LinkedList

Queue：接口，声明了队列的共性功能，表明有队列的特性
Deque：接口，表明有双端队列的特性，
AbstractSequentialList:抽象类，定义了次序访问，表明是链表实现，适合使用迭代器遍历

/*https://www.cnblogs.com/xujian2014/p/4630785.html */
public class LinkedList<E>
        extends AbstractSequentialList<E>
        implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    /*当前有多少个节点，禁止序列化*/
    transient int size = 0;

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

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

    /*空构造方法*/
    public LinkedList() {
    }

    /* 构造方法，初始化给定的集合：当c为空时，抛出NullPointerException * */
    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }

    /*将e置为第一个元素*/
    private void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)//第一次添加元素：空链表的情况下
            last = newNode;
        else  //不是第一次添加元素：不是空链表
            f.prev = newNode;
        size++;//元素的个数++
        modCount++;//修改次数增加
    }

    /*将e置为最后一个元素*/
    void linkLast(E e) {/*允许添加控制*/
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null) //第一次添加元素：空链表的情况下
            first = newNode;
        else    //不是第一次添加元素：不是空链表
            l.next = newNode;
        size++;//元素的个数++
        modCount++;//修改次数增加
    }

    /*将e插入到非空的succ元素前面*/
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;/*保存succ的前节点*/
        /*获得一个前节点为pred，指向元素为e，后置节点为succ的节点*/
        final Node<E> newNode = new Node<>(pred, e, succ);

        succ.prev = newNode;/*将succ的前节点指向新产生的节点*/
        if (pred == null)//如果succ时第一个节点：头节点
            first = newNode;
        else    //如果succ不是第一个节点 ：头节点
            pred.next = newNode;
        size++; //链表节点数++
        modCount++; //链表的修改次数++
    }

    /*去掉第一个节点*/
    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;//返回被去掉的节点元素
    }

    /*删除最后一个节点*/
    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;//尾指针指向前一个节点
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;//被删除的节点里的元素
    }

    /*删除指定元素*/
    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) {/*如果是第一个节点*/
            first = next;
        } else {
            prev.next = next;//把上一个节点指向下一个节点
            x.prev = null;//把当前节点与前面的节点断开
        }

        if (next == null) {/*如果是最后一个节点*/
            last = prev;
        } else {
            next.prev = prev;//把下一个节点的指向前一个节点
            x.next = null;//把当前节点与后面的节点断开
        }

        x.item = null;//当前节点置为空 gc
        size--;
        modCount++;
        return element;/*返回删除的元素的值*/
    }

    /*获得链表的第一个节点的值*/
    public E getFirst() {
        final Node<E> f = first;
        if (f == null)/*如果是空链表*/
            throw new NoSuchElementException();
        return f.item;
    }

    /*获取最后一个元素的值*/
    public E getLast() {
        final Node<E> l = last;
        if (l == null)/*如果链表为空*/
            throw new NoSuchElementException();
        return l.item;
    }

    /*删除第一个节点*/
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

    /*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);
    }

    /*在表头添加一个元素*/
    public void addFirst(E e) {
        linkFirst(e);
    }

    /*在最后添加一个元素*/
    public void addLast(E e) {
        linkLast(e);
    }

    /*判断是否包含这个元素*/
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    /*获得当前链表包含的元素*/
    public int size() {
        return size;
    }

    /*添加元素，最尾部添加*/
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    /*删除第一个指定元素*/
    public boolean remove(Object o) {
        if (o == null) {/*允许有多个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;/*返回是否删除*/
    }

    /*批量添加元素*/
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    /*从指定位置开始批量添加元素*/
    public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);/*检查是否有效index*/

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

        Node<E> pred, succ;
        if (index == size) {/*如果是在尾部添加元素*/
            succ = null;
            pred = last;
        } else {/*在指定位置添加元素*/
            succ = node(index);/*获取位置元素*/
            pred = succ.prev;
        }

        for (Object o : a) {/*遍历要添加的元素集合*/
            @SuppressWarnings("unchecked") E e = (E) o;
            Node<E> newNode = new Node<>(pred, e, null);
            if (pred == null)/*如果是从0位置开始添加*/
                first = newNode;
            else
                pred.next = newNode;
            pred = newNode;
        }

        if (succ == null) {/*如果是从尾部添加*/
            last = pred;
        } else {
            pred.next = succ;
            succ.prev = pred;
        }

        size += numNew;
        modCount++;
        return true;/*返回添加是否成功*/
    }

    /*清除链表*/
    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; ) {/*把所有的元素置为空 gc*/
            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
    /*获取指定节点储存的元素，index必须有效*/
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

    /*设置指定index的节点元素*/
    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;/*返回修改之前的元素*/
    }

    /*在指定的index添加元素*/
    public void add(int index, E element) {
        checkPositionIndex(index);

