LinkedList是使用双向链表实现的List。长处是可以在O(1)复杂度下进行插入、删除操作,弱项是随机访问的复杂度是O(n)。

父类与接口

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

LinkedList实现了List和Deque接口,所以可以把LinkedList当做队列或者双向队列使用。

成员变量

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//保存双向链表的长度
transient int size = 0;
// 指向头元素的引用
// 永远成立的表达式: (first == null && last == null) ||
// (first.prev == null && first.item != null)
transient Node<E> first;
// 指向尾元素的引用
// 永远成立的表达式【有疑问】: (first == null && last == null) ||
// (last.next == null && last.item != null)
transient Node<E> last;

这里我有一个很大的疑问。就是源码注释的永远成立的表达式,感觉是不对的。因为LinkedList是允许添加null的元素的,所以可能出现(first.prev == null && first.item == null)的情况。

这个时候我心里激动了一下,我擦,我是不是发现了jdk的一个小bug,看看jdk最新代码什么情况,唉,可惜已经被人修复了。。。http://hg.openjdk.java.net/jdk9/dev/jdk/rev/cabf2d0876ef

构造函数

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public LinkedList() {
}
//用c集合的元素新建一个LinkedList
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}

节点类

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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;
}
}

这是链表节点类,因为是双向链表,所以有一个next一个prev引用。

内部用插入/删除方法(链表操作方法)

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//添加头部元素
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f);
first = newNode;
//原来first==null,说明当前链表为空链表,则last也指向新加入的头部元素
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
}
//追加尾部元素
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null);
last = newNode;
//原来的last==null,说明当前链表为空链表,则first也指向新加入的尾部元素
if (l == null)
first = newNode;
else
l.next = newNode;
size++;
modCount++;
}
//在某检点前插入元素,因为是内部使用,所以保证了succ!=null,并且保证succ是当前链表上的节点
void linkBefore(E e, Node<E> succ) {
final Node<E> pred = succ.prev;
final Node<E> newNode = new Node<>(pred, e, succ);
succ.prev = newNode;
//指定节点的前缀==null,说明指定的节点为first,则更新first
if (pred == null)
first = newNode;
//否则,指定节点的前驱的后缀为插入的节点
else
pred.next = newNode;
size++;
modCount++;
}
//删除第一个节点,删除节点需要把节点的前驱,元素,后缀都设置为null,帮助GC
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;
}
//删除最后一个节点,删除节点需要把节点的前驱,元素,后缀都设置为null,帮助GC
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;
}
//删除某个节点
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;
}

这些内部方法如果传入的是Node,则需要保证两点:

  • 不为空
  • 是当前LinkedList上的节点

从首尾部获取/添加/删除元素

都是实现Deque接口的方法

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//获取第一个元素,如果列表为空,报错
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
}
//获取最后一个元素,如果列表为空,报错
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
}
//删除第一个元素
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return unlinkFirst(f);
}
//删除最后一个元素
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}
//在首部添加元素
public void addFirst(E e) {
linkFirst(e);
}
//在尾部追加元素
public void addLast(E e) {
linkLast(e);
}

使用上面包装好的链表操作函数,这些函数的实现变得很简单。

这几个方法如果在获取、删除时发现列表是空的会报出NoSuchElementException。

查找与搜索

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//判断列表中是否存在元素
public boolean contains(Object o) {
return indexOf(o) != -1;
}
//获取长度
public int size() {
return size;
}
//追加元素,和addLast是等价的
public boolean add(E e) {
linkLast(e);
return true;
}
//删除链表中第一次出现的指定对象,通过遍历列表实现
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;
}

批量操作

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//添加另外一个集合的元素,如果在添加过程中,外部集合的内容被修改了,行为是不确定的
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
//在指定位置插入另外一个集合的元素,如果在添加过程中,外部集合的内容被修改了,行为是不确定的
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index); //判断插入位置是否合法
Object[] a = c.toArray(); //外部集合转为数组使用,这里涉及到一次拷贝
int numNew = a.length;
if (numNew == 0) //如果外部集合为空,直接返回false
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;
}
//清空所有元素
public void clear() {
// 把元素都设置为null,帮助GC
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++;
}

