Java 恢复主要活动前如何显示admob间隙

我有一个包含2个活动的应用程序。我想在第二次活动结束时展示一则插播广告。你能告诉我这个密码是否正确吗

class secondActivity extends AppCompatActivity {
    private InterstitialAd mInterstitialAd;

    public void loadAd() {
        AdRequest adRequest = new AdRequest.Builder().build();
        InterstitialAd.load(
                this,
                "ca-app-pub-3940256099942544/1033173712",
                adRequest,
                new InterstitialAdLoadCallback() {
                    @Override
                    public void onAdLoaded(@NonNull InterstitialAd interstitialAd) {
                        // The mInterstitialAd reference will be null until
                        // an ad is loaded.
                        mInterstitialAd = interstitialAd;
                    }
                });
    }

    private void showInterstitial() {
        // Show the ad if it's ready. Otherwise toast and restart the game.
        if (mInterstitialAd != null) {
            mInterstitialAd.show(this);
        }
    }

    @Override
    public void onBackPressed() {
        showInterstitial();
        super.onBackPressed();
    }

    @Override
    protected void onResume() {
        super.onResume();
        loadAd();
    }
}

@Override
public void onBackPressed() {
  //Your code here
  super.onBackPressed();
  displayInterstitial();
}

// Invoke displayInterstitial() when you are ready to display an interstitial.
public void displayInterstitial() {
  if (interstitial != null) {
    if (interstitial.isLoaded()) {
      interstitial.show();
    }
  }
}

@Override
protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  //Call loadad on create
  loadAd();
}

public void loadAd() {
  interstitial = new InterstitialAd(this);
  interstitial.setAdUnitId(getString(R.string.fullscreeen_ad_id));
  AdRequest adRequest = new AdRequest.Builder().build();
  interstitial.loadAd(adRequest);
}