Java集合之LinkedList

LinkedList是一个继承AbstractSequentialList的双向链表，它也可以被当做堆栈、队列或者双端队列进行操作。



LinkedList实现了List接口，能对它进行队列操作。
LinkedList实现了Seque接口，能将LinkedList当做双端队列进行使用。
LinkedList实现Cloneable，覆盖了clone函数，可以进行克隆。
LinkedList实现了Serializable，可以被序列化。
LinkedList是非线程安全的。

LinkedList示例程序：

public class Hello {

public static void main(String[] args) {
testLinkedListAPIs() ;
useLinkedListAsLIFO();
useLinkedListAsFIFO();
}

private static void testLinkedListAPIs()
{
String val = null;
LinkedList llist = new LinkedList();
//添加是哪个元素
llist.add("1");
llist.add("2");
llist.add("3");
llist.add(1, "4");//在第一个元素后面插入4
System.out.println("\nTest \"addFirst(), removeFirst(), getFirst()\"");
llist.addFirst("10");//将10插入并作为第一个元素
System.out.println("llist:"+llist);
System.out.println("llist.removeFirst():"+llist.removeFirst());//删除掉第一个元素
System.out.println("llist:"+llist);
System.out.println("llist.getFirst():"+llist.getFirst());//获得第一个元素
System.out.println("\nTest \"offerFirst(), pollFirst(), peekFirst()\"");
llist.offerFirst("10");//添加第一个元素
System.out.println("llist:"+llist);
System.out.println("llist.pollFirst():"+llist.pollFirst());//去掉第一个元素
System.out.println("llist:"+llist);
System.out.println("llist.peekFirst():"+llist.peekFirst());//读取第一个元素
System.out.println("\nTest \"addLast(), removeLast(), getLast()\"");
llist.addLast("20");//链尾添加元素20
System.out.println("llist:"+llist);
System.out.println("llist.removeLast():"+llist.removeLast());//删掉链尾元素20
System.out.println("llist:"+llist);
System.out.println("llist.getLast():"+llist.getLast());//读取链尾元素
System.out.println("\nTest \"offerLast(), pollLast(), peekLast()\"");
llist.offerLast("20");//添加链尾元素20
System.out.println("llist:"+llist);
System.out.println("llist.pollLast():"+llist.pollLast());//删掉链尾元素20
System.out.println("llist:"+llist);
System.out.println("llist.peekLast():"+llist.peekLast());//读取链尾元素
llist.set(2, "300");//替换第三个元素
System.out.println("\nget(3):"+llist.get(2));//获得第三个元素

String[] arr = (String[])llist.toArray(new String[0]);//得到数组
for (String str:arr)
{
System.out.println("str:"+str);
System.out.println("size:"+llist.size());
llist.clear();
System.out.println("isEmpty():"+llist.isEmpty()+"\n");
}
}
private static void useLinkedListAsLIFO()
{
System.out.println("\nuseLinkedListAsLIFO");
LinkedList stack = new LinkedList();
//类似于栈输入
stack.push("1");
stack.push("2");
stack.push("3");
stack.push("4");
System.out.println("stack:"+stack);
System.out.println("stack.pop():"+stack.pop());//出栈
System.out.println("stack.peek():"+stack.peek());//只输出栈顶元素并不出栈
System.out.println("stack:"+stack);
}

private static void useLinkedListAsFIFO()
{
System.out.println("\nuseLinkedListAsFIFO");
LinkedList queue = new LinkedList();
//类似于队列，入队
queue.add("10");
queue.add("20");
queue.add("30");
queue.add("40");
System.out.println("queue:"+queue);
System.out.println("queue.remove():"+queue.remove());//队列出队
System.out.println("queue.element():"+queue.element());//读取队头，并不删除元素
System.out.println("queue:"+queue);
}
}

