集合框架知识系列04 LinkedList的源码分析和使用示例

摘要：一简介内部是通过双向链表存储的，提供顺序访问。继承了，实现在迭代器上的随机访问。四总结本节分析了的源码的用法。实现了接口，内部通过链表实现，能够实现链表队列栈和双端队列等数据结构的功能。

LinkedList内部是通过双向链表存储的，提供顺序访问。继承了AbstractSequentialList，实现在迭代器上的随机访问。并且，还实现了List、Deque、Cloneable,Serializable。Deque是双端队列接口，继承自Queue，Queue是队列接口。LinkedList的定义如下：

public class LinkedList extends AbstractSequentialList
                implements List, Deque, Cloneable, java.io.Serializable {}

数据存储的结构是链表，定义如下：

private static class Node {
    E item;
    Node next;
    Node prev;
    Node(Node prev, E element, Node next) {
        this.item = element;
        this.next = next;
        this.prev = prev;
    }
}

ArrayList提供两个构造方法，定义如下：

//默认构造方法
public LinkedList() {}
//给定初始化集合c的构造方法
public LinkedList(Collection c) {
     this();
     addAll(c);
}

LinkedList通过链表结构实现了双端队列、栈等数据结构，下面具体分析源码：

public class LinkedList
        extends AbstractSequentialList
        implements List, Deque, Cloneable, java.io.Serializable
    {
           //链表大小
        transient int size = 0;
    
        /**
         * 指向第一个节点指针
         */
        transient Node first;
    
        /**
         *指向最后一个节点的指针
         */
        transient Node last;
    
        /**
         * 构造一个空的list.
         */
            public LinkedList() {
                 }
    
        /**
         * 构造一个有特定元素的list
         */
        public LinkedList(Collection c) {
            this();
            addAll(c);
        }
    
        /**
         * 添加一个元素，作为第一个节点
         */
        private void linkFirst(E e) {
            final Node f = first;
            final Node newNode = new Node<>(null, e, f);
            first = newNode;
            if (f == null)
                last = newNode;
            else
                f.prev = newNode;
            size++;
            modCount++;
        }
    
        /**
         * 添加一个节点，作为最后一个元素
         */
        void linkLast(E e) {
            final Node l = last;
            final Node newNode = new Node<>(l, e, null);
            last = newNode;
            if (l == null)
                first = newNode;
            else
                l.next = newNode;
            size++;
            modCount++;
        }
    
        /**
         * 在非空节点succ前插入一个元素
         */
        void linkBefore(E e, Node succ) {
            // assert succ != null;
            final Node pred = succ.prev;
            final Node newNode = new Node<>(pred, e, succ);
            succ.prev = newNode;
            if (pred == null)
                first = newNode;
            else
                pred.next = newNode;
            size++;
            modCount++;
        }
    
        /**
         * 删除一个非空的first节点，将其next节点指向first,如果next节点为空，则last节点也为空
         */
        private E unlinkFirst(Node f) {
            // assert f == first && f != null;
            final E element = f.item;
            final Node 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;
        }
    
        /**
         * 删除一个非空的last节点，将其prev节点指向last,如果prev节点为空，则first节点也为空
         */
        private E unlinkLast(Node l) {
            // assert l == last && l != null;
            final E element = l.item;
            final Node 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;
        }
    
        /**
         * 删除非空节点x，将其next节点的prev节点指向其prev节点，prev节点的next节点指向其next节点
         */
        E unlink(Node x) {
            // assert x != null;
            final E element = x.item;
            final Node next = x.next;
            final Node 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;
        }
    
        /**
         * 返回first节点，如果节点为空，抛出NoSuchElementException 异常
         */
        public E getFirst() {
            final Node f = first;
            if (f == null)
                throw new NoSuchElementException();
            return f.item;
        }
    
        /**
         * 返回last节点，如果节点为空，抛出NoSuchElementException 异常
         */
        public E getLast() {
            final Node l = last;
            if (l == null)
                throw new NoSuchElementException();
            return l.item;
        }
    
        /**
         * 删除第一个节点，如果节点为空，抛出NoSuchElementException 异常
         */
        public E removeFirst() {
            final Node f = first;
            if (f == null)
                throw new NoSuchElementException();
            return unlinkFirst(f);
        }
    
        /**
         * 删除最后一个节点，如果节点为空，抛出NoSuchElementException 异常
         */
        public E removeLast() {
            final Node l = last;
            if (l == null)
                throw new NoSuchElementException();
            return unlinkLast(l);
        }
    
        /**
         * 在链表的头部插入节点e
         */
        public void addFirst(E e) {
            linkFirst(e);
        }
    
        /**
         * 在链表的头部插入节点e
         */
        public void addLast(E e) {
            linkLast(e);
        }
    
