What are the java collections accepts adding null values?

Question

I know Set and Map accepts null values and today I just found out LinkedList also accepts null values like

   map.put(null, 1);
   set.add(null);
   linkedList.add(null)

Is there any other collections that allow null values to be stored?. Just posting to get a comprehensive list in one place and reason for each for them.

Answer 1

Set and Map are interfaces. They have several implementations in Java.

Popular implementations of the Map are:

1. HashMap - accepts one null key
2. Hashtable - doesn't accept any null key
3. TreeMap - doesn't accept any null key
4. LinkedHashMap - accepts one null key

Any number of null values can be added as value in any of above implementations

---

Popular implementations of the Set are:

1. HashSet - accepts one null element
2. TreeSet - doesn't accept any null element
3. LinkedHashSet - accepts one null element

---

Any implementations of List, like ArrayList or LinkedList can accept nulls.