JDK7集合框架源码阅读(二) LinkedList

时间:2021-10-26 17:58:26

基于版本jdk1.7.0_80

java.util.LinkedList

 

代码如下

JDK7集合框架源码阅读(二) LinkedListJDK7集合框架源码阅读(二) LinkedList
/*
* Copyright (c) 1997, 2011, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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*/

package java.util;

/**
* Doubly-linked list implementation of the {
@code List} and {@code Deque}
* interfaces. Implements all optional list operations, and permits all
* elements (including {
@code null}).
*
* <p>All of the operations perform as could be expected for a doubly-linked
* list. Operations that index into the list will traverse the list from
* the beginning or the end, whichever is closer to the specified index.
*
* <p><strong>Note that this implementation is not synchronized.</strong>
* If multiple threads access a linked list concurrently, and at least
* one of the threads modifies the list structurally, it <i>must</i> be
* synchronized externally. (A structural modification is any operation
* that adds or deletes one or more elements; merely setting the value of
* an element is not a structural modification.) This is typically
* accomplished by synchronizing on some object that naturally
* encapsulates the list.
*
* If no such object exists, the list should be "wrapped" using the
* {
@link Collections#synchronizedList Collections.synchronizedList}
* method. This is best done at creation time, to prevent accidental
* unsynchronized access to the list:<pre>
* List list = Collections.synchronizedList(new LinkedList(...));</pre>
*
* <p>The iterators returned by this class's {
@code iterator} and
* {
@code listIterator} methods are <i>fail-fast</i>: if the list is
* structurally modified at any time after the iterator is created, in
* any way except through the Iterator's own {
@code remove} or
* {
@code add} methods, the iterator will throw a {@link
* ConcurrentModificationException}. Thus, in the face of concurrent
* modification, the iterator fails quickly and cleanly, rather than
* risking arbitrary, non-deterministic behavior at an undetermined
* time in the future.
*
* <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification. Fail-fast iterators
* throw {
@code ConcurrentModificationException} on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: <i>the fail-fast behavior of iterators
* should be used only to detect bugs.</i>
*
* <p>This class is a member of the
* <a href="{@
docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
*
@author Josh Bloch
*
@see List
*
@see ArrayList
*
@since 1.2
*
@param <E> the type of elements held in this collection
*/

public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
transient int size = 0;

/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null && first.item != null)
*/
transient Node<E> first;

/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null && last.item != null)
*/
transient Node<E> last;

/**
* Constructs an empty list.
*/
public LinkedList() {
}

/**
* Constructs a list containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
*
@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);
}

/**
* Links e as first element.
*/
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
++;
}

/**
* Links e as last element.
*/
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
++;
}

/**
* Inserts element e before non-null Node succ.
*/
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
++;
}

/**
* Unlinks non-null first node f.
*/
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;
}

/**
* Unlinks non-null last node l.
*/
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;
}

/**
* Unlinks non-null node 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;
}

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

/**
* Returns the last element in this list.
*
*
@return the last element in this list
*
@throws NoSuchElementException if this list is empty
*/
public E getLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return l.item;
}

/**
* Removes and returns the first element from this list.
*
*
@return the first element from this list
*
@throws NoSuchElementException if this list is empty
*/
public E removeFirst() {
final Node<E> f = first;
if (f == null)
throw new NoSuchElementException();
return unlinkFirst(f);
}

/**
* Removes and returns the last element from this list.
*
*
@return the last element from this list
*
@throws NoSuchElementException if this list is empty
*/
public E removeLast() {
final Node<E> l = last;
if (l == null)
throw new NoSuchElementException();
return unlinkLast(l);
}

/**
* Inserts the specified element at the beginning of this list.
*
*
@param e the element to add
*/
public void addFirst(E e) {
linkFirst(e);
}

/**
* Appends the specified element to the end of this list.
*
* <p>This method is equivalent to {
@link #add}.
*
*
@param e the element to add
*/
public void addLast(E e) {
linkLast(e);
}

/**
* Returns {
@code true} if this list contains the specified element.
* More formally, returns {
@code true} if and only if this list contains
* at least one element {
@code e} such that
* <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
*
*
@param o element whose presence in this list is to be tested
*
@return {@code true} if this list contains the specified element
*/
public boolean contains(Object o) {
return indexOf(o) != -1;
}

/**
* Returns the number of elements in this list.
*
*
@return the number of elements in this list
*/
public int size() {
return size;
}

/**
* Appends the specified element to the end of this list.
*
* <p>This method is equivalent to {
@link #addLast}.
*
*
@param e element to be appended to this list
*
@return {@code true} (as specified by {@link Collection#add})
*/
public boolean add(E e) {
linkLast(e);
return true;
}

