import com.google.android.gms.ads.AdRequest;
    import com.google.android.gms.ads.AdView;

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

    // Sample AdMob app ID: ca-app-pub-5289059028582551~3029249239
    MobileAds.initialize(this, "ca-app-pub-5289059028582551~3029249239");
    AdView adView = (AdView)findViewById(R.id.adView);
    AdRequest adRequest = new AdRequest.Builder().build();
    adView.loadAd(adRequest);

    editMessage = findViewById(R.id.editMessageE);
    mDatabase = FirebaseDatabase.getInstance().getReference().child("Messages");
    mMessageList = findViewById(R.id.messageRec);
    mMessageList.setHasFixedSize(true);
    LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
    linearLayoutManager.setStackFromEnd(true);
    mMessageList.setLayoutManager(linearLayoutManager);
    mAuth = FirebaseAuth.getInstance();

    mAuthListener = new FirebaseAuth.AuthStateListener() {
        @Override
        public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
            if (firebaseAuth.getCurrentUser() == null){
                startActivity(new Intent(MainActivity.this,RegisterActivity.class));
            }
        }
    };
}

<meta-data
        android:name="com.google.android.gms.version"
        android:value="@integer/google_play_services_version" />
        <activity android:name="com.google.android.gms.ads.AdActivity"
            android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize"
            android:theme="@android:style/Theme.Translucent" />