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.

What is `EventDto` and how is it related to the elements of your Stream? — Eran, Mar 20 '17 at 08:32
@eran I should use external function to convert `Event` to `EventDto`. I think in explanation can be used any name if you with, or for example `this::convertFromEventToEventDto` — Sergii, Mar 20 '17 at 08:37
I tried this idea also, but it is different story because of some restrictions and I should not change `EventDto` — Sergii, Mar 20 '17 at 19:40

score 15 · Accepted Answer · answered Mar 20 '17 at 08:41

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())));

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

1 Answers1