/**
* Removes the first occurrence of the specified element from this list,
* if it is present. If this list does not contain the element, it is
* unchanged. More formally, removes the element with the lowest index
* {
@code i} such that
* <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
* (if such an element exists). Returns {
@code true} if this list
* contained the specified element (or equivalently, if this list
* changed as a result of the call).
*
*
@param o element to be removed from this list, if present
*
@return {@code true} if this list contained the specified element
*/
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;
}

/**
* Appends all of the elements in the specified collection to the end of
* this list, in the order that they are returned by the specified
* collection's iterator. The behavior of this operation is undefined if
* the specified collection is modified while the operation is in
* progress. (Note that this will occur if the specified collection is
* this list, and it's nonempty.)
*
*
@param c collection containing elements to be added to this list
*
@return {@code true} if this list changed as a result of the call
*
@throws NullPointerException if the specified collection is null
*/
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}

/**
* Inserts all of the elements in the specified collection into this
* list, starting at the specified position. Shifts the element
* currently at that position (if any) and any subsequent elements to
* the right (increases their indices). The new elements will appear
* in the list in the order that they are returned by the
* specified collection's iterator.
*
*
@param index index at which to insert the first element
* from the specified collection
*
@param c collection containing elements to be added to this list
*
@return {@code true} if this list changed as a result of the call
*
@throws IndexOutOfBoundsException {@inheritDoc}
*
@throws NullPointerException if the specified collection is null
*/
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;
}

/**
* Removes all of the elements from this list.
* The list will be empty after this call returns.
*/
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
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

/**
* Returns the element at the specified position in this list.
*
*
@param index index of the element to return
*
@return the element at the specified position in this list
*
@throws IndexOutOfBoundsException {@inheritDoc}
*/
public E get(int index) {
checkElementIndex(index);
return node(index).item;
}

/**
* Replaces the element at the specified position in this list with the
* specified element.
*
*
@param index index of the element to replace
*
@param element element to be stored at the specified position
*
@return the element previously at the specified position
*
@throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
checkElementIndex(index);
Node
<E> x = node(index);
E oldVal
= x.item;
x.item
= element;
return oldVal;
}

/**
* Inserts the specified element at the specified position in this list.
* Shifts the element currently at that position (if any) and any
* subsequent elements to the right (adds one to their indices).
*
*
@param index index at which the specified element is to be inserted
*
@param element element to be inserted
*
@throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
checkPositionIndex(index);

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

/**
* Removes the element at the specified position in this list. Shifts any
* subsequent elements to the left (subtracts one from their indices).
* Returns the element that was removed from the list.
*
*
@param index the index of the element to be removed
*
@return the element previously at the specified position
*
@throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
checkElementIndex(index);
return unlink(node(index));
}

/**
* Tells if the argument is the index of an existing element.
*/
private boolean isElementIndex(int index) {
return index >= 0 && index < size;
}

/**
* Tells if the argument is the index of a valid position for an
* iterator or an add operation.
*/
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));
}

/**
* Returns the (non-null) Node at the specified element index.
*/
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;
}
}

// Search Operations

/**
* Returns the index of the first occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the lowest index {
@code i} such that
* <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*
*
@param o element to search for
*
@return the index of the first occurrence of the specified element in
* this list, or -1 if this list does not contain the element
*/
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;
}

/**
* Returns the index of the last occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the highest index {
@code i} such that
* <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*
*
@param o element to search for
*
@return the index of the last occurrence of the specified element in
* this list, or -1 if this list does not contain the element
*/
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.

/**
* Retrieves, but does not remove, the head (first element) of this list.
*
*
@return the head of this list, or {@code null} if this list is empty
*
@since 1.5
*/
public E peek() {
final Node<E> f = first;
return (f == null) ? null : f.item;
}

/**
* Retrieves, but does not remove, the head (first element) of this list.
*
*
@return the head of this list
*
@throws NoSuchElementException if this list is empty
*
@since 1.5
*/
public E element() {
return getFirst();
}

/**
* Retrieves and removes the head (first element) of this list.
*
*
@return the head of this list, or {@code null} if this list is empty
*
@since 1.5
*/
public E poll() {
final Node<E> f = first;
return (f == null) ? null : unlinkFirst(f);
}

/**
* Retrieves and removes the head (first element) of this list.
*
*
@return the head of this list
*
@throws NoSuchElementException if this list is empty
*
@since 1.5
*/
public E remove() {
return removeFirst();
}

/**
* Adds the specified element as the tail (last element) of this list.
*
*
@param e the element to add
*
@return {@code true} (as specified by {@link Queue#offer})
*
@since 1.5
*/
public boolean offer(E e) {
return add(e);
}

