LinkedList源码及解析

package java.util;

import java.util.function.Consumer;

/**
 * LinkedList基于链表实现
 * 实现了List、Deque、Cloneable、Serializable接口
 */
public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    transient int size = 0;//list默认的长度

    transient Node<E> first;//list第一个节点

    transient Node<E> last;//list最后一个节点

    public LinkedList() {
    }

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

    //在链表的开始连接一个节点
    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++;//修改次数，用于快速失败机制
    }

    //在链表结尾连接一个节点
    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succ节点之前连接一个节点
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null; 断言
        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++;
    }

    //截取f.next节点到尾节点
    //不过这个地方会断言f肯定是头节点，所以此方法是取消连接第一个节点，也即删除第一个节点
    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;
    }

    //取消连接最后一个节点(因为断言)
    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;
    }

    //删除一个非空节点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;
    }

    //得到第一个节点，如果第一个节点为null，会抛出NoSuchElementException
    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }

    //得到最后一个节点，如果最后一个节点为null，会抛出NoSuchElementException
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

    //得到第一个节点，如果第一个节点为null，会抛出NoSuchElementException
    //与getFirst不同的是该方法会删除第一个节点
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

    //得到最后一个节点，如果最后一个节点为null，会抛出NoSuchElementException
    //与getLast不同的是该方法会删除最后一个节点
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

    //向链表头节点连接一个新节点(由元素e决定)
    public void addFirst(E e) {
        linkFirst(e);
    }

    //向链表尾节点连接一个新节点(由元素e决定)
    public void addLast(E e) {
        linkLast(e);
    }

    //判断链表是否包含某一个元素o，会迭代所有元素
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    //得到链表长度，也即链表中元素的个数
    public int size() {
        return size;
    }

    //向链表中新增一个元素e，默认新增位置为尾端
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    //通过迭代的方式去删除某一个元素，如果这个元素有重复，则只删除第一个元素(从头节点开始)
    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;
    }

    //批量增加元素，默认增加到队尾
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    //批量增加元素，增加到index位置之后，可能会抛出IndexOutOfBoundsException异常
    public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)//c中若没有元素的话，则会返回false
            return false;

        Node<E> pred, succ;//插入后c的首节点的前驱节点与尾节点的后驱节点
        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;
    }

    //清除所有元素
    public void clear() {
        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++;
    }

    //通过位置索引的到元素，头节点index=0
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

    //在指定位置插入节点，会覆盖原位置的元素
    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

    //在指定位置插入节点，不会会覆盖原位置的元素
    public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }

    //清除位置index的元素
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

    //判断index是不是一个已存在元素的位置
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

    //判断index是不是迭代器或添加操作的有效位置的索引。
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    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));
    }

    //根据index返回节点，此处index断言isElementIndex
    Node<E> node(int index) {
        // assert isElementIndex(index);

        if (index < (size >> 1)) {//并不是从头开始迭代到尾，而是根据判断止迭代一半
            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;
        }
    }

    //搜索操作

    //返回o在链表中的位置，如果有多个，只返回第一个，从头节点开始
    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;
    }

    //返回o在链表中的位置，如果有多个，只返回第一个，从尾节点开始
    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;
    }

    // 队列操作

    //查看队列的第一个元素，但不移除这个元素，并且不会抛出异常
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    //得到第一个元素，如果为null，抛出NoSuchElementException异常
    public E element() {
        return getFirst();
    }

    //查看队列的第一个元素，会移除这个元素，不会抛出异常
    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    //查看队列的第一个元素，会移除这个元素，会抛出NoSuchElementException异常
    public E remove() {
        return removeFirst();
    }

    //向队列中插入一个元素，默认尾部
    public boolean offer(E e) {
        return add(e);
    }

    //双端队列操作

    //向队列头部中插入一个元素
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    //向队列中插入一个元素，默认尾部，与offer(E e)一致
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

    //从头部查看一个元素，不删除，且不抛出异常
    public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
     }

    //从尾部查看一个元素，不删除，且不抛出异常
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

    //从头部查看一个元素，并删除，不抛出异常
    public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    //从尾部查看一个元素，并删除，不抛出异常
    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

    //栈操作

    //压栈
    public void push(E e) {
        addFirst(e);
    }

    //出栈，若没有元素会抛出异常
    public E pop() {
        return removeFirst();
    }

    //删除第一个出现的元素o
    public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }

    //删除最后一个出现的元素o
    public boolean removeLastOccurrence(Object o) {
        if (o == null) {//很聪明的从后往前遍历
            for (Node<E> x = last; x != null; x = x.prev) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    //迭代器

    //双端链表迭代器
    public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
    }

    private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;
        private Node<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;

        ListItr(int index) {
            // assert isPositionIndex(index);
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }

        public boolean hasNext() {
            return nextIndex < size;
        }

        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.item;
        }

        public boolean hasPrevious() {
            return nextIndex > 0;
        }

        public E previous() {
            checkForComodification();
            if (!hasPrevious())
                throw new NoSuchElementException();

            lastReturned = next = (next == null) ? last : next.prev;
            nextIndex--;
            return lastReturned.item;
        }

        public int nextIndex() {
            return nextIndex;
        }

        public int previousIndex() {
            return nextIndex - 1;
        }

        //删除的是上一个元素
        public void remove() {
            checkForComodification();
            if (lastReturned == null)
                throw new IllegalStateException();

