How to arrange firebase database data in ascending or descending order?

Question

I have setup a firebase database and I was wondering how I'd make it so that my listView shows my data in ascending or descending order.

For example: if I wanted something thats the most expensive I'd have it at the top of my listView and the cheaper ones at the bottom.

Basically I'm trying to create a ranking activity where I've two seperate listviews that rank by cost and count individually.

This is what I have for my count:

  @Override
    protected void onStart() {
        super.onStart();

        itemlists=new ArrayList<String>();
        databaseItems.child("items").addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                for (DataSnapshot itemSnapShot : dataSnapshot.getChildren()) {
                    String itemname=itemSnapShot.child("itemName").getValue().toString();

                    itemlists.add(itemname);
                    adapter = new ArrayAdapter(SearchActivity.this,android.R.layout.simple_list_item_1,itemlists);
                    listFinalCount.setAdapter(adapter);

                }

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
    }

This will be a two step process: 1) store numbers as numbers instead of strings (as you do now) 2) read about how to get descending results from Firebase (e.g. here: https://stackoverflow.com/questions/34156996/firebase-data-desc-sorting-in-android) — Frank van Puffelen, Dec 08 '17 at 21:14
My thought process was retrieving my data from firebase and comparing it with others and then relisting it in listview? — somethingelse, Dec 08 '17 at 21:19

Alex Mamo · Accepted Answer · 2018-11-04T05:52:32.380

If you want to order the results of a Firebase database query, you need to use numbers as values and not Strings. If you keep the Strings, as I see in your screenshot, you need to know that the Strings are ordered lexicographically and I'm sure this is not what you are looking for.

The simplest way to order data in ascending or descending order, would be on client side. Firebase can order the results in ascending order by a given property by default. There is no method in Firebase to order in descending order but there a few tweaks that can be made.

If you are using a RecyclerView to display data, the simplest way would be to use the following code:

LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity());
layoutManager.setReverseLayout(true);
layoutManager.setStackFromEnd(true);
recyclerView.setLayoutManager(layoutManager);

This approch will reverse you the order.

Another approach would be to create your own adapter that extends FirebaseListAdapter and override getItem() method.

Another approach would be to get the data from the database, add it to a Collection, a List would be a good solution and then use Collections.reverse(yourList);.

score 0 · Answer 2 · edited Jul 13 '22 at 10:25

As stated in official documentation:

firebaseDatabase.getReference().child("items").orderByChild("itemCost").addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot snapshot) {
        //list.clear(); clear the list before updating.
        for (DataSnapshot dataSnapshot : snapshot.getChildren()) {
            // todo here
        }
        Adapter.notifyDataSetChanged();
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        System.out.println("The read failed: " + error.getCode());
    }
});

Note: Any node which does not have the child key we're querying on is sorted with a value of null, meaning it will come first in the ordering.

How to arrange firebase database data in ascending or descending order?

2 Answers2

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