code: public static Invoice upcoming() throws StripeException {
      return upcoming((Map<String, Object>) null, (RequestOptions) null);
    }
docs: At any time, you can preview the upcoming invoice for a customer. This will show you all the
  charges that are pending, including subscription renewal charges, invoice item charges, etc. It
  will also show you any discount that is applicable to the customer.
  
  <p>Note that when you are viewing an upcoming invoice, you are simply viewing a preview – the
  invoice has not yet been created. As such, the upcoming invoice will not show up in invoice
  listing calls, and you cannot use the API to pay or edit the invoice. If you want to change the
  amount that your customer will be billed, you can add, remove, or update pending invoice items,
  or update the customer’s discount.
  
  <p>You can preview the effects of updating a subscription, including a preview of what
  proration will take place. To ensure that the actual proration is calculated exactly the same
  as the previewed proration, you should pass a <code>proration_date</code> parameter when doing
  the actual subscription update. The value passed in should be the same as the <code>
  subscription_proration_date</code> returned on the upcoming invoice resource. The recommended
  way to get only the prorations being previewed is to consider only proration line items where
  <code>period[start]</code> is equal to the <code>subscription_proration_date</code> on the
  upcoming invoice resource.