<!--Permissions for internet-->
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

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

    <TextView
        android:id="@+id/idTVHead"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_margin="5dp"
        android:layout_marginTop="30dp"
        android:text="Firebase Firestore as Realtime Database"
        android:textAlignment="center"
        android:textAllCaps="false"
        android:textColor="@color/purple_500"
        android:textSize="20sp" />

    <TextView
        android:id="@+id/idTVUpdate"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/idTVHead"
        android:layout_marginTop="50dp"
        android:text="Updated Text"
        android:textAlignment="center"
        android:textColor="@color/purple_500"
        android:textSize="30sp" />

</RelativeLayout>

import android.os.Bundle;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;

import com.google.firebase.firestore.DocumentReference;
import com.google.firebase.firestore.DocumentSnapshot;
import com.google.firebase.firestore.EventListener;
import com.google.firebase.firestore.FirebaseFirestore;
import com.google.firebase.firestore.FirebaseFirestoreException;

public class MainActivity extends AppCompatActivity {

    // creating a variable for text view.
    TextView updatedTV;

    // initializing th variable for firebase firestore
    FirebaseFirestore db = FirebaseFirestore.getInstance();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // initializing our text view.
        updatedTV = findViewById(R.id.idTVUpdate);

        // creating a variable for document reference.
        DocumentReference documentReference = db.collection("MyData").document("Data");

        // adding snapshot listener to our document reference.
        documentReference.addSnapshotListener(new EventListener<DocumentSnapshot>() {
            @Override
            public void onEvent(@Nullable DocumentSnapshot value, @Nullable FirebaseFirestoreException error) {
                // inside the on event method.
                if (error != null) {
                    // this method is called when error is not null 
                    // and we gt any error
                    // in this cas we are displaying an error message.
                    Toast.makeText(MainActivity.this, "Error found is " + error, Toast.LENGTH_SHORT).show();
                    return;
                }
                if (value != null && value.exists()) {
                    // if th value from firestore is not null then we are getting
                    // our data and setting that data to our text view.
                    updatedTV.setText(value.getData().get("updateValue").toString());
                }
            }
        });
    }
}