Android manifest:

<?xml version="1.0" encoding="utf-8"?>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

    package="com.azim" >
 
 <uses-permission android:name="android.permission.INTERNET"/>

    <application

        android:allowBackup="true"

        android:icon="@drawable/ic_launcher"

        android:label="@string/app_name"

        android:theme="@style/AppTheme" >
        <activity

            android:name=".MainActivity"

            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

setp:2

activity_main.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    android:orientation="vertical"

    android:gravity="center">
 <TextView
     android:layout_width="@+id/textview"
     android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:text="Hello world"
/> </LinearLayout>

package wallpapers.project.andro.istone.neo.com.nwallpapers;

import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.content.UriPermission;
import android.net.Uri;
import android.os.Bundle;
import android.webkit.WebView;
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.google.firebase.database.GenericTypeIndicator;
import com.google.firebase.database.ValueEventListener;

import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Timer;
import java.util.TimerTask;

public class MainActivity extends Activity {
private FirebaseDatabase Version = FirebaseDatabase.getInstance();
    private DatabaseReference Ver = Version.getReference("version");
    private ChildEventListener childEventListener;
    private String package_name ="";
    private String your_version ="";
    private String latest_version = "";
    private HashMap<String, Object> map = new HashMap<>();
    private ArrayList<HashMap<String, Object>> map1 = new ArrayList<>();
    private Timer timer = new Timer();
    private TimerTask _timer;
 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate( savedInstanceState );
        setContentView( R.layout.activity_main );
  
     _timer = new TimerTask() {
            @Override
            public void run() {
                runOnUiThread( new Runnable() {
                    @Override
                    public void run() {
                        updata();

                    }
                });
            }
        };
        timer.schedule( _timer, (3500) );
    }
 private void main(){
        if (new_users.getString( "new", "" ).equals( "" )) {
            new_user.setAction( Intent.ACTION_VIEW );
            new_user.setClass( getApplicationContext(), SingupActivity.class );
            startActivity( new_user );
            finish();
        } else {
            new_user.setAction( Intent.ACTION_VIEW );
            new_user.setClass( getApplicationContext(), Wallpapers.class );
            startActivity( new_user );
            finish();
            new_user.setAction( Intent.ACTION_VIEW );
            new_user.setClass( getApplicationContext(), AdsActivity.class );
            startActivity( new_user );
        }

    }
   private void updata(){
        package_name = "wallpapers.project.andro.istone.neo.com.nwallpapers";
        try {
            android.content.pm.PackageInfo pinfo = getPackageManager().getPackageInfo(
                    package_name, android.content.pm.PackageManager.GET_ACTIVITIES);
            your_version = pinfo.versionName;
        } catch (Exception e){
            Toast.makeText( this, e.toString(), Toast.LENGTH_SHORT ).show();
        }
        DatabaseReference rootRef = Version.getReference();
        rootRef.child("version").addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot snapshot) {
                if (snapshot.exists()) {

                } else {
                    map = new HashMap<>();
                    map.put( "v", your_version );
                    Ver.child( "app" ).updateChildren( map );
                    map.clear();
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        } );
        Ver.addChildEventListener( new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                Ver.addListenerForSingleValueEvent( new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        map1 = new ArrayList<>();
                        try{
                            GenericTypeIndicator<HashMap<String,Object>> ind = new GenericTypeIndicator<HashMap<String, Object>>(){};
                            for (DataSnapshot data: dataSnapshot.getChildren()){
                                HashMap<String,Object> map = data.getValue(ind);
                                map1.add(map);
                            }
                        } catch (Exception e){
                           neoistone.message(MainActivity.this,e,null);
                        }
                        latest_version = map1.get((int)0).get("v").toString();
                        if(Double.parseDouble(latest_version) > Double.parseDouble(your_version)){
                            Intent send = new Intent();
                            send.setAction(Intent.ACTION_VIEW);
                            send.setClass(getApplicationContext(),UpdateActivity.class);
                            startActivity(send);
                            finish();
                        } else {
                            if (Double.parseDouble(your_version) > Double.parseDouble(latest_version)){
                                Ver.child("app").child("v").setValue(your_version);
                            } else {
                                main();
                            }
                        }
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                } );
            }

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

            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {

            }

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

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        } );
    }
}