            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = null;
            expectedModCount++;
        }

        //设置上一个元素的值为e，会覆盖
        public void set(E e) {
            if (lastReturned == null)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.item = e;
        }

        //上一个元素后插入一个的值为e的元素，不会覆盖
        public void add(E e) {
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        //foreach 迭代剩余的元素，并使用Consumer影响
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            while (modCount == expectedModCount && nextIndex < size) {
                action.accept(next.item);
                lastReturned = next;
                next = next.next;
                nextIndex++;
            }
            checkForComodification();//审查快速失败
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    //链表中的实体类--节点
    private static class Node<E> {
        E item;//元素
        Node<E> next;//前驱
        Node<E> prev;//后驱

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

    //单向的迭代器，只需往后不许往前
    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    //单向迭代器
    private class DescendingIterator implements Iterator<E> {
        private final ListItr itr = new ListItr(size());
        public boolean hasNext() {
            return itr.hasPrevious();
        }
        public E next() {
            return itr.previous();
        }
        public void remove() {
            itr.remove();
        }
    }

    @SuppressWarnings("unchecked")
    private LinkedList<E> superClone() {
        try {
            return (LinkedList<E>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    //浅复制链表
    public Object clone() {
        LinkedList<E> clone = superClone();

        // Put clone into "virgin" state
        clone.first = clone.last = null;
        clone.size = 0;
        clone.modCount = 0;

        // Initialize clone with our elements
        for (Node<E> x = first; x != null; x = x.next)
            clone.add(x.item);

        return clone;
    }

    //toArray返回一个Object[]
    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;
    }

    //toArray返回特定类型的数据
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            a = (T[])java.lang.reflect.Array.newInstance(
                                a.getClass().getComponentType(), size);
        int i = 0;
        Object[] result = a;//此处用引用逻辑实现赋值的，可以避免强制转换，只能说太6了
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;

        if (a.length > size)
            a[size] = null;

        return a;
    }

    private static final long serialVersionUID = 876323262645176354L;

    //往一个流中写当前对象
    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);
    }

    //往一个流中读当前对象
    @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());
    }

    //可分裂的迭代器，用于并行计算
    @Override
    public Spliterator<E> spliterator() {
        return new LLSpliterator<E>(this, -1, 0);
    }

    /** A customized variant of Spliterators.IteratorSpliterator */
    static final class LLSpliterator<E> implements Spliterator<E> {
        static final int BATCH_UNIT = 1 << 10;  // batch array size increment  1024
        static final int MAX_BATCH = 1 << 25;  // max batch array size;   33554432
        final LinkedList<E> list; // null OK unless traversed
        Node<E> current;      // 当前节点 默认为null
        int est;              // size的估计值，默认为-1
        int expectedModCount; // 预期的修改数量，用于快速失败
        int batch;            // batch size for splits

        LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
            this.list = list;
            this.est = est;
            this.expectedModCount = expectedModCount;
        }

        final int getEst() {
            int s; // force initialization
            final LinkedList<E> lst;
            if ((s = est) < 0) {
                if ((lst = list) == null)
                    s = est = 0;
                else {
                    expectedModCount = lst.modCount;
                    current = lst.first;
                    s = est = lst.size;
                }
            }
            return s;
        }

        //得到size估计值
        public long estimateSize() { return (long) getEst(); }

        //这就是为Spliterator专门设计的方法，区分与普通的Iterator，该方法会把当前元素划分一部分出去创建一个新的Spliterator作为返回，
        //两个Spliterator变会并行执行，如果元素个数小到无法划分则返回null
        public Spliterator<E> trySplit() {
            Node<E> p;
            int s = getEst();
            if (s > 1 && (p = current) != null) {
                int n = batch + BATCH_UNIT;
                if (n > s)
                    n = s;
                if (n > MAX_BATCH)
                    n = MAX_BATCH;
                Object[] a = new Object[n];
                int j = 0;
                do { a[j++] = p.item; } while ((p = p.next) != null && j < n);
                current = p;
                batch = j;
                est = s - j;
                return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Node<E> p; int n;
            if (action == null) throw new NullPointerException();
            if ((n = getEst()) > 0 && (p = current) != null) {
                current = null;
                est = 0;
                do {
                    E e = p.item;
                    p = p.next;
                    action.accept(e);
                } while (p != null && --n > 0);
            }
            if (list.modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }

        //tryAdvance就是顺序处理每个元素，类似Iterator，如果还有元素要处理，则返回true，否则返回false
        public boolean tryAdvance(Consumer<? super E> action) {
            Node<E> p;
            if (action == null) throw new NullPointerException();
            if (getEst() > 0 && (p = current) != null) {
                --est;
                E e = p.item;
                current = p.next;
                action.accept(e);
                if (list.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }

        //其实就是表示该Spliterator有哪些特性，用于可以更好控制和优化Spliterator的使用
        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

}