How to add element into ArrayList in HashMap

Question

How to add element into ArrayList in HashMap?

    HashMap<String, ArrayList<Item>> Items = new HashMap<String, ArrayList<Item>>(); 

Answer 1

I know, this is an old question. But just for the sake of completeness, the lambda version.

Map<String, List<Item>> items = new HashMap<>(); items.computeIfAbsent(key, k -> new ArrayList<>()).add(item); 

Answer 2

HashMap<String, ArrayList<Item>> items = new HashMap<String, ArrayList<Item>>();  public synchronized void addToList(String mapKey, Item myItem) {     List<Item> itemsList = items.get(mapKey);      // if list does not exist create it     if(itemsList == null) {          itemsList = new ArrayList<Item>();          itemsList.add(myItem);          items.put(mapKey, itemsList);     } else {         // add if item is not already in list         if(!itemsList.contains(myItem)) itemsList.add(myItem);     } }