在android studio中将用户列表按字母顺序排序

Question

I am making a chat application and want to arrange user in users list from firebaseStore in alphabetical order. if new users register with the application, these users will be added to the list automatically and arrange in alphabetical order ( like A to B to C, ..). I try to search the answer but I don't find the solution to my issue. Can anyone help me with how to solve it?

Here is my java code in Userfragment.java:

recyclerView = view.findViewById(R.id.recycler_view);
    recyclerView.setHasFixedSize(true);
    recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));

    mUsers = new ArrayList<>();



    imageProfile = view.findViewById(R.id.profile_image);
    username = view.findViewById(R.id.username);

    storageReference = FirebaseStorage.getInstance().getReference("uploads");

    fuser = FirebaseAuth.getInstance().getCurrentUser();
   
    Query query = FirebaseDatabase.getInstance().getReference("Users").child(fuser.getUid()).orderByChild("username");


    Collections.sort(mUsers, new Comparator<User>() {
        @Override
        public int compare(User lhs, User rhs) {
            return lhs.getUsername().compareTo(rhs.getUsername());
        }
    });

    query.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

            if (isAdded()) {

                User user = dataSnapshot.getValue(User.class);
                username.setText(user.getUsername());
                if (user.getImageUrl().equals("default")) {
                    imageProfile.setImageResource(R.mipmap.ic_launcher);
                } else {
                    Glide.with(getContext()).load(user.getImageUrl()).into(imageProfile);
                }
            }

        }

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

        }
    });

below is some users in realtime database which I want to arrange:

Users 7HMH8nj6AshgYtvD2IMT9a7y2273
id "7HMH8nj6AshgYtvD2IMT9a7y2273" imageUrl "default" search "tony" status "online" username "Tony" AY3gY1eQE1fWJiNUsxxRqpXHBFf2
id "AY3gY1eQE1fWJiNUsxxRqpXHBFf2" imageUrl "default" search "hola" status "offline" username "hola" O6gRe8muq4TGDjQsYkn9Cdpufdy1
id "O6gRe8muq4TGDjQsYkn9Cdpufdy1" imageUrl "default" search "chon" status "offline" username "chon" SGQwWbIZ2vhkPcKqjMuejusKRY13
id "SGQwWbIZ2vhkPcKqjMuejusKRY13" imageUrl "default" search "miko" status "offline" username "miko"

Answer 1

Try this

FirebaseDatabase.getInstance().getReference().child("Users").orderByChild("username");

Learn more here

Answer 2

You're getting the following error:

java.lang.ClassCastException: com.google.firebase.database.Query cannot be cast to com.google.firebase.database.DatabaseReference

Because of the following line of code:

reference = (DatabaseReference) FirebaseDatabase.getInstance().getReference("Users").child(fuser.getUid()).orderByChild("username");

And this is because you are trying to cast an object of type Query to an object of type DatabaseReference , which is actually not possible in Java. The inheritance relationship is as follows, DatabaseReference extends Query and not the opposite.

To solve the problem, you should remove the cast to DatabaseReference and set the type of the object to be Query:

Query query =  FirebaseDatabase.getInstance().getReference("Users").child(fuser.getUid()).orderByChild("username");