基于位置的操作

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//返回特定位置上的元素
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
//替换指定位置上的元素
public E set(int index, E element) {
checkElementIndex(index);
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
}
//在指定位置插入元素
public void add(int index, E element) {
checkPositionIndex(index);
if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
}
//删除指定位置上的元素
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
}
//判断index是否合法
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
}
//判断index是否可以用来add或者用来迭代。多了一个index==size的合法位置,因为可以在size上添加最后一个元素
private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
}
//拼接位置错误信息
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
//检查index是否合法,不合法就报错
private void checkElementIndex(int index) {
if (!isElementIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//判断index是否可以用来add或者用来迭代,不合法就报错
private void checkPositionIndex(int index) {
if (!isPositionIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//返回指定位置上的元素
Node<E> node(int index) {
//必须在外部保证index合法
//判断位置在前半区间还是后半区间
if (index < (size >> 1)) {
//如果在前半区间,从前往后遍历
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
//如果在后半区间,从后往前遍历
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}

搜索操作

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//从前往后搜索指定元素位置,如果找不到,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;
}
//从后往前搜索指定元素位置,如果找不到,返回-1
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;
}

队列操作(Queue接口)

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//返回第一个元素,但是不会删除。如果列表为空,返回null
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
//返回第一个元素,但是不会删除。如果列表为空,报出NoSuchElementException
public E element() {
return getFirst();
}
//返回第一个元素并删除。如果列表为空返回null
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
//返回并删除第一个元素。如果列表为空报NoSuchElementException
public E remove() {
return removeFirst();
}
//追加元素
public boolean offer(E e) {
return add(e);
}

双端队列操作(Deque接口)

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//从头部插入元素
public boolean offerFirst(E e) {
addFirst(e);
return true;
}
//从尾部插入元素
public boolean offerLast(E e) {
addLast(e);
return true;
}
//获取第一个元素,如果列表为空返回null
public E peekFirst() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
//获取最后一个元素,如果列表为空返回null
public E peekLast() {
final Node<E> l = last;
return (l == null) ? null : l.item;
}
//获取并删除第一个元素,如果列表为空返回null
public E pollFirst() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
//获取并删除最后一个元素,如果列表为空返回null
public E pollLast() {
final Node<E> l = last;
return (l == null) ? null : unlinkLast(l);
}
//栈push操作
public void push(E e) {
addFirst(e);
}
//栈pop操作
public E pop() {
return removeFirst();
}
//删除从前往后第一个出现的相等元素
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}
//删除从后往前第一个出现的相等元素
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;
}

迭代器操作

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//获取LinkedList的ListIterator
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
return new ListItr(index);
}
//是fail-fast的
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;
}
//判断是否还有元素,不是通过null而是通过size
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();
}
}
//获取逆向迭代器
public Iterator<E> descendingIterator() {
return new DescendingIterator();
}
//逆向迭代器是基于正向迭代器的简单包装
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();
}
}

克隆与获取数组

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@SuppressWarnings("unchecked")
private LinkedList<E> superClone() {
try {
return (LinkedList<E>) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
}
//克隆LinkedList,会克隆每个Node,但是不会克隆每个元素的内容
public Object clone() {
LinkedList<E> clone = superClone();
// 保持处女的状态(原始注释可就是这句哦)
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;
}
//返回数组
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;
}
//返回指定类型的数组
@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;
}

序列化/反序列化

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private static final long serialVersionUID = 876323262645176354L;
//序列化,写入size,然后是没一个元素,注意LinkedList中size也是transient的
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);
}
//反序列化
@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());
}

Java8中的新方法

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/**
* Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
* and <em>fail-fast</em> {@link Spliterator} over the elements in this
* list.
*
* <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
* {@link Spliterator#ORDERED}. Overriding implementations should document
* the reporting of additional characteristic values.
*
* @implNote
* The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
* and implements {@code trySplit} to permit limited parallelism..
*
* @return a {@code Spliterator} over the elements in this list
* @since 1.8
*/
@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;
}
}

总结

  • LinkedList就是普通的双向链表,但是他可以作为列表,队列,双向队列,栈使用,是一个功能很多的集合类
  • LinkedList在实现了几个链表操作函数后,使用这些函数来保证Deque等接口的方法,核心代码不多
  • ❤️获取指定位置上的元素,LinkedList使用了加速技巧:判断位置在前半区间还是后半区间,如果在前半区间,从前往后遍历,如果在后半区间,从后往前遍历
  • LinkedList因为实现了Deque,所以存在大量冗余的方法,具体可以参考:JDK源码阅读-Queue/Deque | 木杉的博客
  • LinkedList实现了Deque,可以获取逆向迭代器,而逆向迭代器是正向迭代器的简单包装

参考资料