Collection-LinkedList

简单的实现linkedlist：
1、提供ListIterator的实现（包括hasNext、next、hasPrevious、previous、nextIndex、previousIndex、add、set、remove）。

2、提供size方法。

package Collection;

import java.util.AbstractSequentialList;
import java.util.ListIterator;
/**
 *
 * @author lichunguang
 * @param <E>
 */
public class MyLinkedList<E> extends AbstractSequentialList<E> {
    private int size; // 维护此列表的元素个数
    private Node<E> first; // 此列表的第一个节点
    private Node<E> last; // 此列表的最后一个节点

    public MyLinkedList() {
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public ListIterator<E> listIterator(int index) {
        return new ListItr(index);
    }

    // 私有静态的节点类。
    private static class Node<E> {

        private E item; // 此节点持有的数据
        private Node<E> prev;// 上一个节点
        private Node<E> next;// 下一个节点

        Node(Node<E> prev, E item, Node<E> next) {
            this.prev = prev;
            this.item = item;
            this.next = next;
        }
    }

    // 获取第index个节点。
    private Node<E> node(int index) {
        if (index < size >> 1) {
            Node<E> x = first;

            for (int i = 0; i < index; i++) {
                x = x.next;
            }
            return x;
        } else {
            Node<E> x = last;
            for (int i = size - 1; i > index; i--) {
                x = x.prev;
            }
            return x;
        }
    }

    // 迭代器的实现。
    private class ListItr implements ListIterator<E> {

        private Node<E> lastRet; // 上一次返回的节点
        private Node<E> next; // 下一个节点
        private int nextIndex; // 下一节点的索引

        ListItr(int index) {
            // 指定迭代器的开始节点
            next = (index >= size) ? null : node(index);
            nextIndex = index;
        }

        @Override
        public boolean hasNext() {
            return nextIndex < size;
        }

        @Override
        public E next() {
            lastRet = next;
            next = next.next;
            nextIndex++;
            return lastRet.item;
        }

        @Override
        public boolean hasPrevious() {
            return nextIndex > 0;
        }

        @Override
        public E previous() {
            lastRet = next = next.prev;
            nextIndex--;
            return lastRet.item;
        }

        @Override
        public int nextIndex() {
            return nextIndex;
        }

        @Override
        public int previousIndex() {
            return nextIndex - 1;
        }

        @Override
        public void remove() {
            if (lastRet == null) {
                throw new IllegalStateException();
            }
            unlink(lastRet);
            final Node<E> lastNext = lastRet.next;
            // ensure the next not be the removed one
            if (next == lastRet) {
                next = lastNext;
            } else {
                nextIndex--;
            }
            lastRet = null;
        }

        @Override
        public void set(E e) {
            if (lastRet == null) {
                throw new IllegalStateException();
            }
            lastRet.item = e;
        }

        @Override
        public void add(E e) {
            lastRet = null;
            if (next == null) {
                linkLast(e);
            } else {
                linkBefore(e, next);
            }
            nextIndex++;
        }
    }

    /**
     * Links e as last element.
     */
    private void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null) {
            first = newNode;
        } else {
            l.next = newNode;
        }
        size++;
    }

    /**
     * Inserts element e before non-null Node next.
     */
    private void linkBefore(E e, Node<E> next) {
        final Node<E> prev = next.prev;
        final Node<E> newNode = new Node<>(prev, e, next);
        next.prev = newNode;
        if (prev == null) {
            first = newNode;
        } else {
            prev.next = newNode;
        }
        size++;
    }

    /**
     * unlink node e.
     */
    private E unlink(Node<E> x) {
        final E val = x.item;
        final Node<E> prev = x.prev;
        final Node<E> next = x.next;

        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        x.item = null;
        size--;
        return val;
    }
}