NullpointerException in ArrayList,Vector,LinkedList

[rex tony]

import java.util.*;
public class Example
{
public static void main(String args[])
{
//ArrayList AL= new ArrayList();
//Vector AL= new Vector();
LinkedList AL= new LinkedList();
AL.add(null);
AL.add("Rex");
AL.add("");
AL.add("Hema");
AL.add("Antony");
AL.add("Prem");
AL.add("Kamala");
AL.add("Hema");
Collections.sort(AL);
Iterator it=AL.iterator();
 
while(it.hasNext())
{
System.out.println(it.next());
}
 
}
}

In ArrayList,Vector,LinkedList,In which ,Why we cant store the null value but we can store in a "SPACE"?.How the compiler treaded as a "SPACE"?

If i add the null,at the runtime throws NullpointerException?

[Thirugnanam Saravanan]

SPACE ( " " ) , which you have used in your Code is treated as a "Empty String" which is still a valid Java Object for the Compiler.
But null is not a valid Java Object which can not be stored in ArrayList and Vector.

[Jesper de Jong]

As Thirugnanam says, a space is something entirely different than a null reference.

Originally posted by Thirugnanam Saravanan:
But null is not a valid Java Object which can not be stored in ArrayList and Vector.

That is not correct. You can store null in a Vector or ArrayList without problems. The API documentation of ArrayList says:

Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null.

The problem is not in the fact that you try to store null in an ArrayList or Vector. You are calling Collections.sort(...) on the ArrayList, and that method cannot handle null elements.

Look at the stack trace of the exception message that you get when you run your code. You will see that the exception happens in the line with Collections.sort(AL).