code: public Invoice getInvoice(final String invoiceId) {
          if (invoiceId == null || invoiceId.isEmpty())
              throw new RuntimeException("invoiceId cannot be empty!");
  
          return doGET(Invoices.INVOICES_RESOURCE + "/" + invoiceId, Invoice.class);
      }
docs: Lookup an invoice given an invoice id
  
  <p>
  Returns the invoice given a string id.
  The invoice may or may not have acountry code prefix (ex: IE1023).
  For more information on invoicing and prefixes, see:
  https://docs.recurly.com/docs/site-settings#section-invoice-prefixing
  
  @param invoiceId String Recurly Invoice ID
  @return the invoice