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() {    }

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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;    }

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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++;    }

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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++;    }

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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;    }

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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;    }

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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;    }}

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