Java-LinkedList源码分析及示例

纸上得来终觉浅，绝知此事要躬行  --陆游     问渠那得清如许，为有源头活水来  --朱熹

LinkedList是一个继承AbstractSequentialList的双向链表，它也可以被当做堆栈、队列或者双端队列进行操作。

LinkedList实现了List接口，能对它进行队列操作。

LinkedList实现了Seque接口，能将LinkedList当做双端队列进行使用。

LinkedList实现Cloneable，覆盖了clone函数，可以进行克隆。

LinkedList实现了Serializable，可以被序列化。

LinkedList是非线程安全的。

 LinkedList示例程序：

public class Hello {

    public static void main(String[] args) {
        testLinkedListAPIs() ;
        useLinkedListAsLIFO();
        useLinkedListAsFIFO();
    }

    private static void testLinkedListAPIs()
    {
        String val = null;
        LinkedList llist = new LinkedList();
        //添加是哪个元素
        llist.add("1");
        llist.add("2");
        llist.add("3");
        llist.add(1, "4");//在第一个元素后面插入4
        System.out.println("\nTest \"addFirst(), removeFirst(), getFirst()\"");
        llist.addFirst("10");//将10插入并作为第一个元素
        System.out.println("llist:"+llist);
        System.out.println("llist.removeFirst():"+llist.removeFirst());//删除掉第一个元素
        System.out.println("llist:"+llist);
        System.out.println("llist.getFirst():"+llist.getFirst());//获得第一个元素
        System.out.println("\nTest \"offerFirst(), pollFirst(), peekFirst()\"");
        llist.offerFirst("10");//添加第一个元素
        System.out.println("llist:"+llist);
        System.out.println("llist.pollFirst():"+llist.pollFirst());//去掉第一个元素
        System.out.println("llist:"+llist);
        System.out.println("llist.peekFirst():"+llist.peekFirst());//读取第一个元素
        System.out.println("\nTest \"addLast(), removeLast(), getLast()\"");
        llist.addLast("20");//链尾添加元素20
        System.out.println("llist:"+llist);
        System.out.println("llist.removeLast():"+llist.removeLast());//删掉链尾元素20
        System.out.println("llist:"+llist);
        System.out.println("llist.getLast():"+llist.getLast());//读取链尾元素
        System.out.println("\nTest \"offerLast(), pollLast(), peekLast()\"");
        llist.offerLast("20");//添加链尾元素20
        System.out.println("llist:"+llist);
        System.out.println("llist.pollLast():"+llist.pollLast());//删掉链尾元素20
        System.out.println("llist:"+llist);
        System.out.println("llist.peekLast():"+llist.peekLast());//读取链尾元素
        llist.set(2, "300");//替换第三个元素
        System.out.println("\nget(3):"+llist.get(2));//获得第三个元素

        String[] arr = (String[])llist.toArray(new String[0]);//得到数组
        for (String str:arr)
        {
            System.out.println("str:"+str);
            System.out.println("size:"+llist.size());
            llist.clear();
            System.out.println("isEmpty():"+llist.isEmpty()+"\n");
        }
    }
    private static void useLinkedListAsLIFO()
    {
        System.out.println("\nuseLinkedListAsLIFO");
        LinkedList stack = new LinkedList();
        //类似于栈输入
        stack.push("1");
        stack.push("2");
        stack.push("3");
        stack.push("4");
        System.out.println("stack:"+stack);
        System.out.println("stack.pop():"+stack.pop());//出栈
        System.out.println("stack.peek():"+stack.peek());//只输出栈顶元素并不出栈
        System.out.println("stack:"+stack);
    }

    private static void useLinkedListAsFIFO()
    {
        System.out.println("\nuseLinkedListAsFIFO");
        LinkedList queue = new LinkedList();
        //类似于队列，入队
        queue.add("10");
        queue.add("20");
        queue.add("30");
        queue.add("40");
        System.out.println("queue:"+queue);
        System.out.println("queue.remove():"+queue.remove());//队列出队
        System.out.println("queue.element():"+queue.element());//读取队头，并不删除元素
        System.out.println("queue:"+queue);
    }
}