        /**
         * 返回链表中是否包含元素e
         */
        public boolean contains(Object o) {
            return indexOf(o) != -1;
        }
    
        /**
         * 返回链表的大小
         */
        public int size() {
            return size;
        }
    
        /**
         * 在链表尾部添加一个元素
         */
        public boolean add(E e) {
            linkLast(e);
            return true;
        }
    
        /**
         * 从头部遍历，移除链表中第一个o元素
         */
        public boolean remove(Object o) {
            if (o == null) {
                for (Node x = first; x != null; x = x.next) {
                    if (x.item == null) {
                        unlink(x);
                        return true;
                    }
                }
            } else {
                for (Node x = first; x != null; x = x.next) {
                    if (o.equals(x.item)) {
                        unlink(x);
                        return true;
                    }
                }
            }
            return false;
        }
    
        /**
         * 将给定集合中的元素插入到链表尾部
         */
        public boolean addAll(Collection c) {
            return addAll(size, c);
        }
    
        /**
         * 在指定的位置index后面插入集合c中的元素
         */
        public boolean addAll(int index, Collection c) {
            checkPositionIndex(index);
    
            Object[] a = c.toArray();
            int numNew = a.length;
            if (numNew == 0)
                return false;
    
            Node 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 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;
        }
    
        /**
         * 删除list中所有的元素.
         */
        public void clear() {
            //解除节点间的链接是没有必要的，但是，这样做有助于GC和释放内存
            for (Node x = first; x != null; ) {
                Node next = x.next;
                x.item = null;
                x.next = null;
                x.prev = null;
                x = next;
            }
            first = last = null;
            size = 0;
            modCount++;
        }
    
    
        // 下面是位置访问的操作
    
        /**
         * 返回指定位置节点中的元素.
         */
        public E get(int index) {
            checkElementIndex(index);
            return node(index).item;
        }
    
        /**
         * 替换index位置节点中的元素
         */
        public E set(int index, E element) {
            checkElementIndex(index);
            Node x = node(index);
            E oldVal = x.item;
            x.item = element;
            return oldVal;
        }
    
        /**
         * 在指定位置index处插入元素，如果index==size,则在最后位置插入
         */
        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));
        }
    
        /**
         * index处是否有节点
         */
        private boolean isElementIndex(int index) {
            return index >= 0 && index < size;
        }
    
        /**
         * 返回index是否是有效的位置
         */
        private boolean isPositionIndex(int index) {
            return index >= 0 && index <= size;
        }
    
        /**
         * 数组越界的异常详细信息
         */
        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));
        }
    
        /**
         * 返回指定位置的元素
         */
        Node node(int index) {
            // assert isElementIndex(index);
            //如果元素在前半部分，从前查找，否则，从后面开始查找
            if (index < (size >> 1)) {
                Node x = first;
                for (int i = 0; i < index; i++)
                    x = x.next;
                return x;
            } else {
                Node x = last;
                for (int i = size - 1; i > index; i--)
                    x = x.prev;
                return x;
            }
        }
    
        // 下面是搜索操作
    
        /**
         * 从list头部开始搜索，返回遇到的第一个给定元素的下标，如果不存在，返回-1
         */
        public int indexOf(Object o) {
            int index = 0;
            if (o == null) {
                for (Node x = first; x != null; x = x.next) {
                    if (x.item == null)
                        return index;
                    index++;
                }
            } else {
                for (Node x = first; x != null; x = x.next) {
                    if (o.equals(x.item))
                        return index;
                    index++;
                }
            }
            return -1;
        }
    
        /**
         * 返回最后一个指定元素的下标，如果不存在，返回-1
         */
        public int lastIndexOf(Object o) {
            int index = size;
            if (o == null) {
                for (Node x = last; x != null; x = x.prev) {
                    index--;
                    if (x.item == null)
                        return index;
                }
            } else {
                for (Node x = last; x != null; x = x.prev) {
                    index--;
                    if (o.equals(x.item))
                        return index;
                }
            }
            return -1;
        }
    
        // 下面是队列操作
    
        /**
         * 取第一个元素，但是不删除
         */
        public E peek() {
            final Node f = first;
            return (f == null) ? null : f.item;
        }
    
        /**
         * 去第一个元素，但是不删除
         */
        public E element() {
            return getFirst();
        }
    
        /**
         * 取第一个元素，并且删除
         */
        public E poll() {
            final Node f = first;
            return (f == null) ? null : unlinkFirst(f);
        }
    
        /**
         * 返回第一个元素，并且删除
         */
        public E remove() {
            return removeFirst();
        }
    
        /**
         * 在尾部插入元素e
         */
        public boolean offer(E e) {
            return add(e);
        }
    
        // 下面是双端队列的操作
    
        /**
         * 在list的前面插入元素e
         */
        public boolean offerFirst(E e) {
            addFirst(e);
            return true;
        }
    
