Transform a Pair list to a Map grouping by key

PairsToMap.java

import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

import org.apache.commons.lang3.tuple.MutablePair;
import org.apache.commons.lang3.tuple.Pair;

public class Test
{

    /**
     * @param args
     */
    public static void main(String[] args)
    {
        Set<Pair<Integer, String>> pairs = new HashSet<>(Arrays.asList(MutablePair.of(1, "A"), MutablePair.of(1, "B")));

        Map<Integer, List<String>> map = pairs.stream()
            .collect(Collectors.groupingBy(Pair::getKey, Collectors.mapping(Pair::getValue, Collectors.toList())));

        map.forEach((k, v) -> System.out.println("key: " + k + " value:" + v));
    }

}