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Thread: linked list null pointer exception

  1. #1
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    Default linked list null pointer exception

    I am trying to create a Linked List class for my programming homework. I need to make my insert method perform the lookup method first so that I cannot insert any duplicate elements. the code works and compiles fine without my if(lookup==false) conditional, but with my current code below I get a null pointer exception at temp.data.equals(x) in the lookup method. Here is my linkedlist class:
    public class linkedList implements myLinkedList
    {
     
    	public myNode head = null;
    	public myNode tail = null;
    	public myNode temp = null;
     
    	public linkedList()
    	{
    		myNode node = new myNode();
    		node = null;
    	}
    	//inserts objects to the end of the list if not already there
    	public void insert(Object x)
    	{
    		if(lookup(x) == false)
    		{
    			if(head==null)
    			{
    				head = new myNode();
    				tail = new myNode();
    				head.data = x;
    				head.next = tail;
    				tail = head;
    			}
    			else
    			{
    				tail.next = new myNode();
    				tail = tail.next;
    				tail.data = x;
    			}
    		}
    	}
    	public void delete(Object x)
    	{
    	}
    	public boolean lookup(Object x) //interface in lab says public Object, but directions specify for it to return boolean value
    	{
    		temp = head;
    		while(temp != null)
    		{
    			if(temp.data.equals(x)) return true;
    			temp = temp.next;
    		}
    		return false;
    	}
    	public boolean isEmpty()
    	{
    		if(head==null) return true;
    		else return false;
    	}
    	public void printList()
    	{
    		myNode index = head;
    		while(index != null)
    		{
    			System.out.println(index.data.toString());
    			index = index.next;
    		}
    	}
    }
    this will work the first time i try to insert an object, but any subsequent attemps result in the null pointer exception. I am not sure what the problem is, any advice would be extremely helpful
    Last edited by sonicjr; February 16th, 2012 at 09:04 PM.


  2. #2
    Java kindergarten chronoz13's Avatar
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    Default Re: linked list null pointer exception

    where does the nullpointer points out in your line of code? , in that way you/we can determine or atleast have a clue to what causes the exception.. i think its on the constructor of your class

  3. #3
    Super Moderator Norm's Avatar
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    Default Re: linked list null pointer exception

    I get a null pointer exception at temp.data.equals(x) i
    What variable has a null value? Find which one and then backtrack in the code to see why that variable does not have a valid value.

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