public class LinkedList<E>extends AbstractSequentialList<E>implements List<E>, Deque<E>, Cloneable, java.io.Serializable

/*** 当前链表元素数量*/
transient int size = 0;
/*** Pointer to first node.* Invariant: (first == null && last == null) || (first.prev == null && first.item != null)* 链表头部节点           */
transient Node<E> first;
/*** Pointer to last node.* Invariant: (first == null && last == null) ||  (last.next == null && last.item != null)* 链表尾部节点          */
transient Node<E> last;

/*** Constructs an empty list.* 默认空构造器 -- 注意LinkedList并不提供指定容量的构造器*/
public LinkedList() {
}/*** Constructs a list containing the elements of the specified* collection, in the order they are returned by the collection's iterator.* 支持将一个Collection转换成LinkedList** @param  c the collection whose elements are to be placed into this list* @throws NullPointerException if the specified collection is null*/
public LinkedList(Collection<? extends E> c) {this();addAll(c);
}

/*** 存储对象的结构：*  每个Node节点包含了上一个节点和下一个节点的引用，从而构成了双向的链表*/
private static class Node<E> {E item;  //存储元素Node<E> next;  // 指向下一个节点Node<E> prev;  // 指向上一个节点//注意第一个元素是prev，第二个元素才是存储元素即可Node(Node<E> prev, E element, Node<E> next){this.item = element;this.next = next;this.prev = prev;}
}

/*** Appends the specified element to the end of this list.* <p>This method is equivalent to {@link #addLast}.* 插入一个新元素到链表尾部* @param e element to be appended to this list* @return {@code true} (as specified by {@link Collection#add}) 返回插入结果*/
public boolean add(E e) {linkLast(e);return true;
}/*** Inserts the specified element at the specified position in this list.* Shifts the element currently at that position (if any) and any* subsequent elements to the right (adds one to their indices).* 插入一个新元素到指定下标位置，大于该下标的所有元素统一向右移动一位* @param index index at which the specified element is to be inserted* @param element element to be inserted* @throws IndexOutOfBoundsException {@inheritDoc}*/
public void add(int index, E element) {checkPositionIndex(index);//下标边界校验if (index == size) //当下标==链表长度时，尾部插入
        linkLast(element);elselinkBefore(element, node(index));//否则，前部插入（起始位置为index）
}

private void checkPositionIndex(int index) {if (!isPositionIndex(index))throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}/*** Tells if the argument is the index of a valid position for an iterator or an add operation.* 当迭代或插入操作时，需要判断下标的边界*/
private boolean isPositionIndex(int index) {return index >= 0 && index <= size;
} 

/*** Links e as last element.* 将e变为链表的最后一个元素*/
void linkLast(E e) {final Node<E> l = last;final Node<E> newNode = new Node<>(l, e, null);//注意：新建node的next为nulllast = newNode;//将新建node作为链表尾部节点//当原队尾为null时，即链表为空时if (l == null)first = newNode;//将新建node同时作为链表头部节点elsel.next = newNode;//将原链表尾部节点的next引用指向新建node，形成链表结构size++;//当前链表长度+1modCount++;//新增操作属于结构性变动，modCount计数+1
}

/*** Inserts element e before non-null Node succ.* 插入一个新的元素到指定非空节点之前*/
void linkBefore(E e, Node<E> succ) {// assert succ != null;//逻辑与linkLast基本一致，区别在于将last变成prev，将新节点插入到succ节点之前final Node<E> pred = succ.prev;final Node<E> newNode = new Node<>(pred, e, succ);succ.prev = newNode;//唯一的区别，将新节点插入到succ节点之前if (pred == null)first = newNode;elsepred.next = newNode;size++;modCount++;//插入属于结构性变动，modCount计数+1
}

/*** Returns the element at the specified position in this list.* 获取指定下标元素* @param index index of the element to return* @return the element at the specified position in this list* @throws IndexOutOfBoundsException {@inheritDoc}*/
public E get(int index) {checkElementIndex(index);  //判断下标是否存在元素return node(index).item;  //注意返回不是node，而是item；同时node一定不为null，而item允许为null
}

/*** Returns the (non-null) Node at the specified element index.* 返回指定下标的非空node*/
Node<E> node(int index) {// assert isElementIndex(index);//值得一提的是，为了提高查询效率，node查询选择使用二分查找法if (index < (size >> 1)) { Node<E> x = first; //若在前半边，就从前往后找for (int i = 0; i < index; i++)x = x.next;return x;} else {Node<E> x = last; //若在后半边，就从后往前找for (int i = size - 1; i > index; i--)x = x.prev;return x;}
}