        /**
         * 在list的后面插入元素e
         */
        public boolean offerLast(E e) {
            addLast(e);
            return true;
        }
    
        /**
         * 返回list中的第一个元素
         */
        public E peekFirst() {
            final Node f = first;
            return (f == null) ? null : f.item;
         }
    
        /**
         * 返回list中的最后一个元素,
         */
        public E peekLast() {
            final Node l = last;
            return (l == null) ? null : l.item;
        }
    
        /**
         * 返回第一个元素，并且删除
         */
        public E pollFirst() {
            final Node f = first;
            return (f == null) ? null : unlinkFirst(f);
        }
    
        /**
         * 返回最后一个元素，并且删除
         */
        public E pollLast() {
            final Node l = last;
            return (l == null) ? null : unlinkLast(l);
        }
    
        /**
         * 在list头部添加节点
         */
        public void push(E e) {
            addFirst(e);
        }
    
        /**
         * 删除头部节点
         */
        public E pop() {
            return removeFirst();
        }
    
        /**
         * 从头部开始遍历，删除第一个遇到的元素
         */
        public boolean removeFirstOccurrence(Object o) {
            return remove(o);
        }
    
        /**
         * 从尾部开始遍历，删除第一个遇到的元素
         */
        public boolean removeLastOccurrence(Object o) {
            if (o == null) {
                for (Node x = last; x != null; x = x.prev) {
                    if (x.item == null) {
                        unlink(x);
                        return true;
                    }
                }
            } else {
                for (Node x = last; x != null; x = x.prev) {
                    if (o.equals(x.item)) {
                        unlink(x);
                        return true;
                    }
                }
            }
            return false;
        }
    
        /**
         * List迭代器
         */
        public ListIterator listIterator(int index) {
            checkPositionIndex(index);
            return new ListItr(index);
        }
    
             //迭代器内部类
        private class ListItr implements ListIterator {
            private Node lastReturned;
            private Node 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;
            }
    
            public void remove() {
                checkForComodification();
                if (lastReturned == null)
                    throw new IllegalStateException();
    
                Node 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 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();
            }
        }
        //节点定义
        private static class Node {
            E item;
            Node next;
            Node prev;
    
            Node(Node prev, E element, Node next) {
                this.item = element;
                this.next = next;
                this.prev = prev;
            }
        }
    
        /**
         * 降序迭代器，从后向前遍历
         */
        public Iterator descendingIterator() {
            return new DescendingIterator();
        }
    
        /**
         * 降序迭代器实现
         */
        private class DescendingIterator implements Iterator {
            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 superClone() {
            try {
                return (LinkedList) super.clone();
            } catch (CloneNotSupportedException e) {
                throw new InternalError(e);
            }
        }
    
        /**
         * 克隆list对象
         */
        public Object clone() {
            LinkedList 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 x = first; x != null; x = x.next)
                clone.add(x.item);
    
            return clone;
        }
    
        /**
         * 将list转换为数组，顺序遍历，取每一个节点元素，放入数组
         */
        public Object[] toArray() {
            Object[] result = new Object[size];
            int i = 0;
            for (Node x = first; x != null; x = x.next)
                result[i++] = x.item;
            return result;
        }
    
        /**
         * 将list转换为数组，使用示例：String[] y = x.toArray(new String[0])
         * @throws ArrayStoreException if the runtime type of the specified array
         *         is not a supertype of the runtime type of every element in
         *         this list
         * @throws NullPointerException if the specified array is null
         */
        @SuppressWarnings("unchecked")
        public  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 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;
    
        /**
         * 将list写入输出流
         */
        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 x = first; x != null; x = x.next)
                s.writeObject(x.item);
        }
    
        /**
         * 从输出流构造list
         */
        @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());
        }
    
            /**
              *以下方法为1.8新增
               */
    
        /**
         * 
         * @since 1.8
         */
        @Override
        public Spliterator spliterator() {
            return new LLSpliterator(this, -1, 0);
        }
    
        /** A customized variant of Spliterators.IteratorSpliterator */
        static final class LLSpliterator implements Spliterator {
            static final int BATCH_UNIT = 1 << 10;  // batch array size increment
            static final int MAX_BATCH = 1 << 25;  // max batch array size;
            final LinkedList list; // null OK unless traversed
            Node 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 list, int est, int expectedModCount) {
                this.list = list;
                this.est = est;
                this.expectedModCount = expectedModCount;
            }
    
            final int getEst() {
                int s; // force initialization
                final LinkedList 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 trySplit() {
                Node 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 action) {
                Node 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 action) {
                Node 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;
            }
        }
    
