DoublyLinkedList.java

public class DoublyLinkedList<E> {
    private static class Node<E> {
        private E element;
        private Node<E> prev;
        private Node<E> next;

        public Node(E e, Node<E> p, Node<E> n){
            element = e;
            prev = p;
            next = n;
        }

        public E getElement(){
            return element;
        }

        public Node<E> getPrev(){
            return prev;
        }

        public Node<E> getNext(){
            return next;
        }

        public void setPrev(Node<E> p){
            prev = p;
        }
        public void setNext(Node<E> n){
            next = n;
        }
    }

    private Node<E> header;
    private Node<E> trailer;
    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();
    }

    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 void addBetween(E e, Node<E> predecessor, Node<E> successor){
        Node<E> newest = new Node<>(e, predecessor, successor);
        predecessor.setNext(newest);
        successor.setPrev(newest);
    }

    private E remove(Node<E> node){
        Node<E> predecessor = node.getPrev();
        Node<E> successor = node.getNext();
        predecessor.setNext(successor);
        successor.setPrev(predecessor);
        size--;
        return node.getElement();
    }

    public String toString(){
        StringBuilder sb = new StringBuilder("(");
        Node<E> walk = header.getNext();
        while(walk !=  trailer){
            sb.append(walk.getElement());
            walk = walk.getNext();
            if(walk != trailer){
                sb.append(", ");
            }
        }
        sb.append(")");
        return sb.toString();
    }
}