输出结果：

Test "offerLast(), pollLast(), peekLast()"

llist:[1, 4, 2, 3, 20]

llist.pollLast():20

llist:[1, 4, 2, 3]

llist.peekLast():3

get(3):300

str:1

size:4

isEmpty():true

str:4

size:0

isEmpty():true

str:300

size:0

isEmpty():true

str:3

size:0

isEmpty():true

useLinkedListAsLIFO

stack:[4, 3, 2, 1]

stack.pop():4

stack.peek():3

stack:[3, 2, 1]

useLinkedListAsFIFO

queue:[10, 20, 30, 40]

queue.remove():10

queue.element():20

queue:[20, 30, 40]

LinkedList源代码：

public class LinkedList<E> extends AbstractSequentialList<E>
        implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    transient int size = 0; //其实大小为0
    transient Node<E> first; //第一个节点
    transient Node<E> last; //最后一个节点
    public LinkedList() {    //构造一个空LinkedList
    }
    public LinkedList(Collection<? extends E> c) { //构造一个带有输入集合的
        this();
        addAll(c);
    }
    private void linkFirst(E e) { //链接第一个节点
        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++;
    }
    void linkLast(E e) { //链接最后一个节点
        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++;
    }
    void linkBefore(E e, Node<E> succ) { //在节点succ前插入一个e
        // 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++;
    }
    //不再链接第一个非空的节点
    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;
    }
    //删掉最后一个节点
    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;
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;
    }
    //删掉节点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;
    }
    //获得第一个节点
    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }
    //获得最后一个节点
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            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);
    }
    //在头节点插入E
    public void addFirst(E e) {
        linkFirst(e);
    }
    //在尾节点插入E
    public void addLast(E e) {
        linkLast(e);
    }
    //是否包含某个对象
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }
    //链表长度
    public int size() {
        return size;
    }
    //在链表中添加节点
    public boolean add(E e) {
        linkLast(e);
        return true;
    }
    //删掉某个节点
    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;
    }
    //在链表尾追加集合
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }
    //在某个节点之后追加集合
    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;
    }
    //清空链表
    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++;
    }
    //获得第几个节点
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }
    //对某个节点修改
    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }
    //在index节点之前插入一个节点element
    public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }
    //删除掉下标为index的节点
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }
    //测试此index下是否有节点
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }
    //下标位置在链表内
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }
    //下标越界
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }
    //检查下标下的节点
    private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    //判断下标位置
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    //返回下标节点
    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;
        }
    }
    //查找是否存在节点并返回下标，不存在返回-1
    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;
    }
    //返回最后一个相同的节点的下标
    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;
    }
    //返回表头数据并不删除
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }
    //返回表头数据
    public E element() {
        return getFirst();
    }
    //获得表头数据，并删除表头
    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }
    //删除表头
    public E remove() {
        return removeFirst();
    }
    //在表尾添加数据
    public boolean offer(E e) {
        return add(e);
    }
    //在表头添加数据
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }
    //在表尾添加数据
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }
    //获得第一个数据，并不删除
    public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }
    //获得最后一个数据，并不删除
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }
    //获得第一个节点并删除
    public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }
    //获得最后一个节点并删除
    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }
    //入栈
    public void push(E e) {
        addFirst(e);
    }
    //出栈
    public E pop() {
        return removeFirst();
    }
    //删除第一次出现的对象
    public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }
    //删除最后一次出现的对象
    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;
    }
    //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;
        }

        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();
        }
    }

    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;
        }
    }

    /**  * @since 1.6  */  public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    /**  * Adapter to provide descending iterators via ListItr.previous  */  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);
        }
    }
    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;
    }
    //生成对象数组
    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;
    }
    //泛型数组
    @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;
    //序列化写对象
    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);
    }
    //序列化读对象
    @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());
    }
    @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
        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;
        }
    }

}