NullPointerException in TreeSet

[Naresh Chaurasia]

The following code

String s = null;
Set set = new TreeSet();
		set.add(s);
		set.add("s");
		System.out.println(set);

gives null pointer exception. I know that when the element is being added to TreeSet, which has to be maintained in sorted order, each element being added is compared with existing elements in TreeSet. If there is already null or null is added to TreeSet having elements, it will throw null pointer exception.

If such a case happens while using TreeSet then we can never TreeSet collection with null and other elements. If it is so then what is the purpose of allowing null in TreeSet.

[Mohit G Gupta]

From javadocs:


ClassCastException - if the specified object cannot be compared with the elements currently in this set
NullPointerException - if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

for add method

In the above code ,shouldn't add method throw ClassCastException instead of NullPointerException ???

[Wouter Oet]

I recently posted in another topic about this question here. It's because of TreeMap (internally used by the TreeSet) doesn't check for a null value if it's the first element. Otherwise it does. It throws a NullPointerException because the String comparable uses a method on the null reference.

[Arjun Srivastava]

elements in treeset must be comparable(i.e. they must implement the interface comparable).

[Wouter Oet]

Yes but that's not the problem here. The only element used is String and that class implements the Comparable interface.

[Arjun Srivastava]

hi wouter
yeah didn't saw above code clearly,your reason is right.
but saw your previous code where you have written code that any human can understand.