Collection LinkedList

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable       //1. 实现接口List、Deque
{
    transient int size = 0;

    /**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    transient Node<E> first;                   // 2. first 节点

    /**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    transient Node<E> last;                  // 3. last 节点

    /**
     * Constructs an empty list.
     */
    public LinkedList() {                    // 4. 构造函数
    }

 private static class Node<E> {
        E item;           // 1. data
        Node<E> next;     // 2. next potioner
        Node<E> prev;     // 3. prev potioner

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

  public boolean add(E e) {
        linkLast(e);
        return true;
    }

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

  public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

  private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * Tells if the argument is the index of an existing element.
     */
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

　　/**
　　 * Returns the (non-null) Node at the specified element index.
　　 */
    Node<E> node(int index) {     // ！！！折半遍历查找
      // assert isElementIndex(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;
      }
   }

　　 public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

 　 /**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;   // 1. 移动next prev 纸箱即可
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

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

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

        x.item = null;
        size--;
        modCount++;
        return element;
    }