我的 recyclerview 没有显示来自 firebase 的数据

Question

im develping an app with java which uses Firebase. Im trying to show some data from firebase that previously the user has introduced previously, but it does not show anything, and i dont know why. I put my code below:

My class userPojo:

public class userPojo {

    private String tipo;
    private String titulo;
    private String avance;
    private String plataforma; 
    private String uid;

    public userPojo() {

    }

    public String getTipo() {
    return tipo;
    }

    public void setTipo(String tipo) {
    this.tipo = tipo;
    }

     public String getUid() {
    return uid;
    }

    public String getTitulo() {

    return titulo;
    }

    public void setTitulo(String titulo) {
    this.titulo = titulo;
    }

     public String getAvance() {
    return avance;
    }

    public void setAvance (String avance) {
    this.avance = avance;
    }



    public String getPlataforma() {
    return plataforma;
    }

     
    public void setPlataforma(String plataforma) {
    this.plataforma = plataforma;
    }
}

Class Adapter:

public class Adapter extends RecyclerView.Adapter<Adapter.userpojoviewholder>  {

    List<userPojo> datos;
    public Adapter(List<userPojo> datos){
        this.datos=datos;
    }


    @NonNull
    @Override
    public userpojoviewholder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View v = LayoutInflater.from(parent.getContext()).inflate(R.layout.row_recycler, 
    parent,false);
    userpojoviewholder holder= new userpojoviewholder(v);
    return holder;
    }

    @Override
    public void onBindViewHolder(@NonNull userpojoviewholder holder, int position) {
        userPojo userpojo = datos.get(position);
        holder.textViewTipo.setText(userpojo.getTipo());
        holder.textViewTitulo.setText(userpojo.getTitulo());
        holder.textViewAvance.setText(userpojo.getAvance());
        holder.textViewPlataforma.setText(userpojo.getPlataforma());
    }

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

    public static class userpojoviewholder extends RecyclerView.ViewHolder{
        TextView textViewTitulo, textViewTipo,textViewAvance, textViewPlataforma;
         public userpojoviewholder(@NonNull View itemView) {
            super(itemView);
            textViewTipo=itemView.findViewById(R.id.textviewtipo);
             textViewTitulo=itemView.findViewById(R.id.textviewtitulo);
             textViewAvance=itemView.findViewById(R.id.textviewavance);
             textViewPlataforma=itemView.findViewById(R.id.textviewplataforma);
        }
        }
        }
My class visualizar:

       public class Visualizar extends AppCompatActivity {
        List<userPojo> datos;
       Adapter adapter;
       RecyclerView rv;
       String userid;
       @Override
       protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_visualizar);
       int nightModeFlags = this.getResources().getConfiguration().uiMode & 
        Configuration.UI_MODE_NIGHT_MASK;
         switch (nightModeFlags) {
         case Configuration.UI_MODE_NIGHT_YES:
            /* si esta activo el modo oscuro lo desactiva */
            AppCompatDelegate.setDefaultNightMode(
                    AppCompatDelegate.MODE_NIGHT_NO);
            break;
    /*    case Configuration.UI_MODE_NIGHT_NO:
            /* si esta desactivado el modo oscuro lo activa */
           /* AppCompatDelegate.setDefaultNightMode(
                    AppCompatDelegate.MODE_NIGHT_YES);
            break; */
    }
        userid= FirebaseAuth.getInstance().getUid();
         rv=  findViewById(R.id.recycler);
        rv.setLayoutManager(new LinearLayoutManager(this));

    datos=new ArrayList<>();

    FirebaseDatabase database= FirebaseDatabase.getInstance("https://proyecto-daniel-sanchez-default-rtdb.europe-west1.firebasedatabase.app");

    adapter=new Adapter(datos);
    rv.setAdapter(adapter);
    database.getReference().getRoot().addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot datasnapshot) {
            datos.removeAll(datos);
            for(DataSnapshot snapshot : datasnapshot.getChildren() ){
                userPojo user= snapshot.getValue(userPojo.class);
                datos.add(user);

            }
            adapter.notifyDataSetChanged();
        }

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

        }
    });
    }
}

this is my database on Firebase:

LogCat error:

2021-11-29 17:08:51.597 18298-18298/com.example.proyectofinaldanielsanchez 
D/ViewRootImpl: support adaptive color gamut feature!
2021-11-29 17:08:51.598 18298-18298/com.example.proyectofinaldanielsanchez 
V/ViewRootImpl: The specified message queue synchronization  barrier token 
has 
not been posted or has already been removed
2021-11-29 17:08:51.603 18298-18298/com.example.proyectofinaldanielsanchez 
D/ViewRootImpl[Archivos]: windowFocusChanged hasFocus=false 
inTouchMode=true
2021-11-29 17:08:51.627 18298-18298/com.example.proyectofinaldanielsanchez 
W/Choreographer: Already have a pending vsync event.  There should only be 
one 
at a time.
2021-11-29 17:08:51.649 18298-18298/com.example.proyectofinaldanielsanchez 
D/DecorView: onWindowFocusChangedFromViewRoot hasFocus: true, 
DecorView@f1d3935[Visualizar]
2021-11-29 17:08:51.652 18298-18298/com.example.proyectofinaldanielsanchez 
D/ViewRootImpl[Visualizar]: windowFocusChanged hasFocus=true 
inTouchMode=true
2021-11-29 17:09:02.011 18298-18298/com.example.proyectofinaldanielsanchez 
D/CompatibilityChangeReporter: Compat change id reported: 150939131; UID 
10617; state: ENABLED[Visualizar]

Im stucked right now, because i can't find the error, can someone help me? Thank You.

Answer 1

Under the root of your database you have two nested dynamic levels of nodes:

1. First there's a level that seems to be a UID.
2. Then there is a level that is a push key.

Since you read this entire data structure, you also need two nested loops in your onDataChange :

database.getReference().getRoot().addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot datasnapshot) {
        datos.removeAll(datos);
        for(DataSnapshot userSnapshot : datasnapshot.getChildren() ){ // 👈 add another loop
            for(DataSnapshot snapshot : userSnapshot.getChildren() ){
                userPojo user= snapshot.getValue(userPojo.class);
                datos.add(user);
            }
        }
        adapter.notifyDataSetChanged();
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        throw error.toException(); // 👈 never ignore errors
    }
});