LinkedList源码分析

/**
 * 1、是否允许空：允许
 * <p>
 * 2、是否允许重复数据：允许
 * <p>
 * 3、是否有序，有序的意思是读取数据的顺序和存放数据的顺序是否一致：无序
 * <p>
 * 4、是否线程安全：非线程安全
 * <p>
 * 5、底层数据结构：双向链表。实现了Deque这个接口，所以也可以当栈和队列使用
 */
package java.util;

import java.util.function.Consumer;

public class LinkedList<E>
        extends AbstractSequentialList<E>
        implements List<E>, Deque<E>, Cloneable, java.io.Serializable {
    /**
     * 元素个数
     */
    transient int size = 0;

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

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

    /**
     * 无参构造方法
     * Constructs an empty list.
     */
    public LinkedList() {
    }

    /**
     * 带集合参数的构造方法，元素顺序由集合的迭代器返回顺序决定
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @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);
    }

    /**
     * 插入头结点
     * Links e as first element.
     */
    private void linkFirst(E e) {
        // 获取当前首节点引用
        final Node<E> f = first;
        // new一个prev值为null,节点值为e,next值为f的新节点newNode
        final Node<E> newNode = new Node<>(null, e, f);
        // 把新节点作为首节点
        first = newNode;
        // 如果原首节点为null，即原链表为空，把链表尾结点last设置为newNode
        if (f == null)
            last = newNode;
        else // 否则，把原首节点的前驱节点prev设置为newNode
            f.prev = newNode;
        // 元素个数+1
        size++;
        // 修改次数+1
        modCount++;
    }

    /**
     * 插入尾结点
     * Links e as last element.
     */
    void linkLast(E e) {
        // 获取当前尾结点引用
        final Node<E> l = last;
        // new一个prev值为尾结点引用，节点值为e，next值为null的新节点newNode
        final Node<E> newNode = new Node<>(l, e, null);
        // 把新节点作为尾节点
        last = newNode;
        // 如果原尾节点为null，即原链表为空，把链表首节点first设置为newNode
        if (l == null)
            first = newNode;
        else // 否则，把原尾节点的后继节点next设置为newNode
            l.next = newNode;
        // 元素个数+1
        size++;
        // 修改次数+1
        modCount++;
    }

    /**
     * 在非空节点succ前插入元素e
     * Inserts element e before non-null Node succ.
     */
    void linkBefore(E e, Node<E> succ) {
        // 断言succ不为空assert succ != null;
        // 取出节点succ的前驱节点prev
        final Node<E> pred = succ.prev;
        // new一个prev值为pred，节点值为e，next值为succ的新节点newNode
        final Node<E> newNode = new Node<>(pred, e, succ);
        // 把新节点作为cucc的前驱节点prev
        succ.prev = newNode;
        // 如果pred为空，即succ为首节点，把新节点newNode设置为首节点
        if (pred == null)
            first = newNode;
        else // 如果pred不为空，即succ不是首节点
            pred.next = newNode;
        // 元素个数+1
        size++;
        // 修改次数+1
        modCount++;
    }

    /**
     * 删除首节点，返回其存储的元素（内部使用）
     * Unlinks non-null first node f.
     */
    private E unlinkFirst(Node<E> f) {
        // 断言首节点不为空assert f == first && f != null;
        // 获取首节点存储的元素
        final E element = f.item;
        // 获取首节点的后继节点next
        final Node<E> next = f.next;
        // 删除首节点
        f.item = null;
        // 置空便于垃圾回收
        f.next = null; // help GC
        // 把原首节点的后继节点next节点设置为首节点
        first = next;
        // 如果原首节点的后继节点next为空，把尾结点last置空
        if (next == null)
            last = null;
        else // 否则，把原首节点的后继节点next的前驱节点prev置为空
            next.prev = null;
        // 元素个数-1
        size--;
        // 修改次数+1
        modCount++;
        // 返回原首节点存储的元素
        return element;
    }

