Recyclerview won't show items, Firebase Admin SDK, Realtime Database

Question

I'm trying to retrieve some simple data from my database as an Admin, the data has been retrieved, but nothing is shown on the recyclerview.

In the other hand, i was trying to assign a value (String) from the database to a TextView, i was recieving the value, but the TextView was empty, i went to the .xml file and changed the layout_width from android:layout_width="wrap_content" to android:layout_width="match_parent" and it worked! Before that with the simple Firebase Client SDK it was showing the texte even with android:layout_width="wrap_content"

PS: Since i started using the Firebase Admin SDK, I didn't had any issues with writing data to the Realtime database, Or with creating Users. I tried both of them and they works fine.

Here's the retrieving function :

private void RetriveClasses() {
        // Get a reference to our posts
        final FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference ref = database.getReference("Classrooms");

        mclassrooms.clear();
        // Attach a listener to read the data at our posts reference
        ref.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for (DataSnapshot snapshot : dataSnapshot.getChildren())
                {
                    Classroom post = snapshot.getValue(Classroom.class);
                    if (!post.getDeleted().equals("true"))
                    {
                        mclassrooms.add(post);
                    }
                    mAdapter.notifyDataSetChanged();
                    System.out.println(post.getClassName());

                }
            }

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

The System.out.println(post.getClassName()); Shows :

I/System.out: L1 ST 2022-2023
    L1 ST 2021-2022
    L1 ST 2015-2016

These are the class names i'm looking for, which means the reading is succeed, but the recyclerview still won't show any items.

here's my Adapter:

public class ClassroomsAdapter extends RecyclerView.Adapter<ClassroomsAdapter.ViewHolder> {

    private Context c;
    public List<Classroom> mclassrooms;

    public ClassroomsAdapter(List<Classroom> mclassrooms) {
        this.mclassrooms = mclassrooms;
    }


    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        this.c = parent.getContext();
        View view = LayoutInflater.from(c).inflate(R.layout.classroom_item, parent, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        final Classroom classroom = mclassrooms.get(position);

        if (classroom.getClassName().length() >= 20)
        {
            String className = classroom.getClassName();
            holder.classroom_name.setText(className.substring(0,19) + "...");
        }
        else
        {
            holder.classroom_name.setText(classroom.getClassName());
        }

        holder.itemView.setOnLongClickListener(new View.OnLongClickListener() {
            @Override
            public boolean onLongClick(View view) {
                final BottomSheetDialog bottomSheetDialog = new BottomSheetDialog(c, R.style.BottomSheetDialogTheme);
                View eview = LayoutInflater.from(c).inflate(R.layout.bottom_menu_classroom, null);

                eview.findViewById(R.id.Delete_Comment_Btn).setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        final ProgressDialog progressDialog = new ProgressDialog(c, R.style.MyAlertDialogStyle);
                        progressDialog.setTitle("Suppression");
                        progressDialog.setMessage("Traitement...");
                        progressDialog.setCanceledOnTouchOutside(false);
                        progressDialog.show();
                        DatabaseReference mRoot = FirebaseDatabase.getInstance().getReference("Classrooms");
                        mRoot.child(classroom.getClassId()).child("deleted").setValue("true", new DatabaseReference.CompletionListener(){

                            @Override
                            public void onComplete(DatabaseError error, DatabaseReference ref) {
                                progressDialog.dismiss();
                                Toast.makeText(c, "Suppression avec succès!", Toast.LENGTH_SHORT).show();
                                bottomSheetDialog.dismiss();
                            }
                        });
                    }
                });
                eview.findViewById(R.id.copy_comment_button).setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        final BottomSheetDialog bs = new BottomSheetDialog(c, R.style.BottomSheetDialogTheme);
                        View aview = LayoutInflater.from(c).inflate(R.layout.edit_class_name, null);
                        EditText name_et = aview.findViewById(R.id.Edition_Comment_Input_Field);
                        name_et.setText(classroom.getClassName());
                        aview.findViewById(R.id.update_comment).setOnClickListener(new View.OnClickListener() {
                            @Override
                            public void onClick(View view) {
                                if (name_et.getText().toString().isEmpty()) {
                                    Toast.makeText(c, "Ce champ est requis*", Toast.LENGTH_SHORT).show();
                                } else {
                                    final ProgressDialog progressDialog = new ProgressDialog(c, R.style.MyAlertDialogStyle);
                                    progressDialog.setTitle("Mise à jour");
                                    progressDialog.setMessage("Traitement...");
                                    progressDialog.setCanceledOnTouchOutside(false);
                                    progressDialog.show();
                                    DatabaseReference mRoot = FirebaseDatabase.getInstance().getReference("Classrooms");
                                    mRoot.child(classroom.getClassId()).child("className").setValue(name_et.getText().toString(), new DatabaseReference.CompletionListener() {
                                        @Override
                                        public void onComplete(DatabaseError error, DatabaseReference ref) {
                                            progressDialog.dismiss();
                                            Toast.makeText(c, "Mise à jour avec succès!", Toast.LENGTH_SHORT).show();
                                            bs.dismiss();
                                            bottomSheetDialog.dismiss();
                                        }
                                    });
                                }
                            }
                        });
                        aview.findViewById(R.id.close_edit_comment).setOnClickListener(new View.OnClickListener() {
                            @Override
                            public void onClick(View view) {
                                bs.dismiss();
                            }
                        });
                        bs.setContentView(aview);
                        bs.show();
                    }
                });
                bottomSheetDialog.setContentView(eview);
                bottomSheetDialog.show();


                return false;
            }
        });

    }

    @Override
    public int getItemCount() {
        return mclassrooms.size();
    }

    public class ViewHolder extends RecyclerView.ViewHolder {

        private TextView classroom_name;
        public ViewHolder(@NonNull View itemView) {
            super(itemView);

            classroom_name = itemView.findViewById(R.id.class_name);
        }
    }
}

Thanks for your time.