<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>18.0</version>
</dependency>

import com.google.common.collect.HashMultimap;
public class HashMultimapTest {
    public static void main(String [] args) {
        HashMultimap<Long, Long> multimap = HashMultimap.create();
        multimap.put(11L, 20L);
        multimap.put(11L, 32L);
        multimap.put(11L, 41L);
        System.out.println(multimap);
    }
}