    /**
     * 删除尾结点，返回其存储的元素（内部使用）
     * Unlinks non-null last node l.
     */
    private E unlinkLast(Node<E> l) {
        // 断言尾节点不为空assert l == last && l != null;
        // 获取尾结点存储的元素
        final E element = l.item;
        // 获取尾结点的前驱节点
        final Node<E> prev = l.prev;
        // 删除尾结点
        l.item = null;
        // 置空便于垃圾回收
        l.prev = null; // help GC
        // 把原尾结点的前驱节点prev设置为尾结点
        last = prev;
        // 如果原尾结点的前驱节点prev为空，把首节点置空
        if (prev == null)
            first = null;
        else // 否则把原尾结点的前驱节点的后继节点next置空
            prev.next = null;
        // 元素个数-1
        size--;
        // 修改次数+1
        modCount++;
        // 返回原尾结点元素
        return element;
    }

    /**
     * 删除指定非空节点x，返回其存储的元素
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // 断言节点x不为空assert x != null;
        // 获取节点x存储的元素
        final E element = x.item;
        // 获取节点x的后继节点next
        final Node<E> next = x.next;
        // 获取节点x的前驱节点prev
        final Node<E> prev = x.prev;

        // 如果prev为空，即节点x为首节点，把节点x的后继节点next设置为首节点
        // 否则把节点x的后继节点设置为节点x的前驱节点的后继节点，节点x的前驱节点置空
        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        // 如果next为空，即节点x为尾结点，把节点x的前驱节点设置为尾结点
        // 否则把节点x的前驱节点设置节点x的后继节点的前驱节点
        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        // 删除节点x
        x.item = null;
        // 元素个数-1
        size--;
        // 修改次数+1
        modCount++;
        // 返回节点x存储的元素
        return element;
    }

    /**
     * 获取首节点存储的元素
     * Returns the first element in this list.
     *
     * @return the first element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getFirst() {
        // 获取首节点引用
        final Node<E> f = first;
        // 如果首节点引用为空，抛出异常
        if (f == null)
            throw new NoSuchElementException();
        // 否则返回首节点存储的元素
        return f.item;
    }

    /**
     * 返回尾结点存储的元素
     * Returns the last element in this list.
     *
     * @return the last element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getLast() {
        // 获取尾结点引用
        final Node<E> l = last;
        // 如果尾结点为空，抛出异常
        if (l == null)
            throw new NoSuchElementException();
        // 否则返回尾结点存储的元素
        return l.item;
    }

    /**
     * 删除并返回其存储的元素（外部使用）
     * Removes and returns the first element from this list.
     *
     * @return the first element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeFirst() {
        // 获取首节点引用
        final Node<E> f = first;
        // 如果首节点引用为空，抛出异常
        if (f == null)
            throw new NoSuchElementException();
        // 删除首节点并返回其存储的元素
        return unlinkFirst(f);
    }

    /**
     * 删除尾结点并返回其存储的元素（外部使用）
     * Removes and returns the last element from this list.
     *
     * @return the last element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeLast() {
        // 获取尾结点引用
        final Node<E> l = last;
        // 如果尾结点为空，抛出异常
        if (l == null)
            throw new NoSuchElementException();
        // 删除尾结点并返回其存储的元素
        return unlinkLast(l);
    }

    /**
     * 在头部插入指定元素e
     * Inserts the specified element at the beginning of this list.
     *
     * @param e the element to add
     */
    public void addFirst(E e) {
        linkFirst(e);
    }

    /**
     * 在尾部插入指定元素e
     * Appends the specified element to the end of this list.
     * <p>
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     */
    public void addLast(E e) {
        linkLast(e);
    }

    /**
     * 判断是否包含指定元素o
     * <p>
     * Returns {@code true} if this list contains the specified element.
     * More formally, returns {@code true} if and only if this list contains
     * at least one element {@code e} such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested
     * @return {@code true} if this list contains the specified element
     */
    public boolean contains(Object o) {
        // 获取指定元素的索引位置，不存在为-1，再与-1作比较
        return indexOf(o) != -1;
    }

    /**
     * 返回元素个数
     * Returns the number of elements in this list.
     *
     * @return the number of elements in this list
     */
    public int size() {
        return size;
    }