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

    /*删除指定index的节点*/
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));/*返回倍删除节点的元素*/
    }

    /*检查元素index是否在有效范围内*/
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

    /*检查index时候有效*/
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    /*index不合法是的提示信息*/
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

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

    /*检查index是否有效，抛出IndexOutOfBoundsException异常*/
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /*获取指定index的节点*/
    Node<E> node(int index) {
        // assert isElementIndex(index);

        if (index < (size >> 1)) {/*如果从前半部分添加*/
            Node<E> x = first;
            for (int i = 0; i < index; i++)/*从头开始遍历*/
                x = x.next;
            return x;
        } else {/*如果在后半部分添加*/
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)/*从尾部开始遍历*/
                x = x.prev;
            return x;
        }
    }

    // Search Operations

    /*获取指定元素的在链表中的第一个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;/*没找到返回-1*/
    }

    /*获取指定元素的最大的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;/*为找到返回-1*/
    }

    // Queue operations.

    /*获取第一个元素的值*/
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    /*获取第一个节点的值*/
    public E element() {
        return getFirst();
    }

    /*获取第一个节点的值并删除第一个节点*/
    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /*删除第一个节点，返回删除的节点的值*/
    public E remove() {
        return removeFirst();
    }

    /*在链表尾部添加e*/
    public boolean offer(E e) {
        return add(e);
    }

    // Deque operations

    /*在链表头添加一个元素*/
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    /*在链表尾部添加一个元素*/
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

    /*获取链表中第一个节点的值*/
    public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    /*获取最后一个节点的值*/
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

    /*获取第一个节点元素，并删除*/
    public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    /*获取最后一个节点的元素，并删除*/
    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

    /*在链表的头部添加一个节点*/
    public void push(E e) {
        addFirst(e);
    }

    /*获取第一个节点的值，并删除*/
    public E pop() {
        return removeFirst();
    }

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

    /*删除最后一次出现的o*/
    public boolean removeLastOccurrence(Object o) {
        if (o == null) {
            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;
    }


    /*https://www.cnblogs.com/ITtangtang/p/3948610.html#a10*/

    /*创建迭代器指定起始位置*/
    public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
    }

    /*实现ListIterator，使用list迭代对象时使用*/
    private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;/*最近一次返回的节点，也是当前持有的节点*/
        private Node<E> next;/*对下一个元素的引用*/
        private int nextIndex;/*下一个节点的index*/
        private int expectedModCount = modCount;

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

        /*根据nextIndex是否等于size判断时候还有下一个节点*/
        public boolean hasNext() {
            return nextIndex < size;
        }

        /*获取下一个元素*/
        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

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

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

        /* 返回上一个节点*/
        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;
        }

        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++;
        }

        public void set(E e) {
            if (lastReturned == null)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.item = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        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();
        }

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

    /*链表单个节点*/
    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;//本节点储存的元素
            this.next = next;//后节点
            this.prev = prev;//前节点
        }
    }

    /*获取一个反向迭代器*/
    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    /*反向的Iterator*/
    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();
        }
    }

    /*https://blog.csdn.net/m0_37884977/article/details/80467658*/
    /*克隆的辅助方法*/
    @SuppressWarnings("unchecked")
    private LinkedList<E> superClone() {
        try {
            return (LinkedList<E>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    /*重写的clone*/
    public Object clone() {
        LinkedList<E> clone = superClone();

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

        /*把链表中的元素复制过去*/
        // Initialize clone with our elements
        for (Node<E> x = first; x != null; x = x.next)
            clone.add(x.item);

        return clone;
    }

    /*将链表转化成数组*/
    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;
    }

    /*返回指定类型的Array*/
    @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;
        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;


    /*重写的序列化方法*/
    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);

        /*把链表中的所有元素都序列化*/
        // Write out all elements in the proper order.
        for (Node<E> x = first; x != null; x = x.next)
            s.writeObject(x.item);
    }

    /*重写反序列化方法*/
    @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++)/*重新生成链表*/
            linkLast((E)s.readObject());
    }

    /*获取一个可分割迭代器*/
    @Override
    public Spliterator<E> spliterator() {
        return new LLSpliterator<E>(this, -1, 0);
    }


    /*https://blog.csdn.net/jiangmingzhi23/article/details/78927552*/
    static final class LLSpliterator<E> implements Spliterator<E> {
        static final int BATCH_UNIT = 1 << 10;  // batch array size increment
        static final int MAX_BATCH = 1 << 25;  // max batch array size;
        final LinkedList<E> list; // null OK unless traversed
        Node<E> current;      // current node; null until initialized
        int est;              // size estimate; -1 until first needed
        int expectedModCount; // initialized when est set
        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;
        }

        public long estimateSize() { return (long) getEst(); }

        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();
        }

        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;
        }

        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }
}