LinkedList源码浅析（jdk1.8）

LinkedList由双向链表实现的集合，因此可以从头或尾部双向循环遍历。

LinkedList的操作都是对双向链表的操作，理解双向链表的数据结构就很容易理解LinkedList的实现。

双向链表由带前驱和后继的节点构成，简易如下：

如果添加一个c节点，简易步骤如下：

先创建一个新节点。然后把原先的last节点的next指向新节点，在把新节点的pre指向原先的last，最后新节点指为last节点。

具体的双向链表还是参考数据结构的资料。

一、构造方法

1、无参的

    public LinkedList() {
    }

2、集合为参数的

    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }
    public boolean addAll(int index, Collection<? extends E> c) {
        // 检查索引
        checkPositionIndex(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)
            return false;

        Node<E> pred, succ;
        // 索引为尾部还是范围内的
        if (index == size) {
            succ = null;
            pred = last;
        } else {
            succ = node(index);
            pred = succ.prev;
        }

        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;
        }

        size += numNew;
        modCount++;
        return true;
    }
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

二、添加元素

1、add(E e)方法，添加节点到尾部

    public boolean add(E e) {
        linkLast(e);
        return true;
    }
    void linkLast(E e) {
        // 尾节点赋值给l
        final Node<E> l = last;
        // 构造新节点
        final Node<E> newNode = new Node<>(l, e, null);
        // 新节点赋给尾节点
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }

2、addFirst(E e)方法，添加节点到首部

    public void addFirst(E e) {
        linkFirst(e);
    }
    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++;
    }

三、删除元素

1、remove(Object o)方法，删除o节点(只删除一次)，循环链表找到要删除的节点，然后调用unlink()方法进行删除。

    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;
    }
    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;
    }

2、removeFirst()方法，删除首节点，为空则抛异常。

    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }
    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;
    }

四、其他方法源码

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

    /**
     * 首节点
     */
    transient Node<E> first;

    /**
     * 尾节点
     */
    transient Node<E> last;

    /**
     * 无参构造
     */
    public LinkedList() {
    }

    /**
     * 有参构造
     */
    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }

    /**
     * 从头部添加元素（私有的，内部使用）
     */
    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++;
    }

    /**
     * 从尾部添加元素
     */
    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++;
    }

    /**
     * 在指定元素前插入元素
     */
    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++;
    }

    /**
     * 删除首节点(私有的，内部使用)
     */
    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;
    }

    /**
     * 删除尾节点(私有的，内部使用)
     */
    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;
    }

    /**
     * 删除节点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;

        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;
    }

    /**
     * 获取首节点
     */
    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }

    /**
     * 获取尾节点
     */
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

    /**
     * 删除首节点并返回元素值
     */
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

    /**
     * 删除尾节点并返回元素值
     */
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

    /**
     * 把e设为首节点
     */
    public void addFirst(E e) {
        linkFirst(e);
    }

    /**
     * 把e设为尾节点
     */
    public void addLast(E e) {
        linkLast(e);
    }

    /**
     * 判断o是否在集合中
     */
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    /**
     * 集合大小
     */
    public int size() {
        return size;
    }

    /**
     * 添加元素到链表尾部
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    /**
     * 删除o节点
     */
    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;
    }

    /**
     * 添加集合
     */
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    /**
     * 指定位置添加集合
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);

        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;
        }

        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;
        }

        size += numNew;
        modCount++;
        return true;
    }

    /**
     * 清除集合所有元素
     */
    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
        for (Node<E> x = first; x != null; ) {
            Node<E> next = x.next;
            x.item = null;
            x.next = null;
            x.prev = null;
            x = next;
        }
        first = last = null;
        size = 0;
        modCount++;
    }

    // Positional Access Operations

    /**
     * 获取指定位置的元素的值
     */
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

    /**
     * 取代指定位置的元素并返回原元素的值
     */
    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

    /**
     * 添加元素到指定位置并后移原先的元素，如果指定的位置是最后，则直接添加元素在尾部
     */
    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
     */
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

    /**
     * 用于判断元素索引是否正确
     */
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

    /**
     * 用于判断位置索引是否正确
     */
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    /**
     * 越界异常提示
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

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

    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 返回指定位置的节点.
     */
    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

    /**
     * 从头循环返回第一次出现o的index
     */
    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;
    }

    /**
     * 从尾循环返回第一次出现o的index
     */
    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;
    }

    // Queue operations.

    /**
     * 返回首节点的值，但不删除，null也可以返回
     */
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    /**
     * 返回首节点的值，但不删除，如果是null则抛NoSuchElementException异常
     */
    public E element() {
        return getFirst();
    }

    /**
     * 返回首节点并删除
     */
    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /**
     * 返回首节点并删除，如果首节点为null抛异常
     */
    public E remove() {
        return removeFirst();
    }

    /**
     * 添加元素到尾节点
     */
    public boolean offer(E e) {
        return add(e);
    }

    // Deque operations
    /**
     * 添加元素到首节点前面
     */
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    /**
     * 添加元素到尾节点后面
     */
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

    /**
     * 获取首节点不删除
     */
    public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
     }

    /**
     * 获取尾节点不删除
     */
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

    /**
     * 获取首节点并删除
     */
    public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /**
     * 获取尾节点并删除
     */
    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

    /**
     * 添加元素到首节点前面
     */
    public void push(E e) {
        addFirst(e);
    }

    /**
     * 移除并返回首节点
     */
    public E pop() {
        return removeFirst();
    }

    // 节点构造
    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;
        }
    }

    /**
     * 集合转成数组
     */
    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;
    }
}