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ArrayDeque和ArrayList有区别也有相似的地方，他们都是数组存储数组，不同的是ArrayList存储是从数组位置的0开始的，而ArrayDeque有一个head和tail，head和tail可以指向数组的任何位置，head是指向数组的第一个元素，是有值的，tail是指向数组最后一个元素的下一个，是空的，如果tail大于head，那么数组大小就是tail-head，如果tail是小于head，那么数组大小为（head到数组长度+位置0到tail），代码不多，直接上代码
/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
* Written by Josh Bloch of Google Inc. and released to the public domain,
* as explained at http://creativecommons.org/publicdomain/zero/1.0/.
*/
package java.util;
import java.io.Serializable;
import java.util.function.Consumer;
// BEGIN android-note
// removed link to collections framework docs
// END android-note
/**
* Resizable-array implementation of the {@link Deque} interface. Array
* deques have no capacity restrictions; they grow as necessary to support
* usage. They are not thread-safe; in the absence of external
* synchronization, they do not support concurrent access by multiple threads.
* Null elements are prohibited. This class is likely to be faster than
* {@link Stack} when used as a stack, and faster than {@link LinkedList}
* when used as a queue.
*
* <p>Most {@code ArrayDeque} operations run in amortized constant time.
* Exceptions include
* {@link #remove(Object) remove},
* {@link #removeFirstOccurrence removeFirstOccurrence},
* {@link #removeLastOccurrence removeLastOccurrence},
* {@link #contains contains},
* {@link #iterator iterator.remove()},
* and the bulk operations, all of which run in linear time.
*
* <p>The iterators returned by this class's {@link #iterator() iterator}
* method are <em>fail-fast</em>: If the deque is modified at any time after
* the iterator is created, in any way except through the iterator's own
* {@code remove} method, the iterator will generally 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 and its iterator implement all of the
* <em>optional</em> methods of the {@link Collection} and {@link
* Iterator} interfaces.
*
* @author Josh Bloch and Doug Lea
* @since 1.6
* @param <E> the type of elements held in this deque
*/
public class ArrayDeque<E> extends AbstractCollection<E>
implements Deque<E>, Cloneable, Serializable
{
/**
* The array in which the elements of the deque are stored.
* The capacity of the deque is the length of this array, which is
* always a power of two. The array is never allowed to become
* full, except transiently within an addX method where it is
* resized (see doubleCapacity) immediately upon becoming full,
* thus avoiding head and tail wrapping around to equal each
* other. We also guarantee that all array cells not holding
* deque elements are always null.
*/
//存储的元素，并且elements的长度必须是2的n次方，这个和之前看到HashMap很相似，
//之前看的HashMap的数组长度也是2的n次方，因为要用到与运算，详细可以看
//http://blog.csdn.net/abcdef314159/article/details/51165630
transient Object[] elements; // non-private to simplify nested class access
/**
* The index of the element at the head of the deque (which is the
* element that would be removed by remove() or pop()); or an
* arbitrary number equal to tail if the deque is empty.
*/
//指向队列头，这个头并不是数组的第0个元素，如果这样head就没有意义了，这个从
//下面的addFirst(E e)方法也可以看出，如果当head等于0的时候，在添加到first，那么
//会添加到数组的最后，并且head也指向数组的最后，这个下面在分析
transient int head;
/**
* The index at which the next element would be added to the tail
* of the deque (via addLast(E), add(E), or push(E)).
*/
//指向队列尾的下一个空间，可以这样理解，head指向的是第一个元素，tail指向的是最后
//一个元素的下一个，指的是空的。
transient int tail;
/**
* The minimum capacity that we'll use for a newly created deque.
* Must be a power of 2.
*/
//最小存储空间，必须是二的n次方，
private static final int MIN_INITIAL_CAPACITY = 8;
// ****** Array allocation and resizing utilities ******
/**
* Allocates empty array to hold the given number of elements.
*
* @param numElements the number of elements to hold
*/
//分配空间，这个空间大小是比numElements大的最小的2的n次方，比如numElements
//是17，则初始化大小为32，因为2的4次是16小于17,2的5次是32，才大于17，如果传入
//的正好是2的n次方，那么初始化的空间大小的2的n+1次方，
private void allocateElements(int numElements) {
//根据numElements初始化空间
int initialCapacity = MIN_INITIAL_CAPACITY;
// Find the best power of two to hold elements.
// Tests "<=" because arrays aren't kept full.
//如果numElements小于8，直接初始化数组elements，否则，根据numElements
//的大小来计算数组的大小
if (numElements >= initialCapacity) {
initialCapacity = numElements;
//下面代码很简单，其实就是移位把最左边位的1填充到后面，计算的时候你完全可以把
//numElements第一个1后面的所有1都忽略，我举个例子，比如17，二进制是（总共32位，
//前面全是0）10001，就可以转化为（总共32位，前面全是0）11111，就相当于把最左边
//的1全部填充到后面，
initialCapacity |= (initialCapacity >>> 1);
initialCapacity |= (initialCapacity >>> 2);
initialCapacity |= (initialCapacity >>> 4);
initialCapacity |= (initialCapacity >>> 8);
initialCapacity |= (initialCapacity >>> 16);
initialCapacity++;//加1，达到2的n次方
// 越界了，太大了，
if (initialCapacity < 0) // Too many elements, must back off
initialCapacity >>>= 1; // Good luck allocating 2^30 elements
}
elements = new Object[initialCapacity];
}
/**
* Doubles the capacity of this deque. Call only when full, i.e.,
* when head and tail have wrapped around to become equal.
*/
//空间扩大一倍，仅仅当满的时候，这时候head和tail都会指向同一个元素
private void doubleCapacity() {
assert head == tail;//满了
int p = head;
int n = elements.length;//数组的长度
//关键是r不好理解，举个例子，在数组中，head不一定是之前0位置的，他可以指向其他位置，比如
//原来空间大小为16，head为13，也就是第14个元素（一数组是从0开始的），那么r就是16-13=3，也
//就是从head往后还有多少元素，待会copy的时候也是先从最后的r个元素开始
int r = n - p; // number of elements to the right of p
int newCapacity = n << 1;//扩大一倍
if (newCapacity < 0)
throw new IllegalStateException("Sorry, deque too big");
Object[] a = new Object[newCapacity];
System.arraycopy(elements, p, a, 0, r);//先copy后面的r个
System.arraycopy(elements, 0, a, r, p);//在copy前面的p个
elements = a;
//重新调整head和tail的值
head = 0;
tail = n;
}
/**
* Constructs an empty array deque with an initial capacity
* sufficient to hold 16 elements.
*/
