String Operation

[lucky bhatia]

hi kindly tell me why I am getting Nullpointer Exception in following code.

(1) Vector vector=(some resultset from DB);
String str = "";
str = vector.elementAt(0).toString();

==========

Following code not giving exception

(2) Vector vector=(some resultset from DB);
String str="";
str = str+vector.elementAt(0).toString();

Although i have done with coding number (2), but i want to know the technical reason for failure of coding number (1).

Thanks & Regards

[Joyce Lee]

Hi Lucky,

Couldn't understand why the second method work. It doesn't make sense. If the vector is empty and elementAt(0) is called, java.lang.ArrayIndexOutOfBoundsException is thrown. If the vector is not empty and the elementAt(0) is called successfully, an object will be returned. A NullPointerException is thrown when trying to access members of a null object.

I wrote a simple code to experiment it.

  
import java.util.Vector;
 
public class VectorTest {
    public static void main(String[] args) {
        Vector v = new Vector();
        // ArrayIndexOutOfBoundsException is thrown when the following line
        // is commented, i.e. empty vector
        // v.add("Hello"); 
        Object o = v.elementAt(0);    
        String s = o.toString();
    }
}

  
import java.util.Vector;
 
public class VectorTest {
    public static void main(String[] args) {
        // NullPointerException is thrown here 
        Vector v = null;
        v.add("Hello"); // trying to access member of a null object
        Object o = v.elementAt(0);    
        String s = o.toString();
    }
}

Could the problem of NullPointerException caused by other part of the code?

Joyce
[ November 02, 2004: Message edited by: Joyce Lee ]

[Tony Morris]

v.elementAt(0) is returning null.
Your second diagnosis is false.
You shouldn't be using a Vector.