Can HashMap allows multiple null values for key in Java

HashMap allows only one null key, because keys must be unique. However, a HashMap can store multiple null values—either for different keys or inside a collection (like a List) that is stored as a value.

Example Code:

HashMap<Integer, List<Integer>> map = new HashMap<>();

map.put(1, new ArrayList<>()); // key = 1, value = empty list

map.get(1).add(null); // adding first null value
map.get(1).add(null); // adding second null value