Java tutorial
//package com.java2s; import java.util.ArrayList; import java.util.List; import java.util.Map; public class Main { public static <K, V> void addListInMap(Map<K, List<V>> map, K key, V value) { List<V> list = map.get(key); if (list == null) { list = new ArrayList<V>(); map.put(key, list); } list.add(value); } }