Lambda Expressions with Streams magics

RequestHandler.java

@Component
@Scope("prototype")
public class RequestHandler extends BaseRequestHandler<List<PositionEstablishment>> {
	@Autowired
	private PositionEstablishmentRepository positionEstablishmentRepository;
	Date asAtDate;
	private List<com.jemini.base.model.position.PositionEstablishment> establishments = new ArrayList<>();
	private List<PositionDateEffective> positionDateEffectives;

	public Date getAsAtDate() {return asAtDate;}
	public void setAsAtDate(Date asAtDate) {this.asAtDate = asAtDate;}

	@Override
	public boolean validateRequest() throws RequestHandlingException {
		positionDateEffectives = positionEstablishmentRepository.findAllAsAtDate(asAtDate);
		if (positionDateEffectives == null) {
			ResourceNotFoundException resourceNotFoundException = new ResourceNotFoundException(getWsErrorBy(WsErrorEnum.POSITION_ESTABLISHMENT_NOT_FOUND, String.valueOf(asAtDate)));
			throw resourceNotFoundException;
		}
		return !hasValidationErrors();
	}

	@Override
	protected List<PositionEstablishment> handleRequest() throws RequestHandlingException {
		//Convert results (List of `PositionDateEffective`) from DB to the type the `@GET` method operates on (`PositionEstablishment` Entity from jemini.base)
		establishments = positionDateEffectives.stream()
                                 .map(PositionDateEffective::getPositionEstablishment)
                                 .collect(Collectors.toList());

		//Group those `PositionEstablishment` by their `externalID`
		Map<String, List<com.jemini.base.model.position.PositionEstablishment>> establishmentsByExternalId =
				establishments.stream()
						.collect(Collectors.groupingBy(PositionEstablishmentEnt::getExternalId));

		//To avoid getting "IncorrectResultSizeDataAccessException: result returns more than one elements;"
		//(If such `externalId` was passed to single `@GET` method it will blow up exactly on this too.)
		//We need to select entries with just one element in them.
		Set<Map.Entry<String, List<PositionEstablishmentEnt>>> cleanEstablishmentsByExternalId =
				establishmentsByExternalId.entrySet().stream()
						.filter(entry -> entry.getValue().size() == 1)
				        .collect(Collectors.toSet());

		//Create a list of individual `PositionEstablishment` (which is otherwise handled by single `@GET` method)
		//And return it in the results list
		List<PositionEstablishment> results = new ArrayList<>();
		cleanEstablishmentsByExternalId.forEach(entry -> {
			PositionEstablishment result = new PositionEstablishment();
			result.setExternalId(entry.getKey());
			result.setDatedPositionEstablishments(
				entry.getValue().get(0).getPositionDateEffectiveRecords().stream().map(this::buildDatedPositionEstablishment).collect(Collectors.toList())
			);
			results.add(result);}
		);

		//Make sure we return everytime the same results, as the list is build by Streams in nondeterministic order
		results.sort((p1, p2) -> p1.getExternalId().compareTo(p2.getExternalId()));
		return results;
	}
}