Android の通知マネージャー

private void sendNotification() {
    NotificationCompat.Builder builder = new NotificationCompat.Builder(this, CHANNEL_ID)
            .setSmallIcon(R.drawable.notification_icon)
            .setContentTitle("My notification")
            .setContentText("Hello World!")
            .setPriority(NotificationCompat.PRIORITY_DEFAULT);

    NotificationManagerCompat notificationManager = NotificationManagerCompat.from(this);

    // notificationId is a unique int for each notification that you must define
    notificationManager.notify(notificationId, builder.build());
}

if (ContextCompat.checkSelfPermission(this, Manifest.permission.RECEIVE_BOOT_COMPLETED)
                != PackageManager.PERMISSION_GRANTED) {
    // Permission is not granted
    // Request the permission
    ActivityCompat.requestPermissions(this,
            new String[]{Manifest.permission.RECEIVE_BOOT_COMPLETED},
            MY_PERMISSIONS_REQUEST_RECEIVE_BOOT_COMPLETED);
}