App updates data in firebase real-time database but doesn't show anything in recyclerview

Question

I am trying to add data onto firbase realtime database and retrieve that data on a recyclerview.I am successfully able to do that but when i try to retrieve data from firebase its not happening even-though when i try to print my data on logacat window i am successfully able to do that.

MainActivity.java

    import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.shivam.fireapp.model.User;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {
    private  Button button;
    private  Button button2;
    EditText editText;
    EditText editText2;
    String TAG ="MyTag";
    private ChildEventListener mchildeventlistener;
    private RecyclerView mRecyclerView;
    private mModelAdapter modelAdapter;
    private List<User> mDataList;

    DatabaseReference mRootRef= FirebaseDatabase.getInstance().getReference();
    DatabaseReference mRefMain =mRootRef.child("Users");
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        button = (Button) findViewById(R.id.Senddata);
        button2 = (Button) findViewById((R.id.GetData));
        editText = (EditText) findViewById(R.id.Firstname);
        editText2 = (EditText) findViewById(R.id.Agee);
        mDataList = new ArrayList<>();

        mRecyclerView= findViewById(R.id.recyclerView);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
        modelAdapter = new mModelAdapter(this,mDataList);
        mRecyclerView.setAdapter(modelAdapter);
        mchildeventlistener = new ChildEventListener() {
            @Override
            public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {
                User user = dataSnapshot.getValue(User.class);
                mDataList.add(user);
                modelAdapter.notifyDataSetChanged();

                Log.d(TAG,"OnChildAdded : Name    "+ user.getFirst_name());
                Log.d(TAG,"OnChildAdded : ageeee    "+ user.getAgeeee());



            }

            @Override
            public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

            }

            @Override
            public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {

            }

            @Override
            public void onChildMoved(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

            }

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

            }
        };

        mRefMain.addChildEventListener(mchildeventlistener);





    }
    @Override
    protected void onStart(){
        super.onStart();
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                String Name = editText.getText().toString();
                String Age = editText2.getText().toString();

                User user = new User(Name,Age);

                String key = mRefMain.push().getKey();
                mRefMain.child(key).setValue(user);

                Toast.makeText(MainActivity.this,"Done",Toast.LENGTH_SHORT).show();
            }
        });







        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {



            }});



    }
    @Override
    protected void onDestroy(){
        super.onDestroy();
        mRefMain.removeEventListener(mchildeventlistener);
    }


}

activity_main.xml

    <?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <Button
        android:id="@+id/Senddata"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="32dp"
        android:text="Send Data"
        app:layout_constraintBottom_toBottomOf="@+id/GetData"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/GetData" />

    <EditText
        android:id="@+id/Firstname"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="32dp"
        android:layout_marginTop="41dp"
        android:layout_marginEnd="39dp"
        android:ems="10"
        android:hint="Enter Name"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toStartOf="@+id/Agee"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/GetData"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="160dp"
        android:layout_marginEnd="40dp"
        android:text="Get Data"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/Agee"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="41dp"
        android:layout_marginEnd="40dp"
        android:ems="10"
        android:hint="Entre a Age"
        android:inputType="textPersonName"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/Firstname"
        app:layout_constraintTop_toTopOf="parent" />

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="345dp"
        android:layout_marginTop="55dp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/Senddata" />
</androidx.constraintlayout.widget.ConstraintLayout>

mModelAdapter.java

    import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import com.shivam.fireapp.model.User;

import java.util.List;

public class mModelAdapter extends RecyclerView.Adapter<mModelAdapter.MyViewHolder> {
    private Context context;
    private List<User> mDataList;

    public mModelAdapter(Context context, List<User> mDataList) {
        this.context = context;
        this.mDataList = mDataList;
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View rootView= LayoutInflater.from(context).inflate(R.layout.recycler_view_item,parent,false);

        return new MyViewHolder(rootView);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        User user = mDataList.get(position);
        holder.textView.setText(user.getFirst_name()+"  |  "+user.getAgeeee());

    }

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

    public class MyViewHolder extends RecyclerView.ViewHolder{
        TextView textView;

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
            textView = itemView.findViewWithTag(R.id.textView);
        }
    }
}

User.java

    public class User {

    String first_name, ageeee;

    public User(String first_name, String ageeee) {
        this.first_name = first_name;
        this.ageeee = ageeee;
    }

    public User() {
    }

    public String getFirst_name() {
        return first_name;
    }

    public void setFirst_name(String first_name) {
        this.first_name = first_name;
    }

    public String getAgeeee() {
        return ageeee;
    }

    public void setAgeeee(String ageeee) {
        this.ageeee = ageeee;
    }
}

recycler_view_item.xml

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:text="TextView"
        android:textSize="24sp"
        android:gravity="center"/>
</LinearLayout>

Answer 1

I had the same problem. Its reason is that when firebase hasn't read data from realtime database yet, recyclerview has been made; and it hasn't have any data yet. For solving that you should make recyclerview when you are sure that firebase has read data. Then recyclerview will be made with correct data.

Answer 2

First i will recommend you don't pass list in adapter . Create setList method in adapter.

If you still passing list in adapter then TRY THIS

Replace this in line childadded method

  modelAdapter.notifyDataSetChanged();

With this two line

    modelAdapter = new mModelAdapter(this,mDataList); //this will recreate the adapter with new data
    mRecyclerView.setAdapter(modelAdapter); //Again set adapter to recyclerview

Answer 3

java inner class dos't access outer local declared variable.So you need do

 public List<User> mDataList = new ArrayList<>();

OR

use interface