Nullpointer exception during object creation

Question

I am writing a stack program in Java. In the code the push function is causing a null pointer exception. I guess the node is not getting created. Please advice. Thanks in advance

//Stack_using_ll is a stack implementation using linked list 
public class Stack_using_ll{
    private Node first;
    private int count;
    private class Node {
        private String str;
        private Node next;
    }// Node has a value and reference

    public void push(String item){
        Node old_first = first;
        first = new Node();
        first.str = item;
        first.next = old_first.next;
        //first = node;
        count++;
    }//Inserts a new node
    public String pop(){
        String str_pop = first.str;
        first = first.next;
        count--;
        return str_pop;
    }//pops the string out of the stack
    public boolean is_empty(){
        if(first == null)
            return true;
        else
            return false;
    }//check if the stack is empty

    public static void main(String[] args){
        Stack_using_ll stack = new Stack_using_ll() ;
        stack.push("Jeans");
        System.out.println("There are " + stack.count + " elements in stack");
    }
}//End of class Stack_using_ll

-------------The output i get is as follows-----------------------------

java.lang.NullPointerException
    at Stack_using_ll$Node.access$2(Stack_using_ll.java:7)
    at Stack_using_ll.push(Stack_using_ll.java:14)
    at Stack_using_ll.main(Stack_using_ll.java:33)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

You can just use LinkedList for this.

Ingo
– Ingo

2013-10-14 13:25:36 +00:00
Commented Oct 14, 2013 at 13:25 — Ingo
– Ingo, Commented Oct 14, 2013 at 13:25

Ray Stojonic · Accepted Answer · 2013-10-14 13:27:06Z

1

In your code:

public class Stack_using_ll{
    private Node first;

private Node first only declares a reference to a Node named 'first', it does not create an instance of a Node for it to refer to.

So, when you later assign first to oldFirst, you're assigning a null, attempting to access a member of a null results in a NPE

public void push(String item){
        Node old_first = first;
        first = new Node();
        first.str = item;
        first.next = old_first.next; <- NPE here

answered Oct 14, 2013 at 13:27

Ray Stojonic

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Comments

Richard Tingle · Accepted Answer · 2013-10-14 13:27:10Z

0

Within your main method you create a new Stack_using_ll object, this creates any member variables, however you never give first a value (e.g. within the constructor), as such it remains null.

private Node first; //<--- first is null here and you use the blank constructor, so it is never given a non null value

You then call stack.push("Jeans"); in your main method which attempts to use first, but first is null, hence the exception.

public void push(String item){
    Node old_first = first; //<-- the initial null is preserved here
    first = new Node();
    first.str = item;
    first.next = old_first.next; //<-- you attempt to use the preserved null here
    //first = node;
    count++;
}

answered Oct 14, 2013 at 13:27

Richard Tingle

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Comments

Nimrod007 · Accepted Answer · 2013-10-14 13:27:49Z

0

problem is the null pointer in the first time you push to you object (because first item is null and you try to get its .next

see correction below : (in push function )

public void push(String item){
    Node old_first = first;
    first = new Node();
    first.str = item;
    //first time old_first is null!
    if (old_first != null){
       first.next = old_first.next;
    }
    //first = node;
    count++;
}

answered Oct 14, 2013 at 13:27

Nimrod007

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Comments

bogdan.herti · Accepted Answer · 2013-10-14 13:29:26Z

0

first is null at the beginning so when you do: old_first = first, old_first becomes null, so old_first.next gives you exception.

Solution:

 public void push(String item){
        Node old_first = first;
        first = new Node();
        first.str = item;
        if(old_first!=null)
           first.next = old_first.next;
    ...}

edited Oct 14, 2013 at 13:29

answered Oct 14, 2013 at 13:24

bogdan.herti

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