Send Simple text notification in android studio project

activity_main.xml

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



    <Button

        android:id="@+id/simple"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_vertical"
        android:layout_marginLeft="15dp"
        android:clickable="true"
        android:layout_marginTop="5dp"
        android:background="@color/black"
        android:onClick="simple_notification"
        android:text="Simple Notification"
        android:textColor="@color/white" />



</LinearLayout>

MainActivity.java

package com.example.sendnotification;

import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.media.RingtoneManager;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.NotificationCompat;


public class MainActivity extends AppCompatActivity {
    Button btn;

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

        btn = findViewById(R.id.simple);
    }


    public void simple_notification(View view){
        int notificationId = 0;
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this);
        builder.setSmallIcon(R.drawable.ic_launcher_background);
        builder.setContentTitle("Test Notification");
        builder.setContentText("Hello world");
        builder.setAutoCancel(true);
        builder.setDefaults(NotificationCompat.DEFAULT_ALL);
        Uri path = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
        builder.setSound(path);
        NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);

        if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.O){
            String channelId = "test";
            NotificationChannel channel = new NotificationChannel( channelId,"This is channel",NotificationManager.IMPORTANCE_DEFAULT);
            notificationManager.createNotificationChannel(channel);
            builder.setChannelId(channelId);
        }
        notificationManager.notify(notificationId,builder.build());
    }
}