Java双向链表结构

Java双向链表结构（环形）：

/**
 * two way linked list
 * @param 
 */
public class TwoWayLink {

    private Node first, last;
    private int size;

    private void insert(Node insert) {
        if (isEmpty()) {
            first = insert;
            last = insert;
        } else {
            insert.next = first;
            insert.prev = last;
            first.prev = insert;
            last.next = insert;
        }
        size ++;
    }

    /**
     * head node insert
     * @param e
     */
    public void insertFirst (E e) {
        Node insert = new Node(e);
        insert(insert);
        first = insert;
    }

    /**
     * tail node insert
     * @param e
     */
    public void insertLast (E e) {
        Node insert = new Node(e);
        insert(insert);
        last = insert;
    }

    /**
     * check node
     * @return
     */
    public E peek () {
        if (isEmpty()) throw new NullPointerException("the link is empty, error.");
        return first.e;
    }

    public void display () {
        if (isEmpty()) throw new NullPointerException("the link is empty, error.");
        Node tmp = first;
        tmp = tmp.next;
        while (tmp!=first) {
            tmp = tmp.next;
        }

    }

    public boolean isEmpty () {
        return first == null;
    }

    public int getSize (){
        return size;
    }

    class Node {
        E e;
        Node prev, next;
        public Node(E e) {
            this.e = e;
        }
    }

}

Java双向链表结构（普通）：

/**
 * normal two way linked list
 * @param 
 */
public class TwoWayLink2 {

    private int size;
    private Node first, last;

    /**
     * insert head node
     * @param e
     */
    public void linkedHead (E e) {
        Node newNode = new Node(e);
        if (initLinked(newNode)) return;
        newNode.next = first;
        first.prev = newNode;
        first = newNode;
        size ++;
    }

    /**
     * insert tail node
     * @param e
     */
    public void linkedTail (E e) {
        Node newNode = new Node(e);
        if (initLinked(newNode)) return;
        last.next = newNode;
        newNode.prev = last;
        last = newNode;
        size ++;
    }

    /**
     * remove head node
     * @return
     */
    public E removeHead () {
        checkNull();
        Node node = first;
        first = node.next;
        return node.e;
    }

    /**
     * remove tail node
     * @return
     */
    public E removeTail () {
        checkNull();
        Node node = last;
        last = node.prev;
        return node.e;
    }

    private void checkNull () {
        if (size <= 0) throw new IllegalStateException("the linked is empty, error.");
    }

    /**
     * init linked list
     * @param newNode
     * @return
     */
    private boolean initLinked (Node newNode) {
        if (isEmpty()) {
            first = newNode;
            last = newNode;
            size ++;
            return true;
        }
        return false;
    }

    public boolean isEmpty () {
        return first == null;
    }

    public int getSize () {
        return size;
    }

    /**
     * linked iterator
     * @return
     */
    public Iterator getIterator() {
        if (isEmpty()) return null;
        return new Iterator(first);
    }
    
    class  Iterator {
        Node current;
        public Iterator(Node current) {
            this.current = current;
        }
		Node next (){
            return current.next;
        }
    }

    class Node {
        E e;
        Node prev, next;
        public Node(E e) {
            this.e = e;
        }
    }

}