Shift elements in a linked list in java

Question

I have to shift all of the tokens left by one position in a Linked List.

Here's my code for the method:

private LLNode<E> head;     // the first node in the list
private LLNode<E> tail;     // the last node in the list

public void shiftLeft()
{
    LLNode<E> temp = new LLNode<E>();
    temp = head;
    head = head.next;
    tail.next = temp;
}

/*from main method
TopSpinLinkedList<Integer> ll = new TopSpinLinkedList<Integer>(numTokens, spinSize);

//fills LinkedList with tokens
for(int i = 1; i <= numTokens; i++) {
    ll.add(i);
}
*/

A nullpointer error appears during runtime when I call the method. Any help would be appreciated. Thanks.

Answer 1

You have to think about some points:
1) If your link list does not contain any element then what?
2) For all tokens to be shifted you have to use while-loop.

Answer 2

I assume that head and tail are properly updated on insert and remove

public void shiftLeft()
{
    if(head == null || head.next == null){
       return;    
    }
    LLNode<E> temp = new LLNode<E>();     
    temp = head;           
    head = head.next;  
    temp.next = null;   
    tail.next = temp;  
    tail = temp;  
}  

UPDATE:
From the comment I see that the OP mentions about a circular list. This is not mentioned in the OP or apparent from the code. I will leave the answer as is.

Answer 3

If it is circular linked list and your add method works properly.

public void shiftLeft(){
    head = head.next; tail = tail.next;
}