    /**
     * 插入指定元素e，默认在尾部插入
     * Appends the specified element to the end of this list.
     * <p>
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    /**
     * 删除指定元素，默认从首节点开始，删除第一次出现的那个元素
     * 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&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;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).
     *
     * @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) {
        // 如果要删除的对象为null
        if (o == null) {
            // 遍历，查找到指定元素后删除该结点，返回true（用==作比较）
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else { // 如果要删除的对象不为null
            // 遍历，查找到指定元素后删除该结点，返回true（用equals作比较）
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * 将集合插入到链表尾部
     * 开始位置索引为size
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator.  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (Note that this will occur if the specified collection is
     * this list, and it's nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element
     *              from the specified collection
     * @param c     collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException      if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        // 检查索引是否越界
        checkPositionIndex(index);

        // 转换成数组
        Object[] a = c.toArray();
        // 获取数组的长度
        int numNew = a.length;
        // 长度等于0，即集合为空，直接返回false
        if (numNew == 0)
            return false;

        // 定义两个节点pred，succ，pred表示要插入元素的前驱节点，succ表示要插入元素的后继节点
        Node<E> pred, succ;
        // 查询需要插入位置的前驱和后继节点
        // 如果index == size，说明在尾部添加，所以succ = null，pred = last
        if (index == size) {
            succ = null;
            pred = last;
        } else { // 否则，succ为指定index位置的节点，pred为succ的前驱节点
            succ = node(index);
            pred = succ.prev;
        }

        // 遍历数组并插入到链表中
        for (Object o : a) {
            @SuppressWarnings("unchecked") E e = (E) o;
            // 创建一个prev为pred，元素为e，next为null的新节点newNode
            Node<E> newNode = new Node<>(pred, e, null);
            // 如果pred为空，则把新插入的节点作为链表首节点
            if (pred == null)
                first = newNode;
            else // 否则把前驱节点的next指向新节点
                pred.next = newNode;
            // pred指针向后移动，指向下一个待插入节点的前驱节点
            pred = newNode;
        }

        // 如果后继节点为空，那么c集合中最后一个节点就是整个list的最后节点
        if (succ == null) {
            last = pred;
        } else {// 否则，把c中的最后一个节点的后继节点指向succ，把后继节点的前驱节点指向c中的最后一个节点
            pred.next = succ;
            succ.prev = pred;
        }

        // 元素个数为size+numNew
        size += numNew;
        // 修改次数+1
        modCount++;
        return true;
    }

    /**
     * 删除整个链表
     * Removes all of the elements from this list.
     * The list will be empty after this call returns.
     */
    public void clear() {
        // Clearing all of the links between nodes is "unnecessary", but:
        // - helps a generational GC if the discarded nodes inhabit
        //   more than one generation
        // - is sure to free memory even if there is a reachable Iterator
        // 遍历，删除所有节点，便于垃圾回收
        for (Node<E> x = first; x != null; ) {
            Node<E> next = x.next;
            x.item = null;
            x.next = null;
            x.prev = null;
            x = next;
        }
        // 把首节点、尾结点置空
        first = last = null;
        // 重置元素个数
        size = 0;
        // 修改次数+1
        modCount++;
    }


    // Positional Access Operations

    /**
     * 获取指定位置的元素
     * Returns the element at the specified position in this list.
     *
     * @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);
        return node(index).item;
    }

    /**
     * 修改指定位置的元素，返回修改前的元素
     * Replaces the element at the specified position in this list with the
     * specified element.
     *
     * @param index   index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

    /**
     * 在指定位置前插入元素
     * 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).
     *
     * @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);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }

    /**
     * 删除指定位置的元素，并返回该元素
     * 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.
     *
     * @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);
        return unlink(node(index));
    }

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

    /**
     * Tells if the argument is the index of a valid position for an
     * iterator or an add operation.
     */
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    /**
     * Constructs an IndexOutOfBoundsException detail message.
     * Of the many possible refactorings of the error handling code,
     * this "outlining" performs best with both server and client VMs.
     */
    private String outOfBoundsMsg(int index) {
        return "Index: " + index + ", Size: " + size;
    }

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

    /**
     * 检查越界
     *
     * @param index
     */
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 获取指定下标的结点
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // 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;
        }
    }

