PassengerTypeService.java

@Service
@ConfigurationProperties
@PropertySource(value = { "classpath:passenger-type-codes.yml" },  factory = YamlPropertySourceFactory.class)
public class PassengerTypeService {

    private Map<String, String> codes;

    public PassengerType getPassengerType(String code) {
        String value = codes.get(code) == null ? "ADULT" : codes.get(code);
        return PassengerType.valueOf(value);
    }

    public Map<String, String> getCodes() {
        return codes;
    }

    public void setCodes(Map<String, String> codes) {
        this.codes = codes;
    }
}

StatusGateway.java

@Service
public class AirportGateway {
    private Connector connector;
    
    public Airport findByCode(String code) {
         Airport airport = connector.getByCode(code)
         if (airport != null) {
           return airport;
         } else {
            return null;
        }
    }
}

TicketDetailsByNumberMap.java

final class TicketDetailsByNumberMap {
    private Map<String, Optional<TicketDetailsType>> ticketDetailsByNumber;

    TicketDetailsByNumberMap(List<TicketDetailsType> ticketDetails) {
        lastUpdatedTicketDetailsByTicketNumber(ticketDetails);
    }

    private void lastUpdatedTicketDetailsByTicketNumber(List<TicketDetailsType> ticketDetailsTypes) {
        this.ticketDetailsByNumber = ticketDetailsTypes.stream()
                .collect(
                        groupingBy(t -> getETicketNumber(t.getTicketNumber()),
                                maxBy((t1, t2) -> ((TicketDetailsType) t1).getTimestamp().compare(((TicketDetailsType) t2).getTimestamp()))));
    }

    TicketDetailsType get(String ticketNumber) {
        return this.ticketDetailsByNumber.get(getETicketNumber(ticketNumber)).orElseThrow(IllegalArgumentException::new);
    }

    private static String getETicketNumber(String ticketNumber) {
        return ticketNumber.substring(0, 13);
    }
}

TicketMapper.java

Component
class TicketMapper {
    private PassengerTypeService passengerTypeService;
    private AirportGateway airpotGateway;

    List<Ticket> map(List<PassengerPNRB> passengerPNRBs, TicketDetailsByNumberMap ticketDetailsByNumberMap) {
        return passengerPNRBs.stream()
                .filter(TicketMapperHelper::hasTicket)
                .map(passengerPNRB -> {
                    TicketDetailsType ticketDetailsType = ticketDetailsByNumberMap.get(TicketMapperHelper.getTicketNumber(passengerPNRB));
                    return new TicketBuilder()
                            .setPassenger(passengerTypeService.getPassengerType(passengerPNRB.getPassengerType()),
                                    passengerPNRB.getFirstName(),
                                    passengerPNRB.getLastName())
                            .setOrigin(airportGateway.findByCode(ticketDetailsType.getEmissionLocation()))
                            .setTicketNumber(ticketDetailsType.getTicketNumber())
                            .setEmissionDate(ticketDetailsType.getTimestamp())
                            .build();
                })
                .collect(Collectors.toList());
    }
}

I would like to write some unit test for TicketMapper, although it is not part of my domain logic, it is important to verify the mapping logic. As this mapper has some out-of-process dependencies, the first one is reads values from filesystem and the second one does an API call to a 3er party system, i was wondering whether the use of mocks and stubs in this unit test is a good fit or maybe i can use some humble object to separate dependencies from complex logic as you mention so that this complex logic does not depends on o-o-p dependencies. I thought about using humble pattern, i should fetch all airport first, pass them as a parameter and do some search. The thing is that aiport list can become a very large list.