buildscript {
    repositories {
        google()
    }
    dependencies {
        classpath 'com.google.gms:google-services:4.3.8'
    }
}
allprojects {
    repositories {
        google()
    }
}

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'
dependencies {
    implementation platform('com.google.firebase:firebase-bom:26.6.0')
    implementation 'com.google.firebase:firebase-analytics'
    implementation 'com.google.firebase:firebase-messaging:17.3.4'
}

package your.package.name;

import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;

import androidx.core.app.NotificationCompat;

import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;

public class FirebaseMessageReceiver extends FirebaseMessagingService {
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage){
        if(remoteMessage.getNotification() != null){
            String title = remoteMessage.getNotification().getTitle();
            String body = remoteMessage.getNotification().getBody();
            addNotification(body);
        }
    }

    @Override
    public void onNewToken(String token){
    }

    private void addNotification(String msg){
        //String channel_i = "Your Notification";
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this)
                .setSmallIcon(R.drawable.common_google_signin_btn_icon_dark)
                .setContentTitle("Test Notification")
                .setContentText(msg)
                .setAutoCancel(true)
                .setPriority(NotificationCompat.PRIORITY_DEFAULT)
                .setChannelId(AdmActivity.CHANNEL_ID);
        Intent notificationIntent = new Intent(this, MyActivity.class);
        notificationIntent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        notificationIntent.putExtra("message",msg);
        PendingIntent contentIntent = PendingIntent.getActivity(this, 0, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT);
        builder.setContentIntent(contentIntent);
        NotificationManager manager = (NotificationManager)getSystemService(Context.NOTIFICATION_SERVICE);
        manager.notify(0, builder.build());
    }
}

<application
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">
….. ……. ….. …..     …….
<service android:name=".FirebaseMessageReceiver">
            <intent-filter>
                <action android:name="com.google.firebase.MESSAGING_EVENT" />
            </intent-filter>
        </service>
</application>