AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.learnoset.googlerewardedvideoad">

    <uses-permission android:name="android.permission.INTERNET" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.GoogleRewardedVideoAd">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <!---Always test your ads with test app id -->
        <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-3940256099942544~3347511713" />
    </application>

</manifest>

gradle.build (app)

dependencies {
    
    // Implement Google Ads Dependency 
    implementation 'com.google.android.gms:play-services-ads:20.5.0'
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/showAdBtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="Show Ad" />

</RelativeLayout>

MainActivity.java

package com.learnoset.googlerewardedvideoad;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import com.google.android.gms.ads.AdError;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.FullScreenContentCallback;
import com.google.android.gms.ads.LoadAdError;
import com.google.android.gms.ads.MobileAds;
import com.google.android.gms.ads.OnUserEarnedRewardListener;
import com.google.android.gms.ads.initialization.InitializationStatus;
import com.google.android.gms.ads.initialization.OnInitializationCompleteListener;
import com.google.android.gms.ads.rewarded.RewardItem;
import com.google.android.gms.ads.rewarded.RewardedAd;
import com.google.android.gms.ads.rewarded.RewardedAdLoadCallback;

public class MainActivity extends AppCompatActivity {

    private RewardedAd rewardedAd2 = null;

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

        final Button showAdBtn = findViewById(R.id.showAdBtn);

        // Add below line in your starting/launcher activity like SplashScreen etc.
        MobileAds.initialize(this, new OnInitializationCompleteListener() {
            @Override
            public void onInitializationComplete(InitializationStatus initializationStatus) {
            }
        });


        // making ad request to google's servers
        AdRequest adRequest = new AdRequest.Builder().build();

        //replace the test ad unit ID with your original ad unit id. my ad unit id = ca-app-pub-3299203761326949/6263598439
        // loading requested ad
        RewardedAd.load(this, "ca-app-pub-3940256099942544/5224354917",
                adRequest, new RewardedAdLoadCallback() {
                    @Override
                    public void onAdFailedToLoad(@NonNull LoadAdError loadAdError) {
                        rewardedAd2 = null;
                        // ad load failed
                    }

                    @Override
                    public void onAdLoaded(@NonNull RewardedAd rewardedAd) {

                        // assign loaded ad to rewardedAd2
                        rewardedAd2 = rewardedAd;

                        // ad loaded successfully
                        // show ad over the Activity
                        rewardedAd.show(MainActivity.this, new OnUserEarnedRewardListener() {
                            @Override
                            public void onUserEarnedReward(@NonNull RewardItem rewardItem) {

                                // your reward amount
                                int rewardAmount = rewardItem.getAmount();

                                // you can also give custom rewards here Ex.
                                rewardAmount = 200;

                                // your reward item type
                                String rewardType = rewardItem.getType();
                            }
                        });

                        // listening for ad events
                        rewardedAd.setFullScreenContentCallback(new FullScreenContentCallback() {
                            @Override
                            public void onAdShowedFullScreenContent() {
                                Toast.makeText(MainActivity.this, "Ad is Showing", Toast.LENGTH_SHORT).show();
                            }

                            @Override
                            public void onAdFailedToShowFullScreenContent(AdError adError) {
                                rewardedAd2 = null;
                                Toast.makeText(MainActivity.this, "Failed to show ad", Toast.LENGTH_SHORT).show();
                            }

                            @Override
                            public void onAdDismissedFullScreenContent() {
                                Toast.makeText(MainActivity.this, "Ad closed or dismissed", Toast.LENGTH_SHORT).show();
                            }
                        });

                    }
                });

        showAdBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                // showing ad on button click
                // checking if ad is loaded
                if (rewardedAd2 != null) {
                    rewardedAd2.show(MainActivity.this, new OnUserEarnedRewardListener() {
                        @Override
                        public void onUserEarnedReward(@NonNull RewardItem rewardItem) {

                            // getting reward amount
                            int rewardAmount = rewardItem.getAmount();

                            // you can also give custom rewards here Ex.
                            rewardAmount = 200;

                            // your reward item type
                            String rewardType = rewardItem.getType();
                        }
                    });
                }
            }
        });
    }
}