code: public Transaction getTransaction(final String transactionId) {
          if (transactionId == null || transactionId.isEmpty())
              throw new RuntimeException("transactionId cannot be empty!");
  
          return doGET(Transactions.TRANSACTIONS_RESOURCE + "/" + transactionId,
                       Transaction.class);
      }
docs: Lookup a transaction
  
  @param transactionId recurly transaction id
  @return the transaction if found, null otherwise