1. LinkedList的定义
1.1 继承于AbstractSequentialList的双向链表，可以被当作堆栈、队列或双端队列进行操作
1.2 有序，非线程安全的双向链表，默认使用尾部插入法
1.3 适用于频繁新增或删除场景，频繁访问场景请选用ArrayList
1.4 插入和删除时间复杂为O(1)，其余最差O(n)
1.5 由于实现Deque接口，双端队列相关方法众多
2. LinkedList的继承体系
- 继承
AbstractSequentialList，能被当作堆栈、队列或双端队列进行操作 - 实现
List接口，能进行队列操作 - 实现
Deque接口，能将LinkedList当作双端队列使用 - 实现
Cloneable接口，重写clone()，能克隆(浅拷贝) - 实现
java.io.Serializable接口，支持序列化
3. LinkedList的源码实现
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable { /**
* 记录当前链表中元素的个数
*/
transient int size = 0; /**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
* 链表的头部节点
*/
transient Node<E> first; /**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
* 链表的尾部节点
*/
transient Node<E> last; /**
* Constructs an empty list.
* 默认空构造器 -- 注意LinkedList并不提供指定容量的构造器
*/
public LinkedList() {
} /**
* 支持将一个Collection对象转换为linkedList对象
*/
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
} /**
* 从头部开始添加一个元素
*/
private void linkFirst(E e) {
//获取头部节点
final Node<E> f = first; //组建新的节点，新的节点的prev为null,并赋值给头部节点
final Node<E> newNode = new Node<>(null, e, f);
first = newNode; //头节点==null.说明链表中没有元素
if (f == null)
last = newNode;
else
f.prev = newNode; //设置旧的头节点的前一个节点为新建的节点
size++;
modCount++;
} /**
* Links e as last element.
* 从尾部开始添加一个元素
*/
void linkLast(E e) { //获取尾部节点
final Node<E> l = last; //组建新的节点，新节点的next节点为null,并将新的节点赋值给尾部节点
final Node<E> newNode = new Node<>(l, e, null);
last = newNode; //尾部节点为null,说明链表中没有元素
if (l == null)
first = newNode;
else
l.next = newNode; //设置旧的尾节点的next为新建的节点
size++;
modCount++;
} /**
* 在指定的节点之前添加元素
*/
void linkBefore(E e, Node<E> succ) { //获取指定节点的prev
final Node<E> pred = succ.prev; //构建新的节点
final Node<E> newNode = new Node<>(pred, e, succ);
//将新建的节点设置为指定节点的prev
succ.prev = newNode; //判断prev是否为头结点
if (pred == null)
first = newNode;
else
pred.next = newNode; //设置新节点为prev的next节点
size++;
modCount++;
} /**
* 移除头部节点
*/
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
final E element = f.item; //获取头节点的next，并赋值给first
final Node<E> next = f.next;
f.item = null;
f.next = null; // help GC
first = next; //next == null,说明链表中已经没有元素
if (next == null)
last = null;
else
next.prev = null; //设置头结点的prev为null
size--;
modCount++;
return element;
} /**
* 移除尾部节点
*/
private E unlinkLast(Node<E> l) {
// assert l == last && l != null;
final E element = l.item; //获取尾部节点的prev节点，并赋值为last节点
final Node<E> prev = l.prev;
l.item = null;
l.prev = null; // help GC
last = prev; //prev == null，说明链表中已经没有元素
if (prev == null)
first = null;
else
prev.next = null; //设置尾节点的next为空
size--;
modCount++;
return element;
} /**
* 移除链表中的某个节点
*/
E unlink(Node<E> x) { //获取要移除的元素
final E element = x.item;
final Node<E> next = x.next;
final Node<E> prev = x.prev; //要移除的节点是否是头结点
if (prev == null) {
first = next;
} else {
//把当前节点的prev节点的next节点赋值为当前节点的next
prev.next = next;
//当前节点的prev置为null
x.prev = null;
} //要移除的节点是否是尾节点
if (next == null) {
last = prev;
} else {
//当前节点的next节点的prev节点赋值为当前节点的prev
next.prev = prev;
x.next = null;
} //将当前节点置为null
x.item = null;
size--;
modCount++;
return element;
} /**
* 获取链表中的第一个元素
*/
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
} /**
* 获取链表中的最后一个元素
*/
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
} /**
* 移除链表中的第一个元素并返回
*/
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException(); //执行移除头部节点的方法
return unlinkFirst(f);
} /**
* 移除链表中的最后一个元素并返回
*/
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException(); //执行移除尾部节点的方法
return unlinkLast(l);
} /**
* 在链表的头部添加节点
*/
public void addFirst(E e) {
linkFirst(e);
} /**
* 在链表的尾部添加节点
*/
public void addLast(E e) {
linkLast(e);
} /**
* 链表中是否包含某个元素
*/
public boolean contains(Object o) {
return indexOf(o) != -1;
} /**
* 链表中元素的个数
*/
public int size() {
return size;
} /**
* 添加一个元素到链表中
* 默认是尾部添加
* 成功则返回true
*/
public boolean add(E e) {
linkLast(e);
return true;
} /**
* 移除链表中的某个元素
*/
public boolean remove(Object o) {
if (o == null) {
//遍历查找null元素,并进行移除
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null) {
//执行移除的方法
unlink(x);
return true;
}
}
} else {
//遍历找到要移除的元素
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item)) {
//执行移除的方法
unlink(x);
return true;
}
}
}
return false;
} /**
* 将一个collection对象转换为链表中的节点
*/
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
} /**
*
*/
public boolean addAll(int index, Collection<? extends E> c) { //检查索引的合理性
checkPositionIndex(index); //将collection对象转换为数组，数组为空则直接返回
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false; //判断是在指定位置插入节点还是在尾部插入节点
Node<E> pred, succ;
if (index == size) {
succ = null;
pred = last;
} else {
succ = node(index);
pred = succ.prev;
} //遍历数组，进行插入
for (Object o : a) {
@SuppressWarnings("unchecked") E e = (E) o;
Node<E> newNode = new Node<>(pred, e, null);
//判断链表书否为空
if (pred == null)
first = newNode;
else
pred.next = newNode;
pred = newNode;
} //如果succ为null,则是在尾部进行节点的插入
if (succ == null) {
last = pred;
} else {
//将新插入的节点和后面的节点进行链接
pred.next = succ;
