Here's a test where a future has many dependents, that should scale linearly with n:


    /**
     * A single CompletableFuture with many dependents.
     * A demo of scalability - runtime is O(n).
     */
    public void testManyDependents() throws Throwable {
        final int n = 1_000;
        final CompletableFuture<Void> head = new CompletableFuture<>();
        final CompletableFuture<Void> complete = CompletableFuture.completedFuture((Void)null);
        final AtomicInteger count = new AtomicInteger(0);
        for (int i = 0; i < n; i++) {
            head.thenRun(() -> count.getAndIncrement());
            head.thenAccept((x) -> count.getAndIncrement());
            head.thenApply((x) -> count.getAndIncrement());

            head.runAfterBoth(complete, () -> count.getAndIncrement());
            head.thenAcceptBoth(complete, (x, y) -> count.getAndIncrement());
            head.thenCombine(complete, (x, y) -> count.getAndIncrement());
            complete.runAfterBoth(head, () -> count.getAndIncrement());
            complete.thenAcceptBoth(head, (x, y) -> count.getAndIncrement());
            complete.thenCombine(head, (x, y) -> count.getAndIncrement());

            head.runAfterEither(new CompletableFuture<Void>(), () -> count.getAndIncrement());
            head.acceptEither(new CompletableFuture<Void>(), (x) -> count.getAndIncrement());
            head.applyToEither(new CompletableFuture<Void>(), (x) -> count.getAndIncrement());
            new CompletableFuture<Void>().runAfterEither(head, () -> count.getAndIncrement());
            new CompletableFuture<Void>().acceptEither(head, (x) -> count.getAndIncrement());
            new CompletableFuture<Void>().applyToEither(head, (x) -> count.getAndIncrement());
        }
        head.complete(null);
        assertEquals(5 * 3 * n, count.get());
    }