Java 11 CompletableFuture not returning expected results with multiple asynchronous tasks
After trying multiple solutions online, I still can't figure this out... I'm testing a new approach and I'm working with an scenario with using `CompletableFuture` in Java 11 when trying to execute multiple asynchronous tasks. I have a method that fetches user data from an API and another method that processes this data. The question arises when I try to run them concurrently; the output is not what I expect. Instead of returning the processed data, I'm getting a null result, and the debug logs indicate that the tasks are completing exceptionally. Here's a simplified version of what I'm trying to achieve: ```java import java.util.List; import java.util.concurrent.CompletableFuture; import java.util.concurrent.ExecutionException; public class UserDataProcessor { public CompletableFuture<List<String>> fetchUserData() { return CompletableFuture.supplyAsync(() -> { // Simulate API call try { Thread.sleep(1000); } catch (InterruptedException e) { } return List.of("user1", "user2"); }); } public CompletableFuture<List<String>> processUserData(List<String> users) { return CompletableFuture.supplyAsync(() -> { if (users == null) throw new RuntimeException("User list is null"); return users.stream().map(String::toUpperCase).toList(); }); } public void run() throws ExecutionException, InterruptedException { CompletableFuture<List<String>> userDataFuture = fetchUserData(); CompletableFuture<List<String>> processedDataFuture = userDataFuture.thenCompose(this::processUserData); List<String> processedData = processedDataFuture.get(); System.out.println(processedData); } } public class Main { public static void main(String[] args) throws ExecutionException, InterruptedException { new UserDataProcessor().run(); } } ``` When I run this code, I expect to see the processed user data in uppercase, but instead, I get a `java.util.concurrent.CompletionException` along with the message "User list is null". I've confirmed that `fetchUserData` does return a non-null list, but somehow it's not being passed correctly into `processUserData`. I've tried adding logging statements before returning the list in `fetchUserData`, and they confirm the list is populated. Is there something I'm missing in the way I'm chaining the `CompletableFuture`s? Any insights would be greatly appreciated! I'm using Java latest in this project. Any suggestions would be helpful. What's the correct way to implement this?