java.util.LinkedList源码分析

一、LinkedList的继承关系

LinkedList实现了Collection,List,Serializable,Deque等接口

二、LinkedList的结构

通过查看源码发现linkedlist有个静态的内部类Node,Node里面有prev和next成员，说明linkedlist是一个双向链表的结构。代码如下

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;
        }
    }

因此linkedlist结构图如下

这里写了一个MyLinkedList类似于LinkedList 的结构，MyLinkedList还有很多方法没有实现

/**
 * 模仿java.util.LinkedList的MyLinkedList
 * @param <E>
 */
public class MyLinkedList<E> {
    private int size;
    Node<E> first;//头结点
    Node<E> last; // 尾结点
    private static  class Node<E>{
        // 从这可以看出 java.util.LinkedList是双向链表
        private Node<E> prev;
        private Node<E> next;
        private E element;

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

 三、LinkedList的操作

基本操作

LinkedList是一个双向链表。因此可以方便的对元素进行插入删除操作

插入元素，调用add方法

 public boolean add(E e) {
        linkLast(e);
        return true;
    }

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++;
    }

 在插入元素时会往链表的尾部插入元素，过程如下：

1、先用一个变量l指向尾结点，

2、创建新结点

3、尾结点指向新的结点

4、判断原来的尾结点（变量l指向的结点）是否为空，

5 、如果为空说明是个空链表，将头结点指向新的结点；

6 、原来的尾结点不为空，将原来尾结点(l指向的结点)的prev指向新的结点

链表不为空的情况对应如下图

链表为空对应如下

在指定位置前插入元素

  public void add(int index, E element) {
        checkPositionIndex(index);

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

void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

从头部插入元素

public void addFirst(E e) {
        linkFirst(e);
    }

private void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

从尾部插入元素

public void addLast(E e) {
        linkLast(e);
    }

获取元素

由于linkedlist是一个链表，内部没有维护数组，因此获取元素的时候需要循环来获取，代码如下

public E get(int index) {
        checkElementIndex(index);//比较index是否大于等于0并且小于size，否则抛出异常
        return node(index).item;
    }

Node<E> node(int index) {
     
        // 判断要找的元素的位置是在左半部分还是右半部分
        if (index < (size >> 1)) {
            Node<E> x = first;
           //从左边开始找
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
           // 从右开始
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }
                            

获取头结点

public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }

获取尾结点

public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

移除指定元素

移除元素，也要遍历查找然后删除

 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;
    }
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;
    }

根据位置移除元素

public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

移除头元素

public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

移除尾元素

public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

清空链表

 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; ) {
            Node<E> next = x.next;
            x.item = null;
            x.next = null;
            x.prev = null;
            x = next;
        }
        first = last = null;
        size = 0;
        modCount++;
    }

检查元素是否存在链表中

public boolean contains(Object o) {
        return indexOf(o) != -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;
    }

添加其他集合中的元素

public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

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;
    }

改变指定位置的元素

public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

迭代器操作

LinkedList的iterator()方法内部调用了其listIterator()方法，所以可以只分析listIterator()方法。listIterator()提供了两个重载方法。iterator()方法和listIterator()方法的关系如下：

public Iterator<E> iterator() {
        return listIterator();
    }

public ListIterator<E> listIterator() {
        return listIterator(0);
    }

 public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
    }

从上面可以看到三者的关系是iterator()——>listIterator(0)——>listIterator(int index)。最终都会调用listIterator(int index)方法，其中参数表示迭代器开始的位置。在ArrayList源码分析中提到过ListIterator是一个可以指定任意位置开始迭代，并且有两个遍历方法。下面直接看ListItr的实现：

private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;
        private Node<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;//保存当前modCount，确保fail-fast机制

        ListItr(int index) {
            // assert isPositionIndex(index);
            next = (index == size) ? null : node(index);//得到当前索引指向的next节点
            nextIndex = index;
        }

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

        //获取下一个节点
        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

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

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

        //获取前一个节点，将next节点向前移
        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();
        }
    }

用linkedList实现一个栈的结构

栈是先进后出的结构，而linkedlist中有往头结点插入元素，从头结点取出元素的方法，因此可以将linkedlist的头结点作为栈顶，每一次都往头部添加元素，取元素的时候往头部取，实现与先进先出，也可以把尾结点作为栈顶，每一次都往尾部添加，往尾部取数据也可以实现，下面的代码使用尾结点作为栈顶

public class LinkStack<E> {

    private LinkedList<E> stack;

    public LinkStack(){
        stack = new LinkedList<E>();

    }

    //压入数据
    public void push(E e) {
        stack.add(e);
    }

    //弹出数据，在Stack为空时将抛出异常
    public E pop() {
        return stack.removeLast();
    }

    //检索栈顶数据，但是不删除
    public E peek() {
        return stack.getLast();
    }

    public static void main(String[] args){
        LinkStack<Integer> stack = new LinkStack<>();
        stack.push(1);
        stack.push(2);
        System.out.println(stack.pop());
    }
}

LinkedList是基于双端链表的List，其内部的实现源于对链表的操作，所以适用于频繁增加、删除的情况；该类不是线程安全的；另外，由于LinkedList实现了Queue接口，所以LinkedList不止有队列的接口，还有栈的接口，可以使用LinkedList作为队列和栈的实现。