Thread: 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