succ.prev = pred;
} size += numNew;
modCount++;
return true;
} /**
* 清空链表中的元素
*/
public void clear() { //循环设置链表的值为null,加快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++;
} /**
* 获取指定位置的元素
*/
public E get(int index) {
//检查索引的合理性
checkElementIndex(index);
return node(index).item;
} /**
* 替换指定位置上的元素并返回旧的元素
*/
public E set(int index, E element) {
checkElementIndex(index);
//找到指定元素所在的节点
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
} /**
* 在指定的位置添加元素
*/
public void add(int index, E element) {
checkPositionIndex(index); if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
} /**
* 移除指定位置的元素
*/
public E remove(int index) {
//检查索引的合理性
checkElementIndex(index);
//进行指定位置上的节点移除
return unlink(node(index));
} private boolean isElementIndex(int index) {
return index >= 0 && index < size;
} private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
} private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
} /**
* 检查索引是否合理
* @param index
*/
private void checkElementIndex(int index) {
if (!isElementIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
} private void checkPositionIndex(int index) {
if (!isPositionIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
} /**
* 返回某个位置上的节点元素
*/
Node<E> node(int index) { //二分查找
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;
}
} /**
* 元素存在于链表中，则返回所在的节点的位置
* 否则返回-1
*/
public int indexOf(Object o) {
//初始化为0
int index = 0;
//如果元素为null,返回第一个查找到的null的位置
if (o == null) {
//遍历链表，查找null元素，并增加index的值
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null)
return index;
index++;
}
} else {
//遍历链表，查找元素，并增加index的值
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item))
return index;
index++;
}
}
return -1;
} /**
* 从链表的尾节点开始查找指定的元素
* 并返回指定元素的位置
* 没有找到返回-1
*/
public int lastIndexOf(Object o) {
int index = size;
//指定的元素是否为null
if (o == null) {
//遍历查找null元素
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (x.item == null)
return index;
}
} else {
//遍历查找指定的元素
for (Node<E> x = last; x != null; x = x.prev) {
index--;
if (o.equals(x.item))
return index;
}
}
return -1;
} /**
* 进行出队列的操作
*/
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
} /**
* 获取链表中的一个元素
* 默认返回头结点的元素
*/
public E element() {
return getFirst();
} /**
* 进行出队列的操作
* 并移除元素
*/
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
} /**
* 移除链表中的元素
* 默认为头结点的元素
*/
public E remove() {
return removeFirst();
} /**
* 添加元素，默认在尾部添加
*/
public boolean offer(E e) {
return add(e);
} /**
* 在链表的头部添加元素
* 成功则返回true
*/
public boolean offerFirst(E e) {
addFirst(e);
return true;
} /**
* 在尾部添加元素
*/
public boolean offerLast(E e) {
addLast(e);
return true;
} /**
* 获取第一个元素
*/
public E peekFirst() {
final Node<E> f = first;
return (f == null) ? null : f.item;
} /**
* 获取最后一个元素
*/
public E peekLast() {
final Node<E> l = last;
return (l == null) ? null : l.item;
} /**
* 获取第一个元素
* 并移除
*/
public E pollFirst() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
} /**
* 获取最后一个元素
* 并移除
*/
public E pollLast() {
final Node<E> l = last;
return (l == null) ? null : unlinkLast(l);
} /**
* 头部添加元素
*/
public void push(E e) {
addFirst(e);
} /**
* 移除头部元素
*/
public E pop() {
return removeFirst();
} /**
* 从头查找，并移除指定的元素
* 成功返回true
*/
public boolean removeFirstOccurrence(Object o) {
return remove(o);
} /**
* 从尾部查找，并移除指定的元素
* 成功返回true
*/
public boolean removeLastOccurrence(Object o) {
if (o == null) {
//遍历查找指定的null元素
for (Node<E> x = last; x != null; x = x.prev) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
//遍历查找指定的元素
for (Node<E> x = last; x != null; x = x.prev) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
} /**
* 从指定位置开始获取链表的迭代器
*/
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
return new ListItr(index);
} /**
* 链表的内部类
* 一个迭代器对象
* @author Administrator
*
*/
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) {
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();
}
} /**
* 链表中保存的节点对象
* @param <E>
*/
private static class Node<E> {
E item; //当前元素
Node<E> next; //下一个元素
Node<E> prev; //上一个元素 Node(Node<E> prev, E element, Node<E> next) {
this.item = element;
this.next = next;
this.prev = prev;
}
} /**
* @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();
}
} /**
* 调用父类进行链表的克隆
* @return
*/
@SuppressWarnings("unchecked")
private LinkedList<E> superClone() {
try {
return (LinkedList<E>) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
} /**
* 进行链表的克隆
*/
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;
} /**
* 将链表转换为Object类型的数组
*/
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;
} /**
* 将链表转换为指定类型的数组
*/
@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;
} private static final long serialVersionUID = 876323262645176354L; /**
* 序列化链表中的元素
*/
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);
} /**
* 反序列化链表中的元素
*/
@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新增的返回
* 返回一个并行的分片迭代器
*/
@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;
}
} }