I'm not 100% sure on what you mean by an optional interrupt, but I'm guessing it's the boolean flag on the cancel(); Let's look!

    public boolean cancel(boolean mayInterruptIfRunning) {
        boolean cancelled = this.result == null && this.internalComplete(new CompletableFuture.AltResult(new CancellationException()));
        this.postComplete();
        return cancelled || this.isCancelled();
    }

Try it.

    @Test
    public void cannotCancelARunningFuture() {

        // Future that just sleeps
        CompletableFuture<Void> sleeping =
                CompletableFuture.supplyAsync(() -> {
                    int i = 0;
                    while (true) {
                        System.out.println("zzzz: " + i++);
                        threadSleep(300);
                    }
                });

        // Make sure it started
        threadSleep(1000);

        sleeping.cancel(true);
        System.out.println("Sleep was cancelled");

        threadSleep(1000);
        System.out.println("Haha no it wasn't");
    }

  zzzz: 0
  zzzz: 1
  zzzz: 2
  zzzz: 3
  Sleep was cancelled
  zzzz: 4
  zzzz: 5
  zzzz: 6
  Haha no it wasn't