How to run async tasks and combine their results

Short answer

supplyAsync() runs a task in the background, thenApply() says what to do with the result, and join() waits for the outcome:

void main() {
    CompletableFuture<String> greeting = CompletableFuture
            .supplyAsync(() -> "anna")
            .thenApply(String::toUpperCase)
            .thenApply(name -> "Hello, " + name);
 
    IO.println(greeting.join());
}

Output:

Hello, ANNA

Combining two results

thenCombine() waits for two independent tasks and combines their results:

int load(String what, int value) {
    try {
        Thread.sleep(200);
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    }
    return value;
}
 
void main() {
    var price = CompletableFuture.supplyAsync(() -> load("price", 120));
    var discount = CompletableFuture.supplyAsync(() -> load("discount", 20));
 
    IO.println(price.thenCombine(discount, (p, d) -> p - d).join());
}

Output:

Waiting for all

allOf() completes when all tasks are done:

void main() {
    var futures = List.of(
            CompletableFuture.supplyAsync(() -> 1),
            CompletableFuture.supplyAsync(() -> 2),
            CompletableFuture.supplyAsync(() -> 3));
 
    CompletableFuture.allOf(futures.toArray(new CompletableFuture[0])).join();
    IO.println(futures.stream().mapToInt(CompletableFuture::join).sum());
}

Output:

Handling errors

exceptionally() supplies a value if an error happened at some step:

void main() {
    int value = CompletableFuture.supplyAsync(() -> Integer.parseInt("x"))
            .exceptionally(error -> {
                IO.println("Failed: " + error.getCause().getMessage());
                return 0;
            })
            .join();
    IO.println(value);
}

Output:

Failed: For input string: "x"
0

Common mistake

Forgetting to wait for the result. The program ends, and background tasks from the common pool do not get a chance to finish — their output simply disappears:

void main() {
    CompletableFuture.runAsync(() -> {
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
        IO.println("never printed");
    });
    IO.println("main finished");
}

Output:

main finished

Call join() or get() if you need the result before the program exits.

Thanks for reading!

Short answer

Combining two results

Waiting for all

Handling errors

Common mistake

Functions used in this recipe

Thanks for reading!