AndroidManifest.xml

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

    <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.IntegrateGoogleBannerAds">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

        <!--Replace value with your origional App Id-->
        <!-- If you are testing your app then don't use your origional App Id. Always use Test App Id-->
        <!--My Origioanl App Id is  = ca-app-pub-3299203761326949~6191893358-->
        <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-3940256099942544~3347511713"/>

    </application>
</manifest>

build.gradle (App)

dependencies {

    implementation 'androidx.appcompat:appcompat:1.4.0'
    implementation 'com.google.android.material:material:1.4.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.2'
    testImplementation 'junit:junit:'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'

    // Add Google Admob Depandancy
    implementation 'com.google.android.gms:play-services-ads:20.5.0'
}

strings.xml

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

    <!--Replace with your origional Banner Ad Id-->
    <!--My Origional Banner Ad Unit id is  = ca-app-pub-3299203761326949/9141490329-->
    <string name="banner_ad">ca-app-pub-3940256099942544/6300978111</string>
</resources>

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">


    <!--Always use Test Ad Unit Ids while testing your app. When you are about to launch your app
    then you can replace this Test Ad Unit Id with your original Ad unit Id-->
    <com.google.android.gms.ads.AdView xmlns:ads="http://schemas.android.com/apk/res-auto"
        android:id="@+id/adView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        ads:adSize="BANNER"
        ads:adUnitId="@string/banner_ad" />

</RelativeLayout>

MainActivity.java

package com.learnoset.integrategooglebannerads;

import android.os.Bundle;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

import com.google.android.gms.ads.AdListener;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdView;
import com.google.android.gms.ads.LoadAdError;
import com.google.android.gms.ads.MobileAds;
import com.google.android.gms.ads.initialization.InitializationStatus;
import com.google.android.gms.ads.initialization.OnInitializationCompleteListener;

public class MainActivity extends AppCompatActivity {

    private AdView adView;

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

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

        // use below code once in your starting Activity like Splash Screen.
        MobileAds.initialize(this, new OnInitializationCompleteListener() {
            @Override
            public void onInitializationComplete(InitializationStatus initializationStatus) {
            }
        });

        adView.setAdListener(new AdListener() {
            @Override
            public void onAdLoaded() {
                Toast.makeText(MainActivity.this, "Ad Loaded", Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onAdFailedToLoad(LoadAdError adError) {
                Toast.makeText(MainActivity.this, "Failed to load Ad", Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onAdOpened() {
                Toast.makeText(MainActivity.this, "Ad Opened", Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onAdClicked() {
                Toast.makeText(MainActivity.this, "Ad Clicked", Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onAdClosed() {
                Toast.makeText(MainActivity.this, "Ad Closed", Toast.LENGTH_SHORT).show();
            }
        });
    }
}