LinkedList add get 源码解析

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

其中Deque为双端队列

add(E e)方法如下，就是在链表的最后一项，在其next项附上要添加的数据

    /**
     * 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) {
        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)
            //如果last节点为空，说明链表是空的，那么新增的数据既是first又是last
            first = newNode;
        else
            //如果last不为空，把新增的元素链接到原来的last的next
            l.next = newNode;
        size++;
        modCount++;
    }


    private static class Node<E> {
        E item;
        Node<E> next;
        Node<E> prev;

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

add(int index, E e)方法如下，就是在指定位置，把新节点插入到链表，并对应更新其前驱和后续节点的链接关系。

    /**
     * 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) {
        //检查索引是否越界
        checkPositionIndex(index);

        if (index == size)
            //在链表末端添加新节点
            linkLast(element);
        else
            //在指定位置插入新节点
            linkBefore(element, node(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.
     */
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    /**
     * Inserts element e before non-null Node succ.
     */
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        //保留要插入节点位置的前驱节点
        final Node<E> pred = succ.prev;
        
        //新建节点
        final Node<E> newNode = new Node<>(pred, e, succ);
        
        //把新建节点作为原节点的前驱节点
        succ.prev = newNode;
        
        //把新建节点作为pred的后续节点，或者作为链表头结点
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }
    

get方法如下：

    /**
     * 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) {
        //检查索引是否越界
        checkElementIndex(index);
        //根据索引，遍历链表找到数据
        return node(index).item;
    }


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

    /**
     * Tells if the argument is the index of an existing element.
     */
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }
    
    /**
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);

        if (index < (size >> 1)) {
            //索引值小于链表总长度的一半，从前往后遍历链表
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            //索引值大于等于链表总长度的一半，从后往前遍历链表
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

根据索引获取元素时，有个小技巧，就是把链表对半分，当索引在前半段时，从前往后遍历，否则反向遍历，该算法可以节省遍历次数，从一半，减为四分之一。

有个需要注意的地方，Linkedlist里面有两个很相似的方法，一个是获取数据时，检查索引下标是否越界；另外一个是添加数据时，添加位置索引小标是否越界。前者边界条件比后者小1，代码如下：

    /**
     * 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;
    }

remove(Object o)函数，该函数从前往后遍历链表，删除第一个满足条件的数据，到此结束。

    /**
     * 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;
    }

    /**
     * Unlinks non-null node x.
     */
    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;
        
        //前驱节点解绑，如果前驱节点为空，则后续节点成为first
        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        //后续节点解绑，如果后续节点为空，则前驱节点成为last
        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        //节点数值释放，方便gc回收
        x.item = null;
        size--;
        modCount++;
        return element;
    }

remove(int index)方法，先检查索引是否越界，然后根据索引获取node，调用unlink方法解绑该节点。

    /**
     * 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));
    }