LinkedList的实现原理浅析

孟君

发布于 2020-04-22 15:08:01

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发布于 2020-04-22 15:08:01

文章被收录于专栏：孟君的编程札记孟君的编程札记

本文简单分析一下JDK1.7的LinkedList源码，看一下其内部的结构以及典型方法的实现~

LinkedList内部结构

查看LinkedList的源码，发现其继承自AbstractSequentialList，实现了List，Deque，Cloneable以及Serializable接口，如：

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{

}

也就意味着：

LinkedList 是一个继承于AbstractSequentialList的双向链表。它也可以被当作堆栈、队列或双端队列进行操作。
LinkedList 实现 List 接口，能对它进行列表操作。
LinkedList 实现 Deque 接口，即能将LinkedList当作双端队列使用。
LinkedList 实现了Cloneable接口，即覆盖了函数clone()，能克隆。
LinkedList 实现java.io.Serializable接口，这意味着LinkedList支持序列化，能通过序列化去传输。

public class LinkedList<E>    extends AbstractSequentialList<E>    implements List<E>, Deque<E>, Cloneable, java.io.Serializable{    transient int size = 0;
    /**     * Pointer to first node.     * Invariant: (first == null && last == null) ||     *            (first.prev == null && first.item != null)     */    transient Node<E> first;
    /**     * Pointer to last node.     * Invariant: (first == null && last == null) ||     *            (last.next == null && last.item != null)     */    transient Node<E> last;
}

从上述代码可以看出，LinkedList中有size，first以及last全局变量，其作用分别是：

size -- 存放当前链表有多少个节点。 first -- 指向链表的第一个节点的引用 last -- 指向链表的最后一个节点的引用

其中，Node是内部类，内容如下：

    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;        }    }

从上述代码可以看出，

一个节点除了包含元素内容之外，同时包含前一个节点和后一个节点的引用~

各个节点通过指定前一个节点和后一个节点，最终形成了一个链表~

代码示例：

import java.util.LinkedList;

public class LinkedListExample {

    public static void main(String[] args) {
        LinkedList<Integer> values = new LinkedList<>(); 
        
        values.add(23);
        values.add(3);
        values.add(17);
        values.add(9);
        values.add(42);
        
        System.out.println("LinkedList ==> " + values);
        System.out.println("first ==> " + values.getFirst());
        System.out.println("last ==> " + values.getLast());
        
    }
}

输出结果：

LinkedList ==> [23, 3, 17, 9, 42]
first ==> 23
last ==> 42

debug查看LinkedList的结构如下：

形成了一个链表

方法add的实现

源代码

    /**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

add方法会调用linkLast方法，会在链表尾端添加节点。

    /**     * Links e as last element.     */    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++;    }

linkLast方法步骤

获取原来的last节点，然后创建一个新的节点，其prev为原来的last节点，其next节点为null
将last只想新的节点
如果原来的last节点为null，其实就是还没有元素，那么新的节点同样也是first节点；如果不为null，则原来的last节点的next就是新的节点
因为有新元素加入，size加1，且修改次数加1（modCount++）

方法addAll的实现

源代码

    /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator.  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (Note that this will occur if the specified collection is
     * this list, and it's nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

addAll在LinkedList内部其实就是调用了方法addAll(int index, Collection<? extends E> c)

方法addAll(int index, Collection<? extends E> c)

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element
     *              from the specified collection
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    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;
    }

方法addAll(int index, Collection<? extends E> c) 主要包含如下几个步骤：

检查指定index是否合理

    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

index的有效位置是[0,size]

    /**
     * Tells if the argument is the index of a valid position for an
     * iterator or an add operation.
     */
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

定义pred和succ节点，并根据index的大小确定pred和succ节点

        Node<E> pred, succ;
        if (index == size) {
            succ = null;
            pred = last;
        } else {
            succ = node(index);
            pred = succ.prev;
        }

对Collection转换成数组（Object[] a = c.toArray()）的元素进行循环遍历，确定first、pred.next等节点信息

        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;
        }

检查succ是否为空，如果为null，则表示目前的pred节点就是最后一个了，将last节点指向pred；反之，如果不为null，则将prev的next节点指向succ，同时succ的prev节点指向pred。

        if (succ == null) {
            last = pred;
        } else {
            pred.next = succ;
            succ.prev = pred;
        }

最后修改size和modCount的值

        size += numNew;
        modCount++;

上述是往指定位置添加多个元素，那么，往指定位置添加单个元素add(int index, E element) 就变得很简单了。

方法add(int index, E element)

    /**
     * Inserts the specified element at the specified position in this list.
     * Shifts the element currently at that position (if any) and any
     * subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        checkPositionIndex(index);

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

该方法包含如下两个步骤

检查指定index的值是否有效[0,size] 如果index == size 则使用linkLast添加在尾部；如果index ！= size，则使用linkBefore将新元素添加在指定位置之前~

linkBefore方法如下

    /**
     * Inserts element e before non-null Node succ.
     */
    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++;
    }

本文上述已经讲述了linkLast，linkBefore的方法实现思路类似，这里就不再具体给出解释了。

此外，LinkedList还提供了addFirst以及addLast方法，分别用于将元素插在列表头部和尾部~

    /**
     * Inserts the specified element at the beginning of this list.
     *
     * @param e the element to add
     */
    public void addFirst(E e) {
        linkFirst(e);
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     */
    public void addLast(E e) {
        linkLast(e);
    }

其中，linkFirst和linkLast方法如下：

    /**
     * Links e as first element.
     */
    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++;
    }

    /**
     * Links e as last element.
     */
    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++;
    }

方法remove的实现

LinkedList支持多种删除元素的方法~

一起来看看具体是怎么样的

无参数remove方法

    /**
     * Retrieves and removes the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E remove() {
        return removeFirst();
    }

无参数的remove方法其实就是调用了removeFirst方法，也就是移除first元素~

removeFirst方法

    /**
     * Removes and returns the first element from this list.
     *
     * @return the first element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

removeFirst使用了unlinkFirst方法来移除元素

    /**
     * Unlinks non-null first node f.
     */
    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;
    }

unlinkFirst方法处理主要包含如下几个步骤：

获取first元素值，然后获取first的next元素
将first节点指向next，同时原来的first节点的属性值置为null（包括item和next）
如果next节点（原first节点的nex节点）为null，则将last置为null值；如果不为null，则将next节点的prev属性置为null
然后修正元素个数以及修改次数（size和modCount）

同样，也存在移除尾节点的方法removeLast

removeLast方法

    /**
     * Removes and returns the last element from this list.
     *
     * @return the last element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

其使用了unlinkLast方法实现

    /**
     * Unlinks non-null last node l.
     */
    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;
    }

unlinked方法的实现与unlinkedFirst的方法思路类似，就不在这里一一说明了。

方法remove(int index)

    /**
     * Removes the element at the specified position in this list.  Shifts any
     * subsequent elements to the left (subtracts one from their indices).
     * Returns the element that was removed from the list.
     *
     * @param index the index of the element to be removed
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

按照指定位置移除元素，主要包含如下几个部分

检查index是否有效
通过node(index)查找index位置下的节点

/** * Returns the (non-null) Node at the specified element index. */ 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; } }

从上述代码可以看出，方法node(int index)中

先判断index和中间点（size >>1）位置的大小。如果index < (size >> 1), 那么按下标从小到大查找；否则，按下标从大到小查找~

使用unlink(Node<E> x)修改链表的连接关系，达到移除元素的效果

    /**
     * Unlinks non-null node 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;
    }

方法remove(Object o)

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If this list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * {@code i} such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns {@code true} if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if this list contained the specified 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;
    }

按照指定对象的移除，在代码中，区分删除的元素是否为null值，然后从first开始遍历链表，如果元素值和删除的值内容一致，则调用unlink方法移除元素

方法indexOf的实现

源代码

    // Search Operations

    /**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index {@code i} such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    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;
    }

从上述代码可以看出：

LinkedList的indexOf实现区分null和非null值。从first节点开始遍历，如果找到符合条件的元素，则返回元素所在的下标值。如果没有找到，则返回-1

与之对应的还有lastIndexOf方法，该方法和indexOf的思路一致，区别就是，lastIndexOf是以last节点开始往前寻找

    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index {@code i} such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    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;
    }

方法contains的实现

源代码

    /**
     * Returns {@code true} if this list contains the specified element.
     * More formally, returns {@code true} if and only if this list contains
     * at least one element {@code e} such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested
     * @return {@code true} if this list contains the specified element
     */
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

从上述代码可以看出，contains方法内调用了indexOf方法，然后采用获取的结果与-1比较，如果不相等表示有匹配的元素，否则表示没有符合条件的元素

方法clear的实现

源代码

    /**
     * Removes all of the elements from this list.
     * The list will be empty after this call returns.
     */
    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++;
    }

clear方法，从first开始遍历链表，将元素的item、prev和nex属性置为null值，然后将first和last置为null。同时将size置为0，修改次数加1（modCount+）

方法get的实现

LinkedList支持按索引查找以及获取first和last元素的操作~ 如：

方法get(int index)的实现

    /**
     * Returns the element at the specified position in this list.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

此方法包含两个步骤：

检查指定的index的值是否有效
调用node(index)获取节点，返回值node(index).item即可

方法getFirst

    /**     * Returns the first element in this list.     *     * @return the first element in this list     * @throws NoSuchElementException if this list is empty     */    public E getFirst() {        final Node<E> f = first;        if (f == null)            throw new NoSuchElementException();        return f.item;    }

方法getFirst获取first节点的值item即可，得先判断first是否为空~

方法getLast

    /**
     * Returns the last element in this list.
     *
     * @return the last element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

方法getLast获取last节点的值item即可，得先判断last是否为空~

方法listIterator的实现

源代码

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

其使用了内部类ListItr来实现，ListItr类内容如下：

private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned = null;
        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++;
        }

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

listIterator接口继承自Iterator接口，具备更多的方法，如add，set，previous等等

ListIterator示例

import java.util.Iterator;
import java.util.LinkedList;

public class LinkedListIteratorExample {

    public static void main(String[] args) {
        LinkedList<Integer> values = new LinkedList<>(); 
        
        values.add(23);
        values.add(3);
        values.add(17);
        values.add(9);
        values.add(42);
        
        System.out.println("LinkedList ==> " + values);
 
        System.out.println("listIterator~~");
        Iterator<Integer> iter = values.listIterator();
        while(iter.hasNext()) {
            Integer ele = iter.next();
            System.out.println(ele);
        }
        
        System.out.println("listIterator with index~~");
        Iterator<Integer> iterWithIndex = values.listIterator(2);
        while(iterWithIndex.hasNext()) {
            Integer ele = iterWithIndex.next();
            System.out.println(ele);
        }
        
    }
}

输出结果：

LinkedList ==> [23, 3, 17, 9, 42]
listIterator~~
23
3
17
9
42
listIterator with index~~
17
9
42

方法descendingIterator的实现

源代码

    /**     * @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();        }    }

descendingIterator与Iterator的区别在于，Iterator是从first开始往后遍历；而descendingIterator是从last开始往前遍历；

Iterator和descendingIterator示例：

import java.util.Iterator;
import java.util.LinkedList;

public class LinkedListIteratorExample {

    public static void main(String[] args) {
        LinkedList<Integer> values = new LinkedList<>(); 
        
        values.add(23);
        values.add(3);
        values.add(17);
        values.add(9);
        values.add(42);
        
        System.out.println("LinkedList ==> " + values);
 
        System.out.println("iterator~~");
        Iterator<Integer> iter = values.iterator();
        while(iter.hasNext()) {
            Integer ele = iter.next();
            System.out.println(ele);
        }
        
        System.out.println("descendingIter~~");
        Iterator<Integer> descendingIter = values.descendingIterator();
        while(descendingIter.hasNext()) {
            Integer ele = descendingIter.next();
            System.out.println(ele);
        }
        
        
    }
}

输出结果：

LinkedList ==> [23, 3, 17, 9, 42]
iterator~~
23
3
17
9
42
descendingIter~~
42
9
17
3
23

方法toArray的实现

源代码

    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;    }

   @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;    }

从first节点开始，依次遍历，然后得到一个数组对象

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原始发表：2020-04-20，如有侵权请联系 cloudcommunity@tencent.com 删除

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