CompletableFuture runs out of order

Question

I have the following code in Java (I use Oracle Java 17):

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.TimeUnit;

public class Solution {
    public void startCompletableFutureExample() {
        CompletableFuture.runAsync(() -> {
                    System.out.println("async operation");
                })
                .completeOnTimeout(
                        timeout(), 10000, TimeUnit.MILLISECONDS);
    }

    private Void timeout() {
        System.out.println("timeout happens");
        return null;
    }
}

when I call the startCompletableFutureExample() method I have the following output:

timeout happens
async operation

while I'd expect to get the only one as timeout has not been even close:

async operation

Why do I have method completeOnTimeout to be run before runAsync? Am I doing something wrong?

Answer 1

Answering my own question - if it is necessary to run asynchronously CompletableFuture or cancel it by timeout expiration, as Slaw suggested it is better to use handle method. I've got the desired behavior with the following code:

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

public class Solution {
    public void startCompletableFutureExample() {
        CompletableFuture<String> f = CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            return "async operation";
        }).orTimeout(10, TimeUnit.MILLISECONDS);

        f.handleAsync((msg, ex) -> {
            if (ex instanceof TimeoutException) {
                System.out.println("timeout happens");
                f.cancel(true);
            } else {
                System.out.println(msg);
            }
            return null;
        });

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }
}

Now if I run it, I'll get a timeout reached first and thus the output would be only:

timeout happens

However if I set Thread.sleep(5) instead of Tread.sleep(50), I will get only:

async operation

which is an expected value.