Map with LIst using Collectors.toMap in Java 8

Question

I like convert below code to java stream,

HashMap<String, List<Data>> heMap = new HashMap<String, List<Data>>();
for (Data heData : obj) {
    String id = heData.getData().getId() + heData.getPlanData().getCode()
            + heData.getPlanData().getId();
    if (!heMap.containsKey(id)) {
        CitizenHElist = new ArrayList<Data>();

        CitizenHElist.add(heData);
        heMap.put(id, CitizenHElist);

    } else {
        heMap.get(id).add(heData);
    }
}

I tried the below code using stream, but i am not succeed on this.

heMap=obj.stream().collect(Collectors.toMap(t->getKey(t), obj.stream().collect(Collectors.toList())));

private String getKey(Data heData){
    String id = heData.getData().getId() + heData.getPlanData().getCode()
                    + heData.getPlanData().getId();
    return id;
}

Answer 1

This is the job for groupingBy collector:

import static java.util.stream.Collectors.groupingBy;

Map<String, List<Data>> heMap = obj.stream().collect(groupingBy(d -> getKey(d)));

Note that this will use some unspecified implementations of Map and List. Currently, it happens to be HashMap and ArrayList, but that might change in the future.