code: private int numberFromTransactionIdString(ChargingStationId chargingStationId, String protocol, String transactionId) {
          String transactionIdPartBeforeNumber = String.format("%s_%s_", chargingStationId.getId(), protocol);
          try {
              return Integer.parseInt(transactionId.substring(transactionIdPartBeforeNumber.length()));
          } catch (NumberFormatException e) {
              throw new NumberFormatException(String.format("Cannot retrieve transaction number from string [%s]", transactionId));
          }
      }
docs: Retrieves the number from a transaction id string. ChargingStationId and protocol are passed to make a better
  guess at the number.
  
  @param chargingStationId    the charging station's identifier.
  @param protocol             the protocol identifier.
  @param transactionId        the transaction id containing the number.
  @return the transaction number
  @throws NumberFormatException if the number cannot be extracted from {@code transactionId}.