Java 8 Stream function grouping to Map where value is a Map

Question

I am trying to collect result of a list and organise them into a Map where the value is a Map:

private Map<Organisation, Map<LocalDate, Status>> getSummaries(final List<Status> summaries) {
    return summaries
            .stream()
            .collect(groupingBy(Status::getOrganisation,
                    toMap(Status::getProcessedDate, Function.identity())));
}

I get java.lang.IllegalStateException: Duplicate key error as getProcessedDate() is same for different values in the list.

Is there a way I can merge multiple objects with same processeddate into the map?

e.g say I have these objects in the the list:

Summary(ProcesseDate=2020-01-30, Organisation=ABC, status=OK, statusCount=5)
Summary(ProcesseDate=2020-01-30, Organisation=ABC, status=FAILED, statusCount=2)
Summary(ProcesseDate=2020-01-30, Organisation=APPLE, status=OK, statusCount=5)
Summary(ProcesseDate=2020-01-30, Organisation=APPLE, status=REJECTED, statusCount=5)

Values contained in the map should be:

key=ABC
   value { key=2020-01-30, value= Summary(ProcesseDate=2020-01-30, Organisation=ABC, status=OK, statusCount=5), Summary(ProcesseDate=2020-01-30, Organisation=ABC, status=FAILED, statusCount=2) }

When I tried toMap(Status::getProcessedDate, Function.identity(), (v1, v2) -> v2))); it removes one of the entries

Answer 1

You might just be looking for a nested grouping if you don't want to merge data and don't have unique keys:

private Map<Organisation, Map<LocalDate, List<Status>>> getSummaries(final List<Status> summaries) {
    return summaries
            .stream()
            .collect(groupingBy(Status::getOrganisation, groupingBy(Status::getProcessedDate)));
}