Map<String, String> map= new HashMap<>();
    String value;
    value=map.get(null);
    System.out.println(value); // null - there is no pair with key==null
    map.put(null, "Hello");
    value=map.get(null);
    System.out.println(value); // Hello - now there is a value with key = null
    map=null;
    value=map.get(null); // NPE - do not follow the null pointer!!