program.java

public class Main {
  public static void main(String[] args) {
    FirebasePushNotification notificationService = new FirebasePushNotification();
    
    notificationService.sendNotificationToSingleUser("Hello", "123456");
    notificationService.sendNotificationToAllUsers("Hello world");
    
    //...
    notificationService.sendNotificationToSingleUser("Hello", "123456");
    notificationService.sendNotificationToAllUsers("Hello world");
    
    //...
    notificationService.sendNotificationToSingleUser("Hello", "123456");
    notificationService.sendNotificationToAllUsers("Hello world");
  }
}