LinkedList源码解析

LinkedList特点

1.内部通过双向链表存储数据

2.插入、删除不需要移动元素，只需要修改指针

3.实现了队列、双端队列、栈

4.插入、删除操作比较多的时候，推荐使用

5.获取指定index位置的值效率低

所在包

package java.util;

继承AbstractSequentialList抽象类

实现List、Deque、Cloneable、java.io.Serializable

Deque是双端队列接口

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

内部节点定义方式

可以看出是双向链表，只有一个构造器，参数：前驱节点，当前节点值，后继节点，节点构造好后，其前驱和后继节点也构建好

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;// 后继节点
this.prev = prev;// 前驱节点
}
}

三个重要参数：size，first，last

// 元素个数
transient int size = 0;

/**
* 第一个节点
* Invariant: (first == null && last == null) ||
*            (first.prev == null && first.item != null)
*/
transient Node<E> first;

/**
* 最后一个节点
* Invariant: (first == null && last == null) ||
*            (last.next == null && last.item != null)
*/
transient Node<E> last;

两个构造器

/**
* 空构造器
*/
public LinkedList() {
}

/**
* 集合c中元素加入的LinkedList中
* @param  c the collection whose elements are to be placed into this list
* @throws NullPointerException if the specified collection is null
*/
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
}

头插入节点

/**
* 插入节点e作为第一个节点
*/
private void linkFirst(E e) {
final Node<E> f = first;
final Node<E> newNode = new Node<>(null, e, f); // f 前插入 值为e的节点
first = newNode; // first指针指向当前新的节点
if (f == null) // f 为空所有没有元素，则last指向当前新的节点
last = newNode;
else
f.prev = newNode; // 建立f节点的前驱节点
size++;// 个数+1
modCount++;
}

尾部插入节点

/**
* 插入节点e作为最后一个节点
*/
void linkLast(E e) {
final Node<E> l = last;
final Node<E> newNode = new Node<>(l, e, null); // l 后 插入节点值为 e的节点
last = newNode;// last指针指向当前新的最后节点
if (l == null) // null 说明没有元素，first指向当前新的节点
first = newNode;
else
l.next = newNode;// 建立l 的后继节点
size++;
modCount++;
}

succ之前插入节点

/**
* succ 前插入节点
*/
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); // 新建节点 newNode.next = succ newNode.prev = pred
succ.prev = newNode; // 建立前驱
if (pred == null) // 空时候
first = newNode;
else // 建立后继
pred.next = newNode;
size++;
modCount++;
}

删除第一个节点，并返回节点值

/**
* 删除第一个节点，并返回节点值
*/
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
final E element = f.item;
final Node<E> next = f.next; // 后继节点
f.item = null;
f.next = null; // 垃圾回收
first = next; // 更新first
if (next == null) // 空
last = null;
else
next.prev = null; // 前驱为null
size--;
modCount++;
return element;
}

删除最后一个节点，并返回节点值

/**
* 删除最后一个节点，并返回节点值
*/
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
final E element = l.item;
final Node<E> prev = l.prev; // 前驱节点
l.item = null;
l.prev = null; // 垃圾回收
last = prev; // 更新last
if (prev == null) // null
first = null;
else // 后继为null
prev.next = null;
size--;
modCount++;
return element;
}

删除节点 x 并返回节点值

/**
* 删除节点 x 并返回节点值
*/
E unlink(Node<E> x) {
// assert x != null;
final E element = x.item;
final Node<E> next = x.next;// next
final Node<E> prev = x.prev; // prev

if (prev == null) { // prev == null ,first = next
first = next;
} else { // prev.next = next
prev.next = next;
x.prev = null; // 垃圾回收
}

if (next == null) { // next== null ，last = prev
last = prev;
} else { // next.prev = prev
next.prev = prev;
x.next = null; // for GC
}

x.item = null; // for GC
size--;
modCount++;
return element;
}

获取第一个节点值

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

获取最后一个节点值

/**
* 获取最后一个节点值
* @throws NoSuchElementException if this list is empty
*/
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
}

