package java.util; import java.util.function.Consumer; public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
transient int size = 0; transient Node<E> first; transient Node<E> last; public LinkedList() {
}
// 传入一个Collection类型的集合，转换为linkedlist类型
public LinkedList(Collection<? extends E> c) {
this();
addAll(c);
} /**
* 将一个element添加为第一个节点 该方法提供给addFirst调用
*/
private void linkFirst(E e) {
//当前第一个节点暂存
final Node<E> f = first;
//新建一个节点，pre为空，next为之前的first节点
final Node<E> newNode = new Node<>(null, e, f);
//将当前节点设置为第一个节点，并且判断原来第一个节点是否存在，不存在，则新增节点既是first，也是last节点，
//如果添加之前存在第一个节点，（因为linkedList是一个双向列表，所以需要将原第一个节点的pre设置为新增节点），添加完成之后size，modCount（改变次数）自增
first = newNode;
if (f == null)
last = newNode;
else
f.prev = newNode;
size++;
modCount++;
} /**
* 同linkFirst，将添加的节点设置为最后一个
*/
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++;
} /**
* 在指定节点之前添加一个节点
*/
void linkBefore(E e, Node<E> succ) {
// assert succ != null;
//暂存指定节点的前一个节点
final Node<E> pred = succ.prev;
//创建新节点，且pre设置为pred，next设置为指定的节点（该新建只是指定了当前节点的pre和next，尚没有指定前一个节点的next和后一个节点的pre）
final Node<E> newNode = new Node<>(pred, e, succ);
//一下步骤是指定前一个节点的next和后一个节点的pre
succ.prev = newNode;
if (pred == null)
first = newNode;
else
pred.next = newNode;
size++;
modCount++;
} /**
* 清掉第一个非空的节点.
*/
private E unlinkFirst(Node<E> f) {
// assert f == first && f != null;
//暂存要清掉节点的item和next，并将f节点的item和next置空（置空是为了帮助清理无用的空间）
final E element = f.item;
final Node<E> next = f.next;
f.item = null;
f.next = null; // help GC
//将下一节点置为第一个节点（因是双向列表，所以需要设置节点的pre为空，同时size减小，modCount改变次数增加）
first = next;
if (next == null)
last = null;
else
next.prev = null;
size--;
modCount++;
//返回删除节点的item
return element;
} /**
* 同unlinkFirst方法，删除最后一个非空节点.
*/
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;
} /**
* 删除指定节点.
*/
E unlink(Node<E> x) {
// assert x != null;
//暂存要删除节点的 值：item/next/pre
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;
} /**
* 返回列表的第一个节点*/
public E getFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return f.item;
} /**
* 返回节点的最后一个节点*/
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
} /**
* 删除第一个节点，（方法调用私有方法unlinkFirst）*/
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return unlinkFirst(f);
} /**
* 删除最有一个节点*/
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
} /**
* 添加一个首节点*/
public void addFirst(E e) {
linkFirst(e);
} /**
* 添加尾节点
*
* @param e the element to add
*/
public void addLast(E e) {
linkLast(e);
} /**
* 判断列表中是否存在指定节点*/
public boolean contains(Object o) {
return indexOf(o) != -1;
} /**
* 列表大小*/
public int size() {
return size;
} /**
* 添加尾节点
**/
public boolean add(E e) {
linkLast(e);
return true;
} /**删除链表中的指定值*/
public boolean remove(Object o) {
if (o == null) {
//循环列表，将列表中null删除
for (Node<E> x = first; x != null; x = x.next) {
if (x.item == null) {
unlink(x);
return true;
}
}
} else {
//循环列表，删除列表中item为o的节点
for (Node<E> x = first; x != null; x = x.next) {
if (o.equals(x.item)) {
unlink(x);
return true;
}
}
}
return false;
} /**
* 构造函数调用，将一个collection类型的集合转换成一个linkedlist类型
*/
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
} /**
* index：范围在【0-size】，当为0是，将集合中的节点添加的linkedlist的开头
不为0是，则添加到第index位置*/
public boolean addAll(int index, Collection<? extends E> c) {
//校验index是否在[0，size]之间，不再则报错
checkPositionIndex(index); Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
//index==size是 添加到最后
Node<E> pred, succ;
if (index == size) {
succ = null;
pred = last;
} else {
//暂存指定位置的节点，之后将该节点设置到新增集合节点中的后边
succ = node(index);
pred = succ.prev;
}
//循环设置新节点，并将pre的next设置为新增节点
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;
//为下一此循环判断，赋值 准备
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() {
// Clearing all of the links between nodes is "unnecessary", but:
// - helps a generational GC if the discarded nodes inhabit
// more than one generation
// - is sure to free memory even if there is a reachable Iterator
//节点全都置null，是为了方便释放内存
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++;
} // Positional Access Operations /**
* 获取指定位置的节点值*/
public E get(int index) {
//判断index是否合法
checkElementIndex(index);
//node方法内部循环获取节点
return node(index).item;
} /**
* 替换指定位置的节点，并返回旧节点的item*/
public E set(int index, E element) {
checkElementIndex(index);
Node<E> x = node(index);
E oldVal = x.item;
x.item = element;
return oldVal;
} /**在指定位置添加节点*/
public void add(int index, E element) {
//校验位置是否合法
checkPositionIndex(index);
//index==size的话，表明指定的位置是在列表的最后
if (index == size)
linkLast(element);
else
linkBefore(element, node(index));
} /**删除指定位置的节点*/
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
} /**
* 判断指定位置是否存在一个节点（因为size表明链表的大小，只有index在[0,size]之间，表明指定index存在一个节点）.
