CompletableFuture allOf method behavior

Question

I have the following piece of java code:

CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(3);
            } catch (InterruptedException e) {
                throw new IllegalStateException(e);
            }
            return "Result of Future 1";
        });

        CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(3);
            } catch (InterruptedException e) {
                throw new IllegalStateException(e);
            }
            return "Result of Future 2";
        });

        CompletableFuture<String> future3 = CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(3);
            } catch (InterruptedException e) {
                throw new IllegalStateException(e);
            }
            return "Result of Future 3";
        });

        boolean isdone = CompletableFuture.allOf(future1, future2, future3).isDone();

        if (isdone) {
            System.out.println("Future result " + future1.get() + " | " + future2.get() + " | " + future3.get());
        } else {
            System.out.println("Futures are not ready");
        }

When I run this code it always prints "Futures are not ready". I am using allOf method here which should wait for all the futures to get completed, but the main thread is not waiting here and priniting the else part. Can someone please help me understand what is going wrong here?

Answer 1

I am using allOf method here which should wait for all the futures to get completed

That's not what allOf does. It creates a new CompletableFuture that is completed when all of the given CompletableFutures complete. It does not, however, wait for that new CompletableFuture to complete.

This means that you should call some method that waits for this CompletableFuture to be completed, at which point all the given CompletableFutures are guaranteed to be complete.

For example:

CompletableFuture<Void> allof = CompletableFuture.allOf(future1, future2, future3);
allof.get();

if (allof.isDone ()) {
    System.out.println("Future result " + future1.get() + " | " + future2.get() + " | " + future3.get());
} else {
    System.out.println("Futures are not ready");
}

Output:

Future result Result of Future 1 | Result of Future 2 | Result of Future 3