    // Search Operations

    /**
     * 获取正向首次出现指定元素的位置
     * 如果返回结果是-1，则表示不存在该元素
     * <p>
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index {@code i} such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     * this list, or -1 if this list does not contain the element
     */
    public int indexOf(Object o) {
        int index = 0;
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null)
                    return index;
                index++;
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item))
                    return index;
                index++;
            }
        }
        return -1;
    }

    /**
     * 获取逆向首次出现指定元素的位置
     * 如果返回结果是-1，则表示不存在该元素
     * <p>
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index {@code i} such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     * this list, or -1 if this list does not contain the element
     */
    public int lastIndexOf(Object o) {
        int index = size;
        if (o == null) {
            for (Node<E> x = last; x != null; x = x.prev) {
                index--;
                if (x.item == null)
                    return index;
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                index--;
                if (o.equals(x.item))
                    return index;
            }
        }
        return -1;
    }

    // 队列操作Queue operations.

    /**
     * 出队，获取但不删除首元素
     * Retrieves, but does not remove, the head (first element) of this list.
     *
     * @return the head of this list, or {@code null} if this list is empty
     * @since 1.5
     */
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    /**
     * 出队，获取但不删除首元素，若为null会抛出异常而不是返回null
     * Retrieves, but does not remove, the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E element() {
        return getFirst();
    }

    /**
     * 获取并删除首元素
     * Retrieves and removes the head (first element) of this list.
     *
     * @return the head of this list, or {@code null} if this list is empty
     * @since 1.5
     */
    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /**
     * 删除首节点元素并返回该元素，如果不存在会抛出异常而不是返回null
     * Retrieves and removes the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E remove() {
        return removeFirst();
    }

    /**
     * 入队，在末尾添加指定元素
     * Adds the specified element as the tail (last element) of this list.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Queue#offer})
     * @since 1.5
     */
    public boolean offer(E e) {
        return add(e);
    }

    // 双端队列操作Deque operations

    /**
     * 入队，在头部插入指定元素
     * Inserts the specified element at the front of this list.
     *
     * @param e the element to insert
     * @return {@code true} (as specified by {@link Deque#offerFirst})
     * @since 1.6
     */
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    /**
     * 入队，在尾部插入指定元素
     * Inserts the specified element at the end of this list.
     *
     * @param e the element to insert
     * @return {@code true} (as specified by {@link Deque#offerLast})
     * @since 1.6
     */
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

    /**
     * 出队，获取首元素，不存在会返回null，不会删除元素
     * Retrieves, but does not remove, the first element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the first element of this list, or {@code null}
     * if this list is empty
     * @since 1.6
     */
    public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    /**
     * 出队，获得尾元素，不存在会返回null，不会删除元素
     * Retrieves, but does not remove, the last element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the last element of this list, or {@code null}
     * if this list is empty
     * @since 1.6
     */
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

    /**
     * 出队，获取并删除首元素，不存在会返回null
     * Retrieves and removes the first element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the first element of this list, or {@code null} if
     * this list is empty
     * @since 1.6
     */
    public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /**
     * 出队，获取并删除尾元素，不存在返回null
     * Retrieves and removes the last element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the last element of this list, or {@code null} if
     * this list is empty
     * @since 1.6
     */
    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

    /**
     * 入栈，在前面插入指定元素
     * Pushes an element onto the stack represented by this list.  In other
     * words, inserts the element at the front of this list.
     * <p>
     * <p>This method is equivalent to {@link #addFirst}.
     *
     * @param e the element to push
     * @since 1.6
     */
    public void push(E e) {
        addFirst(e);
    }

    /**
     * 出栈，返回栈顶元素并删除
     * Pops an element from the stack represented by this list.  In other
     * words, removes and returns the first element of this list.
     * <p>
     * <p>This method is equivalent to {@link #removeFirst()}.
     *
     * @return the element at the front of this list (which is the top
     * of the stack represented by this list)
     * @throws NoSuchElementException if this list is empty
     * @since 1.6
     */
    public E pop() {
        return removeFirst();
    }

