Convert List of key - value Object pairs to simple Multimap using Java 8 API

Question

Having list of key - values:

public class KeyValue {

    private Long key;

    private Long value;

    public KeyValue(long key, long value) {
        thi         


        

Answer 1

This is exactly what the groupingBy collector allows you to do:

Map<Long, List<Long>> result = values.stream()
     .collect(Collectors.groupingBy(KeyValue::getKey,
         Collectors.mapping(KeyValue::getValue, Collectors.toList())));

Then the mapping collector converts the KeyValue objects into their respective values.