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);
         rewardedAd = new RewardedAd(this, "adUnitId");
         ServerSideVerificationOptions serverSideVerificationOptions =
             new ServerSideVerificationOptions.Builder().setUserId("userId").build();
         rewardedAd.setServerSideVerificationOptions(serverSideVerificationOptions);
         AdRequest adRequest = new AdRequest.Builder().build();
         rewardedAd.loadAd(adRequest, new RewardedAdLoadCallback() {
             @Override
             public void onRewardedAdLoaded() {
              findViewById(R.id.display_button).setVisibility(View.VISIBLE);
             }
         });
     }

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