These Sets allow null. Why can't I add null elements?

Question

I want to know why HashSet, LinkedHashSet and TreeSet implementation does not allow null elements? Whenever i try to run the following code it throws a null pointer exception.

public static void main(String[] args) {      HashSet<Integer> hashSet = new HashSet<Integer>();      hashSet.add(2);     hashSet.add(5);     hashSet.add(1); //  hashSet.add(null);  will throw null pointer      hashSet.add(999);     hashSet.add(10);     hashSet.add(10);     hashSet.add(11);     hashSet.add(9);     hashSet.add(10);     hashSet.add(000);     hashSet.add(999);     hashSet.add(0);      Iterator<Integer> it = hashSet.iterator();     while(it.hasNext()){         int i = it.next();         System.out.print(i+" ");     }     } 

Please guide me.

Answer 1

This is why I don't like to rely on auto-boxing. Java Collections cannot store primitives (for that you will need a third party API like Trove). So, really, when you execute code like this:

hashSet.add(2); hashSet.add(5); 

What is really happening is:

hashSet.add(new Integer(2)); hashSet.add(new Integer(5)); 

Adding a null to the hash set is not the problem, that part works just fine. Your NPE comes later, when you try and unbox your values into a primitive int:

while(it.hasNext()){     int i = it.next();     System.out.print(i+" "); } 

When the null value is encountered, the JVM attempts to unbox it into an the int primitive, which leads to an NPE. You should change your code to avoid this:

while(it.hasNext()){     final Integer i = it.next();     System.out.print(i+" "); }