Cannot find symbol AdView variable in Class ID Android Studio

Question

When I try to compile my project I received the following error:

C:\..\MainActivity.java:47: error: cannot find symbol
    adView = findViewById(R.id.home_banner);
                              ^
symbol:   variable home_banner
location: class id

Part of the code in MainActivity.java is:

public AdView adView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //new code for adds
    MobileAds.initialize(this, new OnInitializationCompleteListener() {
        @Override
        public void onInitializationComplete(@NonNull InitializationStatus initializationStatus) {
        }
    });
    adView = findViewById(R.id.home_banner);
    AdRequest adRequest=new AdRequest.Builder().build();
    adView.loadAd(adRequest);
    ...

Any ideas, please.