working with IllegalArgumentException when using Stream API with Optional in Java 17
I'm relatively new to this, so bear with me... I'm sure I'm missing something obvious here, but I'm getting frustrated with I'm working on a Java 17 application where I'm trying to process a list of users and filter them based on some criteria using the Stream API... I have a method that returns an Optional<User> based on the user ID, and I want to use this in a subsequent stream operation. However, I'm running into an `IllegalArgumentException` when I try to filter the stream. Here's the code snippet: ```java import java.util.List; import java.util.Optional; public class User { private String id; private String name; // Getters and setters } public class UserService { private List<User> users; public Optional<User> findUserById(String id) { return users.stream().filter(user -> user.getId().equals(id)).findFirst(); } public void processUsers(List<String> userIds) { userIds.stream() .map(this::findUserById) // This returns Optional<User> .filter(Optional::isPresent) // This line causes the scenario .forEach(optionalUser -> { User user = optionalUser.get(); // Process user }); } } ``` The behavior I receive is: ``` Exception in thread "main" java.lang.IllegalArgumentException: Invalid argument encountered. ``` I've tried changing the filter line to: ```java .filter(optionalUser -> optionalUser.isPresent()) ``` But then I get a compilation behavior because `optionalUser` is of type `Optional<User>`, and I need to directly reference it in that way. I also tried using `flatMap`, but that doesn't seem to fix the scenario either. Can anyone guide to understand what I'm doing wrong here? Is there a better way to handle this situation? Any suggestions would be greatly appreciated! Any help would be greatly appreciated! Is this even possible? This is for a mobile app running on Ubuntu 20.04. My team is using Java for this desktop app.