Is there a better way of doing this in Java 8?
final List<InstitutionUserConnection> allInstitutionUserConnections = new ArrayList<>(); for (final Institution institution : institutionsOfUser) { allInstitutionUserConnections .addAll(institutionUserConnectionService.getActiveInstitutionUserConnectionsByInstitution(institution)); } Yes, this is what the flatMap operation is for:
List<InstitutionUserConnection> allInstitutionUserConnections = institutionsOfUser.stream() .flatMap(institution -> institutionUserConnectionService.getActiveInstitutionUserConnectionsByInstitution(institution).stream()) .collect(Collectors.toList()); If the method throws a checked exception, then you need to catch it and handle it with logging, rethrowing an unchecked exception and/or returning a default value.
List<InstitutionUserConnection> allInstitutionUserConnections = institutionsOfUser.stream() .flatMap(institution -> { try { return institutionUserConnectionService.getActiveInstitutionUserConnectionsByInstitution(institution).stream(); } catch (TheCheckedThrownException e) { // do something here, like throw an unchecked exception } }) .collect(Collectors.toList()); 
Instead of flat mapping the inner list to a stream, you can also map it directly to a List, and then use a custom collector to append the element. The advantage of using a collector above using the stream, is that using a collector, you can get higher performance because the fact it uses 1 large addAll call, instead of separate small add calls like the Collectors.toList() does.
A solution based on this looks like:
List<InstitutionUserConnection> result = institutionsOfUser.stream(). .map(institutionUserConnectionService::getActiveInstitutionUserConnectionsByInstitution) .collect(Collector.of(ArrayList::new, List::addAll, (left, right) -> { left.addAll(right); return left; })); 
