title: Java集合框架整理(五)——LinkedList源码分析
tag: Java集合
category: Java
LinkedList源码分析
LinkedList：链表实现的集合
先看看类的继承关系
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
//链表长度
transient int size = 0;
//头结点
transient Node<E> first;
//尾节点
transient Node<E> last;
public LinkedList() {
}
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}
...
//结点
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;
}
}
}
LinkedList是通过AbstractSequentialList来实现的，并且同时实现了List、Deque、Cloneable、Serialzable等接口。List前面Java集合框架整理已经看过了，Deque则在Queue中有讲到，Cloneable和Serialzable表示可克隆和序列化，也没啥说的
接着可以看到有三个trasient修饰的全局变量表示不可序列化，然后是构造器。默认实现的是一个空链表，或者根据Collection生成的链表。然后我们看到链表结点定义的是一个私有静态内部类，只有一个构造方法，参数分别是元素，下一个结点和前一个结点（维护了双链表，既可前向遍历也可后向遍历），结点的引用可以让我们直接进行添加、删除操作而不用向数组一样需要进行其他元素的移动，但遍历就相对麻烦一些
接着看看常见的操作方法吧
添加add
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
...
//使用尾插法添加
public boolean add(E e) {
linkLast(e);
return true;
}
//添加到指定位置
public void add(int index, E element) {
checkPositionIndex(index);
if (index == size)
linkLast(element); //添加到链尾
else
linkBefore(element, node(index)); //添加到某个结点前，通过node()获取改位置的结点
}
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
//将集合中的元素依次插入到指定位置
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);
//转为元素数组
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
Node<E> pred, succ;
//整个集合插到链尾
if (index == size) {
succ = null;
pred = last;
} else {
//指定结点后面
succ = node(index);
pred = succ.prev;
}
//遍历插入
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;
}
public void addFirst(E e) {
linkFirst(e);
}
public void addLast(E e) {
linkLast(e);
}
//头插法
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++;
}
//尾插法
//获取尾结点，生成一个新结点，如果没有尾结点，链表为空链的情况，那就成为头结点；如果有，则调整链尾，修改结点个数
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++;
}
//插入，插入在某个结点前；通过节点的前指针pred来进行插入操作
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++;
}
//通过比较size的一半，进行遍历查找，前向还是后向
Node<E> node(int index) {
//减小遍历次数
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;
}
}
//来自Deque接口的方法
public boolean offer(E e) {
return add(e);
}
public boolean offerFirst(E e) {
addFirst(e);
return true;
}
public void push(E e) {
addFirst(e);
}
}
常用的add、addAll方法，通过链表的特性进行添加，由于也实现了Deque接口，所以一些方法逻辑就是基本一致
删除remove
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
...
//删除指定元素
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 {
//通过遍历查找删除
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
}
//删除指定位置的元素
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
}
//默认移除头结点
public E remove() {
return removeFirst();
}
//删除第一次出现的某个元素
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}
//删除最后一次出现的元素，通过反响遍历
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;
}
//删除头结点
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return unlinkFirst(f);
}
//删除尾结点
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}
//删除第一个结点
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;
}
//删除最后一个结点
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;
}
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
public E pollFirst() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
public E pollLast() {
final Node<E> l = last;
return (l == null) ? null : unlinkLast(l);
}
public E pop() {
return removeFirst();
}
}
remove也没啥难的，跟add基本对应类似
get
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
...
public E get(int index) {
checkElementIndex(index); //检查下标
return node(index).item;
}
//得到头结点
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
}
//得到尾节点
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
}
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
public E element() {
return getFirst();
}
public E peekFirst() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
public E peekLast() {
final Node<E> l = last;
return (l == null) ? null : l.item;
}
}
set
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
...
//修改指定位置的值，返回旧值
public E set(int index, E element) {
checkElementIndex(index);
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
}
}
其他方法
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
...
//获取某个元素的位置。后向遍历
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;
}
//最后一次出现的位置
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;
}
//通过比较是否下标判断是否存在某个元素
public boolean contains(Object o) {
return indexOf(o) != -1;
}
public int size() {
return size;
}
//清空链表，遍历置null
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++;
}
//拷贝。浅拷贝
public Object clone() {
LinkedList<E> clone = superClone();
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;
}
//转为元素数组
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;
}
}
然后看看迭代器
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
...
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
return new ListItr(index);
}
public Iterator<E> descendingIterator() {
return new DescendingIterator();
}
@Override
public Spliterator<E> spliterator() {
return new LLSpliterator<E>(this, -1, 0);
}
}
通过index获取普通的迭代器，分割迭代器也是重写了。descendingIterator()提供的是一个逆序的迭代器，看源码可以知道就是以size()来生成的ListItr，next是当前结点的前一个结点
先看ListItr
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) {
...
}
public void add(E e) {
...
}
public void forEachRemaining(Consumer<? super E> action) {
...
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
其实ListItr跟AbstractList中的差不多，核心思想都是间接来操作集合，这里就是间接操作链表了
再看LLSpliterator
static final class LLSpliterator<E> implements Spliterator<E> {
static final int BATCH_UNIT = 1 << 10; // 分批每一批的size的增量
static final int MAX_BATCH = 1 << 25; // 每一批最大size
final LinkedList<E> list;
Node<E> current; // 当前的结点
int est; // size估计值
int expectedModCount; // est修改次数
int batch; // 分割每一批的数量size
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;
}
//估计当前Spliterator实例中将要迭代的元素的数量，如果数量是无限的、未知的或者计算数量的花销太大，则返回Long.MAX_VALUE
public long estimateSize() { return (long) getEst(); }
//分割迭代器，没调用一次，将原来的迭代器等分为两份，并返回索引靠前的那一个子迭代器
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;
}
//通过action批量消费所有的未迭代的数据。
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();
}
//单个对元素执行给定的动作，如果有剩下元素未处理返回true，否则返回false
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;
}
//返回当前对象有哪些特征值
public int characteristics() {
return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
}
}
这里就简单看看Spliterator，具体的请自行查看有关Spliterator的知识
java源码阅读之Spliterator
JDK8源码之Spliterator并行遍历迭代器
总结
LinkedList就是充分利用链表这个数据结构的特定，实现的集合，在1.8加入了分割迭代器用于并发处理和遍历，优化性能。同时在查找某个结点的时候，也是优化算法为O(n/2)的时间复杂度；链表的空间复杂度并不复杂，唯一的缺点就是遍历的问题比较麻烦；LinkedList采用的是双链表的形式，在遍历上比单链表效率又要高许多。还有点需要注意的是LinkedList并没有实现List接口中的replaceAll()和sort()方法，而ArrayList实现了