// Deque operations
/**
* Inserts the specified element at the front of this list.
*
*
@param e the element to insert
*
@return {@code true} (as specified by {@link Deque#offerFirst})
*
@since 1.6
*/
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;
}

/**
* Retrieves, but does not remove, the first element of this list,
* or returns {
@code null} if this list is empty.
*
*
@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;
}

/**
* Retrieves, but does not remove, the last element of this list,
* or returns {
@code null} if this list is empty.
*
*
@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;
}

/**
* Retrieves and removes the first element of this list,
* or returns {
@code null} if this list is empty.
*
*
@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);
}

/**
* Retrieves and removes the last element of this list,
* or returns {
@code null} if this list is empty.
*
*
@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);
}

/**
* Pushes an element onto the stack represented by this list. In other
* words, inserts the element at the front of this list.
*
* <p>This method is equivalent to {
@link #addFirst}.
*
*
@param e the element to push
*
@since 1.6
*/
public void push(E e) {
addFirst(e);
}

/**
* Pops an element from the stack represented by this list. In other
* words, removes and returns the first element of this list.
*
* <p>This method is equivalent to {
@link #removeFirst()}.
*
*
@return the element at the front of this list (which is the top
* of the stack represented by this list)
*
@throws NoSuchElementException if this list is empty
*
@since 1.6
*/
public E pop() {
return removeFirst();
}

/**
* Removes the first occurrence of the specified element in this
* list (when traversing the list from head to tail). If the list
* does not contain the element, it is unchanged.
*
*
@param o element to be removed from this list, if present
*
@return {@code true} if the list contained the specified element
*
@since 1.6
*/
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}

/**
* Removes the last occurrence of the specified element in this
* list (when traversing the list from head to tail). If the list
* does not contain the element, it is unchanged.
*
*
@param o element to be removed from this list, if present
*
@return {@code true} if the list contained the specified element
*
@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;
}

/**
* Returns a list-iterator of the elements in this list (in proper
* sequence), starting at the specified position in the list.
* Obeys the general contract of {
@code List.listIterator(int)}.<p>
*
* The list-iterator is <i>fail-fast</i>: if the list is structurally
* modified at any time after the Iterator is created, in any way except
* through the list-iterator's own {
@code remove} or {@code add}
* methods, the list-iterator will throw a
* {
@code ConcurrentModificationException}. Thus, in the face of
* concurrent modification, the iterator fails quickly and cleanly, rather
* than risking arbitrary, non-deterministic behavior at an undetermined
* time in the future.
*
*
@param index index of the first element to be returned from the
* list-iterator (by a call to {
@code next})
*
@return a ListIterator of the elements in this list (in proper
* sequence), starting at the specified position in the list
*
@throws IndexOutOfBoundsException {@inheritDoc}
*
@see List#listIterator(int)
*/
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
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() {
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
++;
}

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();
}
}

/**
* 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());
}
}
View Code

 

代码量还是一千行出头,读起来并不费力,还是只简单分析一下要点

 

1. 接口分析

LinkedList继承于AbstractSequentialList抽象类(说明这是一个顺序访问的数据结构,与ArrayList继承的RandomAccess接口不同)

List,Deque(LinkedList原理跟Deque完全相同,所以顺手支持了),Cloneable,java.io.Serializable接口

 

2. 实现原理

其实现原理是标准的双向链表,节点定义如下

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;
}
}

向前向后指针+元素指针,无需过多解释

 

3. ConcurrentModificationException

与ArrayList相同,LinkedList也是线程不安全的,但是LinkedList的listIterator方法上有一段很好的注释,清晰的说明了它的设计思路,我且引用一下

* The list-iterator is <i>fail-fast</i>: if the list is structurally
* modified at any time after the Iterator is created, in any way except
* through the list-iterator's own {@code remove} or {@code add}
* methods, the list-iterator will throw a
* {@code ConcurrentModificationException}. Thus, in the face of
* concurrent modification, the iterator fails quickly and cleanly, rather
* than risking arbitrary, non-deterministic behavior at an undetermined
* time in the future.

大意是说LinkedList的迭代器是快速失败(fail-fast)的,只要迭代器在被创建之后所属的list受到了改动,无论这个改动是什么原因,迭代器都会抛出ConcurrentModificationException异常,逼迫程序员进行处理,避免在未来发生一些难以追查的异常行为。

 

4. LinkedList的遍历问题

ArrayList可以在循环里通过下标遍历,但是这个操作不能移植到LinkedList里来

因为LinkedList的get(int index)方法会调用node(int index)方法,而这个方法会试图从头部或者尾部遍历链表来定位元素,时间复杂度高。源码如下

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;
}
}

所以最好还是用迭代器来遍历数组元素