Androidのプッシュ通知

サイト参考に下記のようにコードを書いてみたのですが通知が表示されません。
何か間違っている箇所ありますでしょうか？

どうかご教授よろしくお願いします。

import android.app.AlarmManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;

import java.util.Calendar;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

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

        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(System.currentTimeMillis());
        calendar.add(Calendar.SECOND,1);
        scheduleNotification("通知",calendar);
    }

    private void scheduleNotification(String content, Calendar calendar){
        Intent notificationIntent = new Intent(this, AlarmReceiver.class);
        notificationIntent.putExtra(AlarmReceiver.NOTIFICATION_ID, 1);
        notificationIntent.putExtra(AlarmReceiver.NOTIFICATION_CONTENT, content);
        PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 0, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT);

        AlarmManager alarmManager = (AlarmManager)getSystemService(Context.ALARM_SERVICE);
        alarmManager.setExact(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), pendingIntent);
    }

}

import android.app.Notification;
import android.app.NotificationManager;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

public class AlarmReceiver extends BroadcastReceiver {
    public static String NOTIFICATION_ID = "notificationId";
    public static String NOTIFICATION_CONTENT = "content";

    @Override
    public void onReceive(Context context, Intent intent) {
        NotificationManager notificationManager = (NotificationManager)context.getSystemService(Context.NOTIFICATION_SERVICE);
        int id = intent.getIntExtra(NOTIFICATION_ID, 0);
        String content = intent.getStringExtra(NOTIFICATION_CONTENT);
        notificationManager.notify(id, buildNotification(context, content));
    }

    private Notification buildNotification(Context context, String content) {
        Notification.Builder builder = new Notification.Builder(context);
        builder.setContentTitle("Notification!!")
                .setContentText(content)
                .setSmallIcon(android.R.drawable.sym_def_app_icon);

        return builder.build();
    }
}