删除第一个节点，并返回其值，可抛出异常

/**
* 删除第一个节点，并返回其值，可抛出异常
* @throws NoSuchElementException if this list is empty
*/
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return unlinkFirst(f);
}

删除最后一个节点，并返回节点值，可抛出异常

/**
* 删除最后一个节点，并返回节点值，可抛出异常
* @throws NoSuchElementException if this list is empty
*/
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}

头尾插入节点

/**
* 第一个位置插入节点
*/
public void addFirst(E e) {
linkFirst(e);
}

/**
* 最后一个位置插入节点
*/
public void addLast(E e) {
linkLast(e);
}

是否包含元素 o

/**
* 是否包含元素 o
*/
public boolean contains(Object o) {
return indexOf(o) != -1;
}

获取元素的个数

/**
* 获取元素的个数
*/
public int size() {
return size;
}

尾部加入节点

/**
* 尾部加入节点
*/
public boolean add(E e) {
linkLast(e);
return true;
}

删除节点 o

/**
* 删除节点 o
* 顺序遍历查找，找到后删除
*/
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;
}

集合c中元素插入到LinkedList中

/**
* 集合c中元素加入的LinkedList中
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}

index开始 集合c中元素加入的LinkedList中

/**
* index开始 集合c中元素加入的LinkedList中
* @throws IndexOutOfBoundsException {@inheritDoc}
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index); // 检查index是否合法

Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;

Node<E> pred, succ;
// 找到插入位置的前驱 和后继
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);// 新节点 e.prev = pred e.next = null
if (pred == null)
first = newNode;
else
pred.next = newNode;
pred = newNode;
}

if (succ == null) {
last = pred;
} else { // 建立双向链接
pred.next = succ;
succ.prev = pred;
}

size += numNew;
modCount++;
return true;
}

清空

/**
* 清空
*/
public void clear() {
//设为null 方便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++;
}

获取index位置的元素

/**
* 获取index位置的元素
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
checkElementIndex(index);// index合法性检查
return node(index).item;
}

index位置元素的值更新

/**
* index位置元素的值更新
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
checkElementIndex(index); // index合法性检查
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
}

index位置插入新节点

/**
* index位置插入新节点
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
checkPositionIndex(index);

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

删除index位置的节点

/**
* 删除index位置的节点
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
checkElementIndex(index);// index合法性检查
return unlink(node(index));
}

下标合法性的方法

/**
* 是不是下标
*/
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
}

/**
* 是否是合法的插入位置，插入可以查在最后一个节点的后面
*/
private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
}

/**
* 越界输出信息
*/
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
/**
* 检查index是否合法位置
*/
private void checkElementIndex(int index) {
if (!isElementIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
/**
* 检查index是否是合法的插入位置
*/
private void checkPositionIndex(int index) {
if (!isPositionIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

返回index位置的节点

通过前后分开查找的方式

平均查找次数应该是：n/2

/**
* 返回index位置的节点
*/
Node<E> node(int index) {
// assert isElementIndex(index);

if (index < (size >> 1)) { // 查找前一半
Node<E> x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else { // 查找后一半
Node<E> x = last;
for (int i = size - 1; i > index; i--)
x = x.prev;
return x;
}
}

获取节点o第一次出现的下标

/**
* 获取节点o第一次出现的下标
*/
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;
}

获取节点o最后一次出现的下标

/**
* 获取节点o最后一次出现的下标
*/
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;
}

队列的相关操作

/**
* 查看队顶元素
*/
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}

/**
* 查看队顶元素
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
public E element() {
return getFirst();
}

/**
* 获取队顶元素，并删除该元素
* @since 1.5
*/
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}

/**
* 删除队顶元素，并返回该值
* @throws NoSuchElementException if this list is empty
* @since 1.5
*/
public E remove() {
return removeFirst();
}

/**
* 队尾部加入元素
* @since 1.5
*/
public boolean offer(E e) {
return add(e);
}

双端队列相关操作

/**
* 第一个位置插入节点
* @since 1.6
*/
public boolean offerFirst(E e) {
addFirst(e);
return true;
}

/**
* 最后一个位置插入节点
* @since 1.6
*/
public boolean offerLast(E e) {
addLast(e);
return true;
}

/**
* 查看队顶元素
*
* @return the first element of this list, or {@code null}
*         if this list is empty
* @since 1.6
*/
public E peekFirst() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}

/**
* 查看队尾元素
* @return the last element of this list, or {@code null}
*         if this list is empty
* @since 1.6
*/
public E peekLast() {
final Node<E> l = last;
return (l == null) ? null : l.item;
}

/**
*队顶元素出队列
* @return the first element of this list, or {@code null} if
*     this list is empty
* @since 1.6
*/
public E pollFirst() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}

/**
* 队尾元素出队列
* @return the last element of this list, or {@code null} if
*     this list is empty
* @since 1.6
*/
public E pollLast() {
final Node<E> l = last;
return (l == null) ? null : unlinkLast(l);
}

栈的相关操作

/**
* 入栈
* @param e the element to push
* @since 1.6
*/
public void push(E e) {
addFirst(e);
}

/**
* 出栈
* @throws NoSuchElementException if this list is empty
* @since 1.6
*/
public E pop() {
return removeFirst();
}

删除元素 o，只删除第一次出现的那个

/**
* 删除元素 o，只删除第一次出现的那个
* @since 1.6
*/
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}

删除元素o，删除最后一次出现的那个

/**
*删除元素o，删除最后一次出现的那个
* @since 1.6
*/
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;
}

返回从index开始的迭代器

/**
* 返回从index开始的迭代器
* @throws IndexOutOfBoundsException {@inheritDoc}
* @see List#listIterator(int)
*/
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);// 下标检查
return new ListItr(index);
}

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() { // hasNext
return nextIndex < size;
}

public E next() { // next
checkForComodification();
if (!hasNext())
throw new NoSuchElementException();

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

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

public E previous() { // previous
checkForComodification();
if (!hasPrevious())
throw new NoSuchElementException();

lastReturned = next = (next == null) ? last : next.prev;
nextIndex--;
return lastReturned.item;
}

public int nextIndex() { // nextIndex
return nextIndex;
}

public int previousIndex() { //previousIndex
return nextIndex - 1;
}

public void remove() { // 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) { //set
if (lastReturned == null)
throw new IllegalStateException();
checkForComodification();
lastReturned.item = e;
}

public void add(E e) { // add
checkForComodification();
lastReturned = null;
if (next == null)
linkLast(e);
else
linkBefore(e, next);
nextIndex++;
expectedModCount++;
}

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

后向前的迭代器

/**
*后向前的迭代器
* @since 1.6
*/
public Iterator<E> descendingIterator() {
return new DescendingIterator();
}

/**
* 后向前的迭代器
*/
private class DescendingIterator implements Iterator<E> {
private final ListItr itr = new ListItr(size());
public boolean hasNext() { // 前驱
return itr.hasPrevious();
}
public E next() {
return itr.previous();
}
public void remove() {
itr.remove();
}
}

获取LinkedList的clone

@SuppressWarnings("unchecked")
private LinkedList<E> superClone() {
try {
return (LinkedList<E>) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}

/**
* 获取的一个clone
*
* @return a shallow copy of this {@code LinkedList} instance
*/
public Object clone() {
LinkedList<E> clone = superClone();

// Put clone into "virgin" state
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;
}

转换成数组

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;
}

LinkedList元素值更新数组a

/**
* @throws ArrayStoreException if the runtime type of the specified array
*         is not a supertype of the runtime type of every element in
*         this list
* @throws NullPointerException if the specified array is null
*/
@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;
}

序列号

private static final long serialVersionUID = 876323262645176354L;

输入输出流

/**
* Saves the state of this {@code LinkedList} instance to a stream
* (that is, serializes it).
*
* @serialData The size of the list (the number of elements it
*             contains) is emitted (int), followed by all of its
*             elements (each an Object) in the proper order.
*/
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);
}

/**
* Reconstitutes this {@code LinkedList} instance from a stream
* (that is, deserializes it).
*/
@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());
}