Thread: can someone help me understand why i am getting this error.

This is a program to assign global variable with command line args:

class B
{
int x;
B ref;
B (int data)
{
this.x=data;
}
}

public class MyClass {
public static void main(String args[]) {
B b1 = null;
for (String s1 : args)
{
if (b1 == null)
{
b1 = new B(Integer.parseInt(s1));
}
else
{
B element = b1;
while (element != null)
{
element=element.ref;
}
element.ref=new B(Integer.parseInt(s1));
}
}
B element=b1;
while (element != null)
{
System.out.println(element.x + ",");
element=element.ref;

}

}
}

-the program is throwing one exception;i.e; Exception in thread "main" java.lang.NullPointerException: Cannot assign field "ref" because "<local6>" is null
at MyClass.main(MyClass.java:27).
this exception was caused because it should have been "element.ref" in the validation statement of the while loop which is inside the else block.

Can someone explain me why?

by input:
10 20
 
you get:
  new b1( x=10; ref=null)
 
  element = b1
  while() {
    element = b1.ref // null
  }
  elelement.ref =    // error element is null