为什么在HashSet中添加null不会引发异常，而在TreeSet中添加null会引发异常

Question

Why adding null in HashSet does not throw Exception ,but adding null in TreeSet throw Exception.

Set<String>  s = new TreeSet<String>();
        s.add(null);

throws NullPointerException

Set<String>  s = new HashSet<String>();

Allow Null value to be added.

Answer 1

Because the underlying data structure of a TreeSet is a Red-Black tree , which is a binary search tree and thus is sorted. For it to be sorted there must be a Comparator that determines whether a value is equal, lower or greater than another value. The default Comparator is not null-safe, if you'd however write your own Comparator that has support for null it would be no problem to use null as a key.

Answer 2

Simply put, that is how it has been implemented. According to the Java specification for HashSet ,

This class permits the null element

And according to the javadoc for TreeSet in the add method it throws:

NullPointerException - if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements