Why null key is not allowed in TreeMap?

TreeMap does allow null keys. The default natural ordering comparator is the one that throws the exception.

From the documentation of TreeMap.put:

NullPointerException - if the specified key is null and this map uses natural ordering, or its comparator does not permit null keys

The easiest way to allow null values is to create the TreeMap with a comparator like Comparator.nullsFirst(Comparator.naturalOrder()) or Comparator.nullsLast(Comparator.naturalOrder())