1. 替换@OnLifecycleEvent(ON_START)为@OnLifecycleEvent(Lifecycle.Event.ON_START)。

2. 在onCreate()方法中初始化AppOpenAd，并在onStart()或onResume()方法中加载广告。以下是示例代码：

public class MainActivity extends AppCompatActivity implements LifecycleObserver {

    private AppOpenAd appOpenAd;
    private AppOpenAd.AppOpenAdLoadCallback loadCallback;

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

        // 初始化AppOpenAd
        MobileAds.initialize(this, new OnInitializationCompleteListener() {
            @Override
            public void onInitializationComplete(InitializationStatus initializationStatus) {
                AdRequest adRequest = new AdRequest.Builder().build();
                AppOpenAd.load(MainActivity.this, getString(R.string.admob_app_open_ad_id), adRequest, AppOpenAd.APP_OPEN_AD_ORIENTATION_PORTRAIT, new AppOpenAd.AppOpenAdLoadCallback() {
                    @Override
                    public void onAdLoaded(@NonNull AppOpenAd ad) {
                        appOpenAd = ad;
                        if (loadCallback != null) {
                            loadCallback.onAdLoaded(ad);
                        }
                    }

                    @Override
                    public void onAdFailedToLoad(@NonNull LoadAdError error) {
                        Log.e("AppOpenAd", "onAdFailedToLoad: " + error.getMessage());
                        if (loadCallback != null) {
                            loadCallback.onAdFailedToLoad(error);
                        }
                    }
                });
            }
        });      
    }

    // 加载广告
    private void loadAd() {
        if (appOpenAd != null) {
            appOpenAd.show(this);
        } else {
            loadCallback = new AppOpenAd.AppOpenAdLoadCallback() {
                @Override
                public void onAdLoaded(@NonNull AppOpenAd ad) {
                    appOpenAd = ad;
                    if (loadCallback != null) {
                        loadCallback.onAdLoaded(ad);
                    }
                }

                @Override
                public void onAdFailedToLoad(@NonNull LoadAdError error) {
                    Log.e("AppOpenAd", "onAdFailedToLoad: " + error.getMessage());
                    if (load