why this code has NullPointerException in java? - nullpointerexception

Question

I am new to java.
The following code has NullPointerException , I really don't know what's going on. Seems like I was trying to access a null pointer. But I have no idea where I was accessing the null pointer. (If it is the "head" node part, I didn't use its object slot in this code I think.)
Thanks so much!
class Deque<Base>
{
private class Node
{
private Base object;
private Node right; //point to the front Node
private Node left; //Point to the rear Node
//Constrcutor for Node
private Node(Base object, Node left, Node right)
{
this.object = object;
this.left = left;
this.right = right;
}
}
private Node head;
//Constructor for Deque
public Deque()
{
head = new Node(null,head,head);
}
//--------------------------------------
public void enqueueFront(Base object)
{
head.right = new Node(object, head, head.right);
head.right.right.left = head.right;
//Node temp = head.right;
//temp.left = head.right.right;
}
//--------------------------------------------
public void enqueueRear(Base object)
{
//Node temp = head.left;
head.left = new Node(object, head.left, head);
//temp.right = head.left;
head.left.left.right = head.left;
}
//------------------------------------------------
public Base dequeueFront()
{
if (isEmpty())
{
throw new IllegalStateException();
}
else
{
Base a = head.right.object;
Node temp = head.right.right;
temp.left = head;
head.right = temp;
return a;
}
}
//---------------------------------------------------
public Base dequeueRear()
{
if (isEmpty())
{
throw new IllegalStateException();
}
else
{
Base b = head.left.object;
Node temp = head.left.left;
temp.right = head;
head.left = temp;
return b;
}
}
//----------------------------------------------------
public boolean isEmpty()
{
return ((head.left == head)&&(head.right == head));
}
}
class Driver10
{
public static void main(String [] args)
{
Deque<Integer> a = new Deque<Integer>();
//int temp;
//a.dequeueFront();
a.enqueueFront(2);
//a.enqueueFront(3);
//a.enqueueFront(4);
//a.enqueueRear(6);
//a.enqueueRear(7);
//front: 4,3,2
//rear: 7,6
//temp = a.dequeueRear();
// System.out.println(temp); // 7
//System.out.println(a.dequeueRear()); // 6
//System.out.println(a.dequeueRear()); // 2
//System.out.println(a.dequeueFront()); //4
//System.out.println(a.dequeueRear()); // 3
//a.dequeueRear();
}
}

Answer 1

This statement looks very suspicious:
head.right.right.left = head.right;
It doesn't look likely that head.right.right would be anything other than null at the point that the code tries to access its left member.
It looks like you need to assign a new node to head.right.right before trying to assign to head.right.right.left

Answer 2

Multiple Issues:
The most relevants are:
head = new Node(null,head,head);
When you instantiate Dequeue, you assign bydefault to right and left as null as head is null. So post assigning right and left as null, you are assigning node to head.
Post that you try to enqueue an element and you try:
head.right = new Node(object, head, head.right);
head.right.right.left = head.right;
Here you are setting right as new node but see that head.right is still null as mentioned above. Post this statement your queue with right elements (just right for now) would be like:
head -> node -> null
Then you do
head.right.right.left = head.right;
Here you have valid node.right, but node.right.right is null as above and you are trying to access left for null which is where you are getting NullPointerException.
You might need to debug and make sure if you want something like or not:
head.right.left = head.right;