    /**
     * 删除正向首次出现的指定元素
     * Removes the first occurrence of the specified element in this
     * list (when traversing the list from head to tail).  If the list
     * does not contain the element, it is unchanged.
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if the list contained the specified element
     * @since 1.6
     */
    public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }

    /**
     * 删除逆向首次出现的指定元素
     * Removes the last occurrence of the specified element in this
     * list (when traversing the list from head to tail).  If the list
     * does not contain the element, it is unchanged.
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if the list contained the specified element
     * @since 1.6
     */
    public boolean removeLastOccurrence(Object o) {
        if (o == null) {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    /**
     * Returns a list-iterator of the elements in this list (in proper
     * sequence), starting at the specified position in the list.
     * Obeys the general contract of {@code List.listIterator(int)}.<p>
     * <p>
     * The list-iterator is <i>fail-fast</i>: if the list is structurally
     * modified at any time after the Iterator is created, in any way except
     * through the list-iterator's own {@code remove} or {@code add}
     * methods, the list-iterator will throw a
     * {@code ConcurrentModificationException}.  Thus, in the face of
     * concurrent modification, the iterator fails quickly and cleanly, rather
     * than risking arbitrary, non-deterministic behavior at an undetermined
     * time in the future.
     *
     * @param index index of the first element to be returned from the
     *              list-iterator (by a call to {@code next})
     * @return a ListIterator of the elements in this list (in proper
     * sequence), starting at the specified position in the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see List#listIterator(int)
     */
    public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
    }

    private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;
        private Node<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;

        ListItr(int index) {
            // assert isPositionIndex(index);
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }

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

        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.item;
        }

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

        public E previous() {
            checkForComodification();
            if (!hasPrevious())
                throw new NoSuchElementException();

            lastReturned = next = (next == null) ? last : next.prev;
            nextIndex--;
            return lastReturned.item;
        }

        public int nextIndex() {
            return nextIndex;
        }

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

        public void remove() {
            checkForComodification();
            if (lastReturned == null)
                throw new IllegalStateException();

            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = null;
            expectedModCount++;
        }

        public void set(E e) {
            if (lastReturned == null)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.item = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            while (modCount == expectedModCount && nextIndex < size) {
                action.accept(next.item);
                lastReturned = next;
                next = next.next;
                nextIndex++;
            }
            checkForComodification();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    /**
     * 节点的数据结构
     *
     * @param <E>
     */
    private static class Node<E> {
        E item;// 存储的元素
        Node<E> next;// 后继节点
        Node<E> prev;// 前驱节点

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

    /**
     * @since 1.6
     */
    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    /**
     * Adapter to provide descending iterators via ListItr.previous
     */
    private class DescendingIterator implements Iterator<E> {
        private final ListItr itr = new ListItr(size());

        public boolean hasNext() {
            return itr.hasPrevious();
        }

        public E next() {
            return itr.previous();
        }

        public void remove() {
            itr.remove();
        }
    }

    @SuppressWarnings("unchecked")
    private LinkedList<E> superClone() {
        try {
            return (LinkedList<E>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    /**
     * 克隆，浅拷贝
     * Returns a shallow copy of this {@code LinkedList}. (The elements
     * themselves are not cloned.)
     *
     * @return a shallow copy of this {@code LinkedList} instance
     */
    public Object clone() {
        LinkedList<E> clone = superClone();

        // Put clone into "virgin" state
        clone.first = clone.last = null;
        clone.size = 0;
        clone.modCount = 0;

        // Initialize clone with our elements
        for (Node<E> x = first; x != null; x = x.next)
            clone.add(x.item);

        return clone;
    }

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     * <p>
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     * <p>
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list
     * in proper sequence
     */
    public Object[] toArray() {
        Object[] result = new Object[size];
        int i = 0;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;
        return result;
    }