/*** Retrieves and removes the head (first element) of this list.* 默认删除头部节点* @return the head of this list* @throws NoSuchElementException if this list is empty* @since 1.5*/
public E remove() {return removeFirst();
}
/*** Removes the element at the specified position in this list.  Shifts any* subsequent elements to the left (subtracts one from their indices).* Returns the element that was removed from the list.* 根据下标删除元素* @param index the index of the element to be removed* @return the element previously at the specified position* @throws IndexOutOfBoundsException {@inheritDoc}*/
public E remove(int index) {checkElementIndex(index);//边界校验 index >= 0 && index < sizereturn unlink(node(index));//解绑操作
}
/*** Removes the first occurrence of the specified element from this list,if it is present. * If this list does not contain the element, it is unchanged.* More formally, removes the element with the lowest index {@code i}  such that* <tt>(o==null?get(i)==null:;o.equals(get(i)))</tt> (if such an element exists).* Returns {@code true} if this list contained the specified element (or equivalently, * if this list changed as a result of the call).* 直接移除某个元素：*     当该元素不存在，不会发生任何变化*     当该元素存在且成功移除时，返回true，否则false*     当有重复元素时，只删除第一次出现的同名元素 ：*        例如只移除第一次出现的null（即下标最小时出现的null）* @param o element to be removed from this list, if present* @return {@code true} if this list contained the specified element*/
public boolean remove(Object o) {//虽然跟ArrayList一样需要遍历，但由于不需要调用耗时的`System.arraycopy`，效率更高if (o == null) {for (Node<E> x = first; x != null; x = x.next) {if (x.item == null) {unlink(x);return true;}}} else {for (Node<E> x = first; x != null; x = x.next) {if (o.equals(x.item)) {unlink(x);return true;}}}return false;
}

/*** Unlinks non-null node x.* 解除node链接，主要干了三件事情：*     1.解绑当前元素的前后节点链接，前后节点重新绑定关系*     2.当前元素的所有属性清空，help gc*     3.链表长度-1，modCount计数+1(help fail-fast)* @return 返回元素本身 注意是item，而不是node*/
E unlink(Node<E> x) {// assert x != null;final E element = x.item;final Node<E> next = x.next;//后一位节点final Node<E> prev = x.prev;//前一位节点//解绑前一位节点if (prev == null) {//当前节点位于链表头部first = next;//后一位节点放链表头部} else {//非链表头部prev.next = next;//将前一位节点的next指向下一位节点x.prev = null;//当前节点的前一位节点清空 ，help gc
    }//解绑后一位节点if (next == null) {//当前节点位于链表尾部last = prev;//前一位节点放链表尾部} else {//非链表尾部next.prev = prev;//将后一位节点的prev指向前一位节点x.next = null;//当前节点的后一位节点清空 ，help gc
    }x.item = null;//当前节点元素清空size--;//链表长度-1modCount++;//删除操作属于结构性变动，modCount计数+1return element;//返回元素本身
}

/*** 堆栈(Stack)的LinkedList版本简单实现* 这里使用first（使用last原理也一样，保证只在一端操作即可）*/
class Stack<T> {LinkedList<T> linkedList = new LinkedList<T>();/*** 入栈 */public void push(T v) {linkedList.addFirst(v);}/*** 出栈，不删除栈顶元素*/public T peek() {return storage.getFirst();}/*** 出栈 ，删除栈顶元素*/public T pop() {return storage.removeFirst();}
}    

/*** 队列的LinkedList版本简单实现* 这里使用队尾插入，对头删除的写法（反过来原理一致，只要保证插入和删除各占一端即可）*/
class Queue<T> {LinkedList<T> linkedList = new LinkedList<T>();/*** 入队，将指定的元素插入队尾*/public void offer(T v) {linkedList.offer(v);}/*** 出队，获取头部元素，但不删除，如果此队列为空，则返回 null*/public T peek() {return linkedList.peek();}/*** 出队，获取头部元素，但不删除，如果此队列为空，则抛异常*/public T element() {return linkedList.element();}/*** 出队，获取头部元素并删除，如果队列为空，则返回 null*/public T poll() {return linkedList.poll();}/*** 出队，获取头部元素并删除，如果队列为空，则抛异常*/public T remove() {return linkedList.remove();}
}