要实现Admob视频奖励只播放一次的功能，可以使用SharedPreferences来记录视频奖励的播放状态。以下是一个示例代码，演示了如何使用SharedPreferences来记录和检查视频奖励的播放状态：

import android.content.Context;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.widget.Toast;

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.AdRequest.Builder;
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.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 rewardedAd;
    private SharedPreferences sharedPreferences;

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

        // 初始化AdMob
        MobileAds.initialize(this, initializationStatus -> {});

        // 创建SharedPreferences实例，用于记录视频奖励的播放状态
        sharedPreferences = getSharedPreferences("MyPrefs", Context.MODE_PRIVATE);

        // 检查是否已经播放过视频奖励
        boolean isRewardPlayed = sharedPreferences.getBoolean("isRewardPlayed", false);
        if (!isRewardPlayed) {
            loadRewardedAd();
        } else {
            Toast.makeText(this, "视频奖励已播放过", Toast.LENGTH_SHORT).show();
        }
    }

    // 加载视频奖励广告
    private void loadRewardedAd() {
        Builder adRequestBuilder = new AdRequest.Builder();
        AdRequest adRequest = adRequestBuilder.build();

        RewardedAd.load(this, "YOUR_REWARDED_AD_UNIT_ID", adRequest, new RewardedAdLoadCallback() {
            @Override
            public void onAdLoaded(RewardedAd ad) {
                rewardedAd = ad;
                showRewardedAd();
            }

            @Override
            public void onAdFailedToLoad(LoadAdError error) {
                Toast.makeText(MainActivity.this, "加载视频奖励广告失败", Toast.LENGTH_SHORT).show();
            }
        });
    }

    // 播放视频奖励广告
    private void showRewardedAd() {
        if (rewardedAd != null) {
            rewardedAd.setFullScreenContentCallback(new FullScreenContentCallback() {
                @Override
                public void onAdShowedFullScreenContent() {
                    // 广告显示时，将播放状态标记为已播放
                    SharedPreferences.Editor editor = sharedPreferences.edit();
                    editor.putBoolean("isRewardPlayed", true);
                    editor.apply();
                }

                @Override
                public void onAdFailedToShowFullScreenContent(AdError adError) {
                    Toast.makeText(MainActivity.this, "播放视频奖励广告失败", Toast.LENGTH_SHORT).show();
                }

                @Override
                public void onAdDismissedFullScreenContent() {
                    rewardedAd = null;
                }
            });

            rewardedAd.show(this, new OnUserEarnedRewardListener() {
                @Override
                public void onUserEarnedReward(RewardItem rewardItem) {
                    // 用户完成了观看广告的要求，可以给予奖励
                    Toast.makeText(MainActivity.this, "获得奖励: " + rewardItem.getAmount() + " " + rewardItem.getType(), Toast.LENGTH_SHORT).show();
                }
            });
        }
    }
}

请注意，上述代码中的 "YOUR_REWARDED_AD_UNIT_ID" 需要替换为您自己的广告单元ID。此外，还需要在AndroidManifest.xml文件中添加适当的AdMob应用ID和权限声明。