    /**
     * Returns an array containing all of the elements in this list in
     * proper sequence (from first to last element); the runtime type of
     * the returned array is that of the specified array.  If the list fits
     * in the specified array, it is returned therein.  Otherwise, a new
     * array is allocated with the runtime type of the specified array and
     * the size of this list.
     * <p>
     * <p>If the list fits in the specified array with room to spare (i.e.,
     * the array has more elements than the list), the element in the array
     * immediately following the end of the list is set to {@code null}.
     * (This is useful in determining the length of the list <i>only</i> if
     * the caller knows that the list does not contain any null elements.)
     * <p>
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     * <p>
     * <p>Suppose {@code x} is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of {@code String}:
     * <p>
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     * <p>
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of the list
     * @throws ArrayStoreException  if the runtime type of the specified array
     *                              is not a supertype of the runtime type of every element in
     *                              this list
     * @throws NullPointerException if the specified array is null
     */
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            a = (T[]) java.lang.reflect.Array.newInstance(
                    a.getClass().getComponentType(), size);
        int i = 0;
        Object[] result = a;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;

        if (a.length > size)
            a[size] = null;

        return a;
    }

    private static final long serialVersionUID = 876323262645176354L;

    /**
     * 序列化
     * Saves the state of this {@code LinkedList} instance to a stream
     * (that is, serializes it).
     *
     * @serialData The size of the list (the number of elements it
     * contains) is emitted (int), followed by all of its
     * elements (each an Object) in the proper order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out size
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (Node<E> x = first; x != null; x = x.next)
            s.writeObject(x.item);
    }

    /**
     * 反序列化
     * Reconstitutes this {@code LinkedList} instance from a stream
     * (that is, deserializes it).
     */
    @SuppressWarnings("unchecked")
    private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();

        // Read in size
        int size = s.readInt();

        // Read in all elements in the proper order.
        for (int i = 0; i < size; i++)
            linkLast((E) s.readObject());
    }

    /**
     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
     * list.
     * <p>
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
     * {@link Spliterator#ORDERED}.  Overriding implementations should document
     * the reporting of additional characteristic values.
     *
     * @return a {@code Spliterator} over the elements in this list
     * @implNote The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
     * and implements {@code trySplit} to permit limited parallelism..
     * @since 1.8
     */
    @Override
    public Spliterator<E> spliterator() {
        return new LLSpliterator<E>(this, -1, 0);
    }

    /**
     * A customized variant of Spliterators.IteratorSpliterator
     */
    static final class LLSpliterator<E> implements Spliterator<E> {
        static final int BATCH_UNIT = 1 << 10;  // batch array size increment
        static final int MAX_BATCH = 1 << 25;  // max batch array size;
        final LinkedList<E> list; // null OK unless traversed
        Node<E> current;      // current node; null until initialized
        int est;              // size estimate; -1 until first needed
        int expectedModCount; // initialized when est set
        int batch;            // batch size for splits

        LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
            this.list = list;
            this.est = est;
            this.expectedModCount = expectedModCount;
        }

        final int getEst() {
            int s; // force initialization
            final LinkedList<E> lst;
            if ((s = est) < 0) {
                if ((lst = list) == null)
                    s = est = 0;
                else {
                    expectedModCount = lst.modCount;
                    current = lst.first;
                    s = est = lst.size;
                }
            }
            return s;
        }

        public long estimateSize() {
            return (long) getEst();
        }

        public Spliterator<E> trySplit() {
            Node<E> p;
            int s = getEst();
            if (s > 1 && (p = current) != null) {
                int n = batch + BATCH_UNIT;
                if (n > s)
                    n = s;
                if (n > MAX_BATCH)
                    n = MAX_BATCH;
                Object[] a = new Object[n];
                int j = 0;
                do {
                    a[j++] = p.item;
                } while ((p = p.next) != null && j < n);
                current = p;
                batch = j;
                est = s - j;
                return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Node<E> p;
            int n;
            if (action == null) throw new NullPointerException();
            if ((n = getEst()) > 0 && (p = current) != null) {
                current = null;
                est = 0;
                do {
                    E e = p.item;
                    p = p.next;
                    action.accept(e);
                } while (p != null && --n > 0);
            }
            if (list.modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super E> action) {
            Node<E> p;
            if (action == null) throw new NullPointerException();
            if (getEst() > 0 && (p = current) != null) {
                --est;
                E e = p.item;
                current = p.next;
                action.accept(e);
                if (list.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }

        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

}

来源：oschina

链接：https://my.oschina.net/naisitumitiu/blog/4285501