Question

I am new at java and I am trying to write a Linked-List Stack..

public class Stack {

    private Node first;

    private class Node {
        int item;
        Node next;
    }

    public boolean IsEmpty()
    {
        return first==null;
    }

    public void push(int item)
    {
        Node oldfirst=first;
        first=new Node();
        first.item=item;
        first.next=oldfirst;
    }

    public int pop ()
    {
        int item=first.item;
        first=first.next;
        return item;
    }
}

---

import javax.swing.*;

public class main {

    public static void main(String[] args) {
        Stack ob=null;
        int num=0;
        while (true)
        {
            num=Integer.parseInt(JOptionPane.showInputDialog("Enter the number"));
            ob.push(num);
            if (num==0)
                break;
        }
        int k;
        k=ob.pop();
        JOptionPane.showMessageDialog(null, k);     
    }

now when I enter a number the compiler through
an Execption java.lang.NullPointerException
at main.main(main.java:18)

Why this is happening and how to avoid it
Please be patient and thanks in advance

Answer 1

Your stack ob is null when you call push. You need to instantiate it. Instead of

Stack ob=null;

you need to have

Stack ob = new Stack();