이건 MainActivity .java이구요.

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;

//광고로드
import com.google.android.gms.ads.AdListener;
import com.google.android.gms.ads.AdRequest;

//배너광고
import com.google.android.gms.ads.AdView;

//전면광고
import com.google.android.gms.ads.InterstitialAd;

public class MainActivity extends AppCompatActivity {

    //배너광고
    private AdView mAdView;

    //전면광고
    private InterstitialAd mInterstitialAd;

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

        //배너광고
        mAdView = findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder().build();
        mAdView.loadAd(adRequest);

        //전면광고
        String Interstitial_id = MyApplication.Interstitial_ad_unit_id;

        mInterstitialAd = new InterstitialAd(this);
        mInterstitialAd.setAdUnitId(Interstitial_id);
        mInterstitialAd.loadAd(new AdRequest.Builder().build());

    }

    @Override
    public void onBackPressed() {
        // 혹시 웹 뷰를 사용하는 액티비티일 경우 웹뷰 뒤로가기를 구현해준다.
                      /*   if (webView.canGoBack()) {
                             webView.goBack();
                         } else { */
        if (mInterstitialAd.isLoaded()) {
            mInterstitialAd.show();
            mInterstitialAd.setAdListener(new AdListener() {
                @Override
                public void onAdClosed() {
                    // 사용자가 광고를 닫으면 뒤로가기 이벤트를 발생시킨다.
                    finish();
                }
            });
        } else {
            super.onBackPressed();
        }
    }

}

이건 activity_main.xlm입니다.

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="9">


    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!" />

</LinearLayout>

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:layout_weight="1">


    <com.google.android.gms.ads.AdView
        android:id="@+id/adView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_gravity="center"
        ads:adSize="BANNER"
        ads:adUnitId="@string/banner_ad_unit_id">
    </com.google.android.gms.ads.AdView>

</LinearLayout>

string.xml

<resources>
    <string name="app_name">kidsyube</string>


    <!-- 테스트용-->
    <string name="APPLICATION_ID">ca-app-pub-3940256099942544~3347511713</string>
    <string name="banner_ad_unit_id">ca-app-pub-3940256099942544/6300978111</string>
    <string name="Interstitial_ad_unit_id">ca-app-pub-3940256099942544/1033173712</string>



</resources>

MyApplication.java

import android.app.Application;

public class MyApplication extends Application {

    public static String Interstitial_ad_unit_id;
    public static String APPLICATION_ID;
    @Override
    public void onCreate(){
        super.onCreate();
        //테스트용
        MyApplication.Interstitial_ad_unit_id = "ca-app-pub-3940256099942544/1033173712";
        MyApplication.APPLICATION_ID = "ca-app-pub-3940256099942544~3347511713";


    }
}

이렇게 했는데 에러가납니다. 이거 말고 다른 앱을 만들때는 이런 일이 없었거든요. 참고로 메니페스토에도

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

이거 추가했습니다.