输出结果：
Test "offerLast(), pollLast(), peekLast()"
llist:[1, 4, 2, 3, 20]
llist.pollLast():20
llist:[1, 4, 2, 3]
llist.peekLast():3
get(3):300
str:1
size:4
isEmpty():true
str:4
size:0
isEmpty():true
str:300
size:0
isEmpty():true
str:3
size:0
isEmpty():true

useLinkedListAsLIFO
stack:[4, 3, 2, 1]
stack.pop():4
stack.peek():3
stack:[3, 2, 1]
useLinkedListAsFIFO
queue:[10, 20, 30, 40]
queue.remove():10
queue.element():20
queue:[20, 30, 40]

LinkedList源代码：

public class LinkedList<E> extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
transient int size = 0; //其实大小为0
transient Node<E> first; //第一个节点
transient Node<E> last; //最后一个节点
public LinkedList() {    //构造一个空LinkedList
}
public LinkedList(Collection<? extends E> c) { //构造一个带有输入集合的
this();
addAll(c);
}
private void linkFirst(E e) { //链接第一个节点
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++;
}
void linkLast(E e) { //链接最后一个节点
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++;
}
void linkBefore(E e, Node<E> succ) { //在节点succ前插入一个e
// 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++;
}
//不再链接第一个非空的节点
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;
}
//删掉最后一个节点
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;
}
//删掉节点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;
}
//获得第一个节点
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
}
//获得最后一个节点
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
}
//删除第一个节点
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
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}
//在头节点插入E
public void addFirst(E e) {
linkFirst(e);
}
//在尾节点插入E
public void addLast(E e) {
linkLast(e);
}
//是否包含某个对象
public boolean contains(Object o) {
return indexOf(o) != -1;
}
//链表长度
public int size() {
return size;
}
//在链表中添加节点
public boolean add(E e) {
linkLast(e);
return true;
}
//删掉某个节点
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;
}
//在链表尾追加集合
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
//在某个节点之后追加集合
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;
}
//清空链表
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++;
}
//获得第几个节点
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}
//对某个节点修改
public E set(int index, E element) {
checkElementIndex(index);
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
}
//在index节点之前插入一个节点element
public void add(int index, E element) {
checkPositionIndex(index);

if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
}
//删除掉下标为index的节点
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
}
//测试此index下是否有节点
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
}
//下标位置在链表内
private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
}
//下标越界
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
//检查下标下的节点
private void checkElementIndex(int index) {
if (!isElementIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//判断下标位置
private void checkPositionIndex(int index) {
if (!isPositionIndex(index))
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
//返回下标节点
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;
}
}
//查找是否存在节点并返回下标，不存在返回-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;
}
//返回最后一个相同的节点的下标
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;
}
//返回表头数据并不删除
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
//返回表头数据
public E element() {
return getFirst();
}
//获得表头数据，并删除表头
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
//删除表头
public E remove() {
return removeFirst();
}
//在表尾添加数据
public boolean offer(E e) {
return add(e);
}
//在表头添加数据
public boolean offerFirst(E e) {
addFirst(e);
return true;
}
//在表尾添加数据
public boolean offerLast(E e) {
addLast(e);
return true;
}
//获得第一个数据，并不删除
public E peekFirst() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}
//获得最后一个数据，并不删除
public E peekLast() {
final Node<E> l = last;
return (l == null) ? null : l.item;
}
//获得第一个节点并删除
public E pollFirst() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}
//获得最后一个节点并删除
public E pollLast() {
final Node<E> l = last;
return (l == null) ? null : unlinkLast(l);
}
//入栈
public void push(E e) {
addFirst(e);
}
//出栈
public E pop() {
return removeFirst();
}
//删除第一次出现的对象
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}
//删除最后一次出现的对象
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
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
return new ListItr(index);
}

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();
}
}

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;
}
}

/**
* @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();
}
}

@SuppressWarnings("unchecked")
private LinkedList<E> superClone() {
try {
return (LinkedList<E>) super.clone();
} catch (CloneNotSupportedException e) {
throw new InternalError(e);
}
}
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;
}
//生成对象数组
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;
}
//泛型数组
@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;
//序列化写对象
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);
}
//序列化读对象
@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());
}
@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
;
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;
}
}

}