Is it possible to insert a null value into a TreeSet, and if so, under what conditions?

Inserting a null value into a TreeSet is possible but with restrictions. TreeSet is based on a red-black tree and requires elements to be comparable to maintain order.

• If TreeSet is created without a comparator, elements must implement Comparable, and inserting null will throw NullPointerException.
• If TreeSet is created with a comparator that can handle nulls, then inserting null is possible.

However, in standard Java implementation, inserting null into a TreeSet usually results in an exception because null cannot be compared with other elements.

Example:

TreeSet<String> set = new TreeSet<>();
set.add(null); // throws NullPointerException

TreeSet<String> setWithComparator = new TreeSet<>(Comparator.nullsFirst(String::compareTo));
setWithComparator.add(null); // allowed