Java 8 lambda Collectors.groupingBy with map in Collector.toList()

Question

I have implemented the following example:

Map<String, List<Event>> map = events.getItems().stream()
        .collect(Collectors.groupingBy(Event::getStatus, Collectors.toList()));

How can I get an output of Map<String, List<EventDto>> map instead?

An EventDto can be obtained by executing an external method which converts an Event to an EventDto. For example - this::convertFromEventToEventDto.

Answer 1

You need a mapping Collector to map the Event elements to EventDto elements :

Map<String, List<EventDto>> map = 
    events.getItems()
          .stream()
          .collect(Collectors.groupingBy(Event::getStatus, 
                                         Collectors.mapping(this::convertFromEventToEventDto,
                                                            Collectors.toList())));