*/
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
} /**判断指定位置是否合法（加入要新增节点，节点的位置只能在0-size之间，首尾位置也是可插入位置，合法）
*/
private boolean isPositionIndex(int index) {
return index >= 0 && index <= size;
} /**
* Constructs an IndexOutOfBoundsException detail message.
* Of the many possible refactorings of the error handling code,
* this "outlining" performs best with both server and client VMs.
*/
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
} 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));
} /**
* 返回指定位置的节点
*/
Node<E> node(int index) {
// assert isElementIndex(index);
//使用二分模式 （减少内存耗费）
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;
}
} // Search Operations /**
* 返回指定对象在链表中第一次出现的位置，没有则返回-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;
} /**
* 返回一个对象在链表中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;
} // Queue operations. /**
* 返回一个链表的第一个节点
* @since 1.5
*/
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
} /**
* 获取第一个节点 如果链表为空则会爆出异常*/
public E element() {
return getFirst();
} /**
* 返回第一个节点，并且将此在链表中删除*/
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
} /**
* 删除第一个节点*/
public E remove() {
return removeFirst();
} /**
* 在链表末尾添加一个节点*/
public boolean offer(E e) {
return add(e);
} // Deque operations
/**
* 在链表开头添加一个节点*/
public boolean offerFirst(E e) {
addFirst(e);
return true;
} /**
* Inserts the specified element at the end of this list.
*
* @param e the element to insert
* @return {@code true} (as specified by {@link Deque#offerLast})
* @since 1.6
*/
public boolean offerLast(E e) {
addLast(e);
return true;
} /**
* 返回第一个节点的item*/
public E peekFirst() {
final Node<E> f = first;
return (f == null) ? null : f.item;
} /**
* 返回最有一个节点的item*/
public E peekLast() {
final Node<E> l = last;
return (l == null) ? null : l.item;
} /**
* 删除第一个节点，并返会删除的节点*/
public E pollFirst() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
} /**
* 删除最有一个节点，并返回节点*/
public E pollLast() {
final Node<E> l = last;
return (l == null) ? null : unlinkLast(l);
} public void push(E e) {
addFirst(e);
} public E pop() {
return removeFirst();
} /**
* 删除链表中的第一个o*/
public boolean removeFirstOccurrence(Object o) {
return remove(o);
} /**
* 删除链表中最后一个*/
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;
} /**
* 返回指定位置之后的一个迭代器*/
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);
}
} /**
* Returns a shallow copy of this {@code LinkedList}. (The elements
* themselves are not cloned.)
*
* @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;
} /**
* Returns an array containing all of the elements in this list
* in proper sequence (from first to last element).
*
* <p>The returned array will be "safe" in that no references to it are
* maintained by this list. (In other words, this method must allocate
* a new array). The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this list
* in proper sequence
*/
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;
} /**
* Returns an array containing all of the elements in this list in
* proper sequence (from first to last element); the runtime type of
* the returned array is that of the specified array. If the list fits
* in the specified array, it is returned therein. Otherwise, a new
* array is allocated with the runtime type of the specified array and
* the size of this list.
*
* <p>If the list fits in the specified array with room to spare (i.e.,
* the array has more elements than the list), the element in the array
* immediately following the end of the list is set to {@code null}.
* (This is useful in determining the length of the list <i>only</i> if
* the caller knows that the list does not contain any null elements.)
*
* <p>Like the {@link #toArray()} method, this method acts as bridge between
* array-based and collection-based APIs. Further, this method allows
* precise control over the runtime type of the output array, and may,
* under certain circumstances, be used to save allocation costs.
*
* <p>Suppose {@code x} is a list known to contain only strings.
* The following code can be used to dump the list into a newly
* allocated array of {@code String}:
*
* <pre>
* String[] y = x.toArray(new String[0]);</pre>
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @param a the array into which the elements of the list are to
* be stored, if it is big enough; otherwise, a new array of the
* same runtime type is allocated for this purpose.
* @return an array containing the elements of the list
* @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;
} private static final long serialVersionUID = 876323262645176354L; /**
* 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());
} /**
* 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;
}
} }
遍历：
链表的遍历过程也很简单，和查找过程类似，我们从头节点往后遍历就行了。但对于 LinkedList 的遍历还是需要注意一些，不然可能会导致代码效率低下。通常情况下，我们会使用 foreach 遍历 LinkedList，而 foreach 最终转换成迭代器形式。所以分析 LinkedList 的遍历的核心就是它的迭代器实现
我们都知道 LinkedList 不擅长随机位置访问，如果大家用随机访问的方式遍历 LinkedList，效率会很差。比如下面的代码：
List<Integet> list = new LinkedList<>();
list.add(1)
list.add(2)
......
for (int i = 0; i < list.size(); i++) {
Integet item = list.get(i); //这种遍历方式 每次获取指定索引的元素 都会从开头重新寻找到指定位置，验证影响效率
// do something
}