Use of grouping by in java8 streams

Question

I have a List of Event objects i want to transform to JSON.

An event is defined as below :

public class Event {

    private final String name;
    private final Date date;

    public Event(String name, Date date) {
        this.name = name;
        this.date = date;
    }

    public String getName() {
        return name;
    }

    public Date getDate() {
        return date;
    }
}

the code below :

List<Event> events = ...;
        SimpleDateFormat dayFormatter=new SimpleDateFormat("dd/MM/yyyy");
        Map<String,List<JsonNode>> result = events.stream().map(event -> {
            ObjectNode jsonMatch = jsonNodeFactory.objectNode();
            jsonMatch.put("name",event.getName());
            jsonMatch.put("day",dayFormatter.format(event.getDate()));
            return jsonMatch;
        }).collect(Collectors.groupingBy(item -> item.get("day").asText(),
                Collectors.toList()));

creates me a json that i want, which looks like this :

{
    "10/03/2014": [{name:"event1",day:"10/03/2014"},{name:"event2",day:"10/03/2014"}]
}

but to me, the day attribute in each event node is a redundant information that i want to bypass, in order to obtain this json :

{
        "10/03/2014": [{name:"event1"},{name:"event2"}]
    }

but if i dont put it in the object, i cant do a groupingby on it. Is there any workaround to make it happen?

Answer 1

Don't do the mapping step before the collect operation, do it only for the values of the groups.

SimpleDateFormat dayFormatter=new SimpleDateFormat("dd/MM/yyyy");
Map<String,List<JsonNode>> result = events.stream()
    .collect(Collectors.groupingBy(event -> dayFormatter.format(event.getDate()),
        Collectors.mapping(event -> {
            ObjectNode jsonMatch = jsonNodeFactory.objectNode();
            jsonMatch.put("name", event.getName());
            return jsonMatch;
        }, Collectors.toList())));

Answer 2

You can use Collectors.toMap() method:

Map<String,List<JsonNode>> result = events.stream()
    .map(event -> {
        ObjectNode jsonMatch = jsonNodeFactory.objectNode();
        jsonMatch.put("name",event.getName());
        jsonMatch.put("day",dayFormatter.format(event.getDate()));
        return jsonMatch;
    })  
    .collect(Collectors.toMap(
        item -> item.get("day").asText(),
        item -> new ArrayList<>(Arrays.asList(item.get("name"))),
        (i1, i2) -> {
            i1.addAll(i2);
            return i1;
        }
    ));