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

  1. #1
    Junior Member
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    Unhappy 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?

  2. #2
    Super Moderator Norm's Avatar
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    Default Re: can someone help me understand why i am getting this error.

    Please edit your post and wrap your code with code tags:

    [code]
    **YOUR CODE GOES HERE**
    [/code]

    to get highlighting and preserve formatting.


    element is null when the code exits the while loop. It can not be used to reference the variable: ref
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: can someone help me understand why i am getting this error.

    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

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