Java中的LinkedList集合详解

    public LinkedList() {
    }

    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }

    private static class Node<E> {
        //元素
        E item;
        //后驱指针
        Node<E> next;
        //前驱指针
        Node<E> prev;
        //构造方法
        Node(Node<E> prev, E element, Node<E> next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }

    public boolean add(E e) {
        //将元素添加到链表的尾部
        linkLast(e);
        return true;
    }

    void linkLast(E e) {
        final Node<E> l = last;
        //创建新节点
        final Node<E> newNode = new Node<>(l, e, null);
        //把之前的尾指针节点指向新节点
        last = newNode;
        //如果尾节点为空，则代表是新链表，直接赋值给头指针；如果不为空，则把尾指针指向新节点
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        //长度++
        size++;
        //代表对集合的操作次数
        modCount++;
    }

    public void add(int index, E element) {
        //检查索引位置
        checkPositionIndex(index);
        //如果和当前长度size相等，则直接添加元素到末尾，否则就将元素插入到指定的位置
        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));        //node用来获取给定index处的元素节点
    }
    //索引校验
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }
    //linkLast()前面已经介绍，这里只展示linkBefore()
    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++;
    }

    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;
        }
    }

    //调用addAll(index, c)
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }
    public boolean addAll(int index, Collection<? extends E> c) {
        //检查index是否越界
        checkPositionIndex(index);
        //将集合转为数组
        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)
            return false;
        Node<E> pred, succ;
        if (index == size) {
            succ = null;
            pred = last;
        } else {
            succ = node(index);
            pred = succ.prev;
        }
        //遍历数组，将数组中的元素创建为节点，并按照顺序连接起来
        for (Object o : a) {
            @SuppressWarnings("unchecked") E e = (E) o;
            Node<E> newNode = new Node<>(pred, e, null);
            if (pred == null)
                first = newNode;
            else
                pred.next = newNode;
            pred = newNode;
        }
        if (succ == null) {
            last = pred;
        } else {
            pred.next = succ;
            succ.prev = pred;
        }
        //修改当前节点个数size的值
        //操作次数modCount+1
        size += numNew;
        modCount++;
        return true;
    }

    //移除指定位置的元素
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }
    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;
        //如果移除的是头节点，则头节点后移
        if (prev == null) {
            first = next;
        } else {
            prev.next = next;    //否则就释放节点的迁移借宿
            x.prev = null;
        }
        //如果溢出的是尾节点，则尾节点前移
        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;        //否则就释放节点的后一个元素
            x.next = null;
        }
        //节点数据置为空
        x.item = null;
        size--;
        modCount++;
        return 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;
    }

    public E set(int index, E element) {
        //检查索引是否越界
        checkElementIndex(index);
        //获取到要修改的元素的下标
        Node<E> x = node(index);
        //获取旧值
        E oldVal = x.item;
        //修改
        x.item = element;
        return oldVal;
    }

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