This class is used to request and display a rewarded ad.

 public class MainActivity extends AppCompatActivity {

     private RewardedAd rewardedAd;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         FullScreenContentCallback fullScreenContentCallback =
             new FullScreenContentCallback() {
                 @Override
                 public void onAdShowedFullScreenContent() {
                     // Code to be invoked when the ad showed full screen content.
                 }

                 @Override
                 public void onAdDismissedFullScreenContent() {
                     rewardedAd = null;
                     // Code to be invoked when the ad dismissed full screen content.
                 }
             };

         RewardedAd.load(
             this,
             "adUnitId",
             new AdRequest.Builder().build(),
             new RewardedAdLoadCallback() {
                 @Override
                 public void onAdLoaded(RewardedAd ad) {
                     findViewById(R.id.display_button).setVisibility(View.VISIBLE);
                     rewardedAd = ad;
                     rewardedAd.setFullScreenContentCallback(fullScreenContentCallback);
                 }
             });
     }

     public void onDisplayButtonClicked(View view) {
         if (rewardedAd != null) {
             rewardedAd.show(
                 this,
                 new OnUserEarnedRewardListener() {
                     @Override
                     public void onUserEarnedReward(@NonNull RewardItem rewardItem) {
                         Toast.makeText(
                             MainActivity.this,
                             "onRewarded! currency: "
                                 + rewardItem.getType() + "    amount: "
                                 + rewardItem.getAmount(), Toast.LENGTH_SHORT).show();
                     }
                 });
         }
     }
 }