I seem to fail miserably at an apparently simple example with CompletableFuture.

I have a CompletableFuture that is blocked in a different thread, something like:

        Supplier<CompletableFuture<String>> one = () -> CompletableFuture.supplyAsync(() -> {
            System.out.println("started cf");
            LockSupport.parkNanos(TimeUnit.SECONDS.toNanos(3));
            System.out.println("done cf");
            return "";
        });
        ExecutorService service = Executors.newSingleThreadExecutor();
        service.submit(() -> {
            try {
                future = one.get();
                future.get();
            } catch (Exception e) {
                System.out.println("caught");
                future.completeExceptionally(e);
            }
        });

I want to be able to cancel it, and then join to wait until it is completed in a thread that is different then the one above, so something like this:

    static CompletableFuture<String> future;

    public static void main(String[] args) {

        Supplier<CompletableFuture<String>> one = () -> CompletableFuture.supplyAsync(() -> {
            System.out.println("started cf");
            LockSupport.parkNanos(TimeUnit.SECONDS.toNanos(3));
            System.out.println("done cf");
            return "";
        });
        ExecutorService service = Executors.newSingleThreadExecutor();
        service.submit(() -> {
            try {
                future = one.get();
                future.get();
            } catch (Exception e) {
                System.out.println("caught");
                future.completeExceptionally(e);
            }
        });

        LockSupport.parkNanos(TimeUnit.SECONDS.toNanos(1));
        future.cancel(true);
        System.out.println("before join");
        String s = future.join();
        System.out.println("s value : " + s);
        System.out.println("done");
    }

To my surprise, this one String s = future.join(); blocks and I don’t understand why.

More surprising, if I change my code to (instead of String s = future.join();):

        String s = null;
        try {
            s = future.join();
        } catch(Exception e) {
            System.out.println("here : " + e.getClass());
        }

this one does not block. Can someone please shed some light here?