Our implementation of a doubly list relies on two sentinel nodes, header and trailer. Reemployment the DoublyLinkedList without using these nodes.

this is the original class:

public class DoublyLinkedList { private Node header; private Nodetrailer; private int size=0; public DoublyLinkedList() { header=new Node<>(null,null,null); trailer=new Node<>(null,header, null); header.setNext(trailer); } public int size() { return size;} public boolean isEmpty(){ return size==0;} public E first() { if (isEmpty()) return null; return header.getNext().getElement(); } public E last() { if (isEmpty()) return null; return trailer.getPrev().getElement(); } private void addBetween(E e, Node predecessor,Nodesuccessor) { Node newest=new Node<>(e,predecessor, successor); predecessor.setNext(newest); successor.setPrev(newest); size++; } private E remove(Node e) { Node predecessor=e.getPrev(); Node successor=e.getNext(); predecessor.setNext(successor); successor.setPrev(predecessor); size--; return e.getElement(); } public void addFirst(E e) { addBetween(e,header,header.getNext()); } public void addLast(E e) { addBetween(e,trailer.getPrev(),trailer); } public E removeFirst() { if(isEmpty()) return null; return remove(header.getNext()); } public E removeLast() { if(isEmpty()) return null; return remove(trailer.getPrev()); } private static class Node{ private E element; private Node next; private Node prev; public Node(E e,Node p, Node n) { element=e; prev=p; next=n; } public E getElement(){ return element; } public Node getPrev() { return prev;} public Node getNext() { return next;} public void setPrev(Node n) {prev=n;} public void setNext(Node n) {next=n;} } }