👀 Views: 86 💬 Answers: 1 📅 Created: 2025-06-20

java completablefuture asynchronous Java

I've been working on this all day and I've been struggling with this for a few days now and could really use some help... I'm running into an issue with `CompletableFuture` in Java 8 when trying to execute multiple asynchronous tasks in parallel and then combine their results. I've implemented the following code to fetch data from two different APIs that return `CompletableFuture<String>`. However, when I try to combine the results using `thenCombine`, I'm getting unexpected results. Here's the code I used: ```java import java.util.concurrent.CompletableFuture; import java.util.concurrent.ExecutionException; public class ApiFetcher { public CompletableFuture<String> fetchApi1() { return CompletableFuture.supplyAsync(() -> { // Simulate API call try { Thread.sleep(1000); } catch (InterruptedException e) { } return "Result from API 1"; }); } public CompletableFuture<String> fetchApi2() { return CompletableFuture.supplyAsync(() -> { // Simulate API call try { Thread.sleep(1000); } catch (InterruptedException e) { } return "Result from API 2"; }); } public void fetchData() { CompletableFuture<String> future1 = fetchApi1(); CompletableFuture<String> future2 = fetchApi2(); future1.thenCombine(future2, (result1, result2) -> { return result1 + " and " + result2; }).thenAccept(finalResult -> { System.out.println(finalResult); }); } } public class Main { public static void main(String[] args) { new ApiFetcher().fetchData(); } } ``` When I run this, I see the output, but it’s often just showing `Result from API 1 and null` or vice versa, and it seems to be happening intermittently. I suspect it may be an issue with how the `CompletableFuture` instances are being combined. I also tried using `allOf` to wait for both futures, but that didn't solve my problem either. Is there a specific reason why I'm getting `null` for one of the results? How can I ensure both results are properly combined without issues? Any insights would be greatly appreciated! Has anyone else encountered this? I'm working in a Windows 11 environment. This is part of a larger web app I'm building. Thanks for taking the time to read this!