    }

//获取第一个元素，不删除节点
public E getFirst()
//获取最后一个元素，不删除节点
public E getLast()
//删除第一个节点
public E removeFirst()
//删除最后一个节点
public E removeLast()
//在头部添加一个节点
public void addFirst(E e)
//在尾部添加一个节点
public void addLast(E e)
//在尾部添加一个节点
public boolean add(E e)
//顺序遍历，删除链表中第一个元素的节点
public boolean remove(Object o)
//将集合c中的元素添加到尾部
public boolean addAll(Collection c)
//将集合c中的元素添加到index位置后面
public boolean addAll(int index, Collection c)
//获取位置index处的元素，不删除节点
public E get(int index)
//替换位置index处的位置           
public E set(int index, E element)
//在位置index前插入节点
public void add(int index, E element)
//删除位置index处的节点
public E remove(int index)
//从头部遍历，返回第一个元素的位置
public int indexOf(Object o)
//从尾部遍历，返回最后一个元素的位置
public int lastIndexOf(Object o)
//返回第一个节点元素，不删除节点
public E peek() 
//返回第一个节点元素
public E element()
//返回第一个节点元素，并且删除节点
public E poll()
//删除第一个节点 
public E remove()
//在尾部添加节点
public boolean offer(E e)
//在头部添加节点
public boolean offerFirst(E e)
//在尾部添加节点
public boolean offerLast(E e)
//获取第一个节点元素，不删除节点
public E peekFirst()
//获取最后一个节点元素，不删除节点
public E peekLast()
//获取第一个节点元素，删除节点
public E pollFirst()
//获取最后一个节点元素，删除节点
public E pollLast()
//在头部添加节点
public void push(E e)
//返回第一个节点元素，删除节点
public E pop()

由于LikedList实现了接口List、Queue、Deque,内部通过链表存储，所以支持链表、队列、双端队列和栈等数据结构，下面分别介绍作为这几种数据结构的使用方法：

作为队列使用

队列是一种先进先出的数据结构，可以通过add()和poll()两个方法实现，具体代码如下：

        /**
         * LinkedList作为队列的使用
         */
        public static void queueTest(){
            Queue queue = new LinkedList<>();
            Integer[] arrays = new Integer[10];
            int size = 10;
            //队尾加入元素
            for (int i = 0; i < size; i++){
                queue.add(i);
            }
            System.out.println("queue:" + queue.toString());
            //取队头元素--不删除
            for (int i = 0; i < size; i++){
                arrays[i] = queue.peek();
            }
            System.out.println("peek:" + Arrays.toString(arrays));
            System.out.println("queue:" + queue.toString());
            arrays = new Integer[10];
            //取队头元素--删除
            for (int i = 0; i < size; i++){
                arrays[i] = queue.poll();
            }
            System.out.println("poll" + Arrays.toString(arrays));
            System.out.println("queue:" + queue.toString());
        }

运行结果如下：

作为栈使用

栈是一种先进后出的数据结构，可以通过push()和pop()两个方法实现，具体代码如下：

        /**
         * LinkedList作为栈的使用
         */
            public static void stackTest(){
                Deque stack = new LinkedList<>();
                Integer[] arrays = new Integer[10];
                int size = 10;
                //元素入栈
                for (int i = 0; i < size; i++){
                    stack.push(i);
                }
                System.out.println("stack:" + stack.toString());
        
                //元素出栈
                for (int i = 0; i < size; i++){
                    arrays[i] = stack.pop();
                }
                System.out.println("pop:" + Arrays.toString(arrays));
                System.out.println("stack:" + stack.toString());
            }

运行结果如下：

作为双端队列使用

双端队列是一种在两端都可以进出的数据结构，可以通过offerFirst()、offerLast() 和pollFirst()、pollLast()等方法实现，具体代码如下：

            /**
             * LinkedList作为双端队列的使用
             */
            public static void dequeTest(){
                Deque deque = new LinkedList<>();
                Integer[] arrays = new Integer[10];
                int size = 5;
        
                //从队头和队尾各入队五个
                for (int i = 0; i < size; i++){
                    deque.offerFirst(i);
                    deque.offerLast(i);
                }
                System.out.println("deque:" + deque.toString());
                int n = 0;
                //从队头和队尾各出队五个
                for (; n < 10;){
                    arrays[n++] = deque.pollFirst();
                    arrays[n++] = deque.pollLast();
                }
                System.out.println("poll:" + Arrays.toString(arrays));
                System.out.println("deque:" + deque.toString());
            }

运行结果如下：

遍历方式

和ArrayList一样，进行四种遍历方式的比较，遍历代码和ArrayList一样，运行结果如下：

从上图中的结果可以看出，通过下标遍历LinkedList效率是非常低的。遍历中，get(i)方法每次都从头部或者尾部遍历，找到位置i的节点，取出节点中的元素，所以导致效率低。

本节分析了LinkedList的源码的用法。LinkedList实现了List、Queue、Deque接口，内部通过链表实现，能够实现链表、队列、栈和双端队列等数据结构的功能。