/**
   * Returns the {@link Throwable} that caused this service to fail.
   *
   * @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
   * @since 14.0
   */
  Throwable failureCause();

  /**
   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
   * executor. The listener will have the corresponding transition method called whenever the

        };
    Callable<@Nullable Void> completeExceptionallyRunnable =
        new Callable<@Nullable Void>() {
          final Exception failureCause = new Exception("setException");

          @Override
          public @Nullable Void call() {
            if (currentFuture.get().setException(failureCause)) {
              numSuccessfulSetCalls.incrementAndGet();
            }
            awaitUnchecked(barrier);
            return null;

        };
    Callable<@Nullable Void> completeExceptionallyRunnable =
        new Callable<@Nullable Void>() {
          final Exception failureCause = new Exception("setException");

          @Override
          public @Nullable Void call() {
            if (currentFuture.get().setException(failureCause)) {
              numSuccessfulSetCalls.incrementAndGet();
            }
            awaitUnchecked(barrier);
            return null;