理解 LinkedList

java -version ：jdk 1.8.0_191

构造

类内参数，方法

实现

基于双向链表实现。

插入时间复杂度 O(1)
查找时间复杂度 O(n)
删除时间复杂度 O(1)
修改时间复杂度 O(n)

链表不存在扩容的问题。

静态参数

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    /**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    transient Node<E> first;
    链表的第一个，不能被序列化

    /**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    transient Node<E> last;
    链表的最后一个，不能被序列化

节点参数

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    private static class Node<E> {
        //数据
        E item;
        //下一个节点
        Node<E> next;
        //上一个节点
        Node<E> prev;

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

add方法

public boolean add(E e)

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    /**
     * Appends the specified element to the end of this list.
     *
     * <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) {
        //插入到链表的尾端
        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);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }

public void add(int index, E element)

将 e 添加到指定位置

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    /**
     * 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) {
        //先判断是否会越界
        checkPositionIndex(index);

        //刚好是最后一个
        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }

get方法

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    /**
     * 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;
    }

    for 循环遍历下标，找到对应的节点，根据下标的大小判断是从头遍历还是从尾部遍历
    /**
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // assert isEle 大专栏  理解 LinkedListmentIndex(index);

        从头遍历
        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            从尾部遍历
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

remove方法

循环遍历找到对应的节点，然后将上一个节点的next指向该节点的下一个节点。

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    /**
     * 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) {
        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;
    }

序列化

LinkedList 和 ArrayList 一样，不能通过一般方式的序列化

序列化writeObject(java.io.ObjectOutputStream s)

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    /**
     * 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readObject(java.io.ObjectInputStream s)

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    /**
     * 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());
    }

其他

LinkedList 和 ArrayList 一样，都不是线程安全的数据结构。

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