            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

With firebase you should do some of the query logic client side. descending order and ascending orders are not available on Firebase.

Answer 2

Here is what I do to retrieve data from Firebase:

1) Make helping methods in order to attach and detach your Listener. This code will be called whenever something has changed in your real time database.

// Firebase instance variables
private FirebaseDatabase mFirebaseDatabase;
private DatabaseReference mNotificationsDatabaseReference;
private ChildEventListener mChildEventListener;

private void attachDatabaseReadListener() {
        if (mChildEventListener == null) {
            mChildEventListener = new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {

                    Notification currentNotification= dataSnapshot.getValue(Notification.class); //deserialize data (make it an object again)
                    mNotificationAdapter.add(currentNotification);

                    //Store your data here in what order you like.
                }

                public void onChildChanged(DataSnapshot dataSnapshot, String s) {
                }

                public void onChildRemoved(DataSnapshot dataSnapshot) {
                }

                public void onChildMoved(DataSnapshot dataSnapshot, String s) {
                }

                public void onCancelled(DatabaseError databaseError) {
                }
            };
            mNotificationsDatabaseReference.addChildEventListener(mChildEventListener);
        }
    }



private void detachDatabaseReadListener() {
    if (mChildEventListener != null) {
        mNotificationsDatabaseReference.removeEventListener(mChildEventListener);
        mChildEventListener = null;
    }
}

2) Don't forget to attach and detach the listener onResume and onPause.

@Override
protected void onResume() {
    super.onResume();
    attachDatabaseReadListener();  
}

@Override
protected void onPause() {
    super.onPause();
    mNotificationAdapter.clear(); //Delete data in adapter which populates a listview. Otherwise you will end up with double items.
    detachDatabaseReadListener();
}

For extra information and help you should read the official documentation

and if you really want to understand the basics of Firebase I recommend you see this amazing tutorial (made from Google).