Java tutorial
//package com.java2s; //License from project: LGPL import java.util.Map; import java.util.Set; import java.util.TreeSet; public class Main { public static <K, V> void addToMap(K key, V value, Map<K, Set<V>> map) { if (key != null && value != null) if (map.containsKey(key)) map.get(key).add(value); else { Set<V> values = new TreeSet<V>(); values.add(value); map.put(key, values); } } }