LinkedList 源码分析

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

    private static class Node<E> {
        E item; //当前结点存储的元素
        Node<E> next; //下一个节点
        Node<E> prev; //上一个节点

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

    //集合大小
    transient int size = 0;

    /**
     * Pointer to first node.
     * - 头节点
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     * - 固定式：1.头节点为空，尾节点必然为空；2.头节点中上一个节点为空，元素不为空
     */
    transient Node<E> first;

    /**
     * Pointer to last node.
     * - 尾节点
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     * - 固定式：1.头节点为空，尾节点必然为空；2.尾节点中下一个节点为空，元素不为空
     */
    transient Node<E> last;

    /**
     * Constructs an empty list.
     * - 构造一个空列表。
     */
    public LinkedList() {
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     * - 构造一个包含指定 collection 的元素的列表，这些元素是按照该 collection 的迭代器返回它们的顺序排列的。
     *
     * @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);
    }

    /**
     * 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.)
     * - 将指定集合中的所有元素按照该 collection 的迭代器返回它们的顺序追加到此列表的末尾。如果在操作过程中修改了指定的集合，则此操作的行为未定义。(注意，如果指定的集合是这个列表，并且它不是空的，则会发生这种情况。)
     *
     * @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.
     * - 从指定位置开始，将指定集合中的所有元素插入此列表。将当前位于该位置的元素(如果有)和任何后续元素向右移动(增加它们的索引)。新元素将按照指定集合的迭代器返回它们的顺序出现在列表中。
     *
     * @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) {
        // 检查索引是否月结
        checkPositionIndex(index);
        // 集合转换数组
        Object[] a = c.toArray();
        int numNew = a.length;
        // 集合长度为空，直接返回
        if (numNew == 0)
            return false;

        // pred：指代待添加节点的前一个节点。
        // succ：指代添加节点的位置。
        Node<E> pred, succ;
        // index == size：待添加的元素永远在最后
        // index != size：先获取指定索引对应的Node节点，在把Node节点赋值给Pred
        if (index == size) {
            succ = null;
            pred = last;
        } else {
            // node(index)：获取指定索引对应的Node节点
            succ = node(index);
            pred = succ.prev;
        }

        for (Object o : a) {
            //  @SuppressWarnings("unchecked")
            // 告诉编译器忽略 unchecked 警告信息
            @SuppressWarnings("unchecked") E e = (E) o;
            Node<E> newNode = new Node<>(pred, e, null);
            // pred == null：头节点
            // pred != null：非头节点，上一个节点，指向下一个节点
            if (pred == null)
                first = newNode;
            else
                pred.next = newNode;
            pred = newNode;
        }

        // succ == null：表示新添加的元素，是最后一个元素。
        // succ != null：新添加的元素不是最后一个元素。
        if (succ == null) {
            last = pred;
        } else {
            pred.next = succ;
            succ.prev = pred;
        }
        // 更新长度
        size += numNew;
        // 修改的次数
        modCount++;
        return true;
    }

    /**
    * - 检查索引是否越界
    */
    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.
     * - 检查索引是否越界
     */
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    /**
     * Returns the (non-null) Node at the specified element index.
     * - 返回指定元素索引处的(非空)节点。
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);
        // 二分法：判断是在前一半还是在后一半,尽量提高效率。
        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;
        }
    }

    /**
     * Links e as first element.
     * - 设置头节点元素
     */
    private void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

    /**
     * Inserts the specified element at the beginning of this list.
     *
     * @param e the element to add
     */
    public void addFirst(E e) {
        linkFirst(e);
    }

    /**
     * Links e as last element.
     * - 设置为节点元素
     */
    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++;
        modCount++;
    }

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

    /**
     * Inserts element e before non-null Node succ.
     * - 在非空节点之前插入元素e。
     */
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

    /**
     * Unlinks non-null first node f.
     * - 删除头节点
     */
    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null; // help GC
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }

    /**
     * Removes and returns the first element from this list.
     * - 删除头节点,并且返回删除的节点的值。
     * @return the first element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

    /**
     * Unlinks non-null last node l.
     * - 删除尾节点，并且返回尾节点的值。
     */
    private E unlinkLast(Node<E> l) {
        // assert l == last && l != null;
        final E element = l.item;
        final Node<E> prev = l.prev;
        l.item = null;
        l.prev = null; // help GC
        last = prev;
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;
    }

   /**
     * Removes and returns the last element from this list.
     * - 删除指定节点，该节点不为空。
     * @return the last element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

    /**
     * Unlinks non-null node x.
     * - 删除指定节点，该节点不为空。
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        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;
    }

    /**
     * Returns the first element in this list.
     * - 返回头节点元素。
     * @return the first element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }

    /**
     * Returns the last element in this list.
     *
     * @return the last element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

    /**
     * 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 ? e==null : o.equals(e))</tt>.
     * - 如果此列表包含指定的元素，则返回true。
     * @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) {
        return indexOf(o) != -1;
    }

    /**
     * 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 ? get(i)==null : o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     * - 返回该列表中指定元素第一次出现的索引，如果该列表不包含该元素，则返回-1。
     * @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;
    }

    /**
     * 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.
     * - 获取指定元素最后一次出现的索引，如果此列表不包含该元素，则返回-1。
     * @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;
    }

    /**
     * Retrieves, but does not remove, the head (first element) of this list.
     * - 检索头节点，但不删除头节点。
     * @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;
    }