LinkedList的实现原理浅析

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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~

Linked List

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

Linked List

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:

    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;
    }
        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++;

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:

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:

    /**
     * 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~

    /**
     * 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:

  1. 检查指定的index的值是否有效
  2. 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~

其它的方法就不一一列举了。

本次LinkedList源码阅读分析就到这里,有兴趣的朋友可以实际去读一下,读源码,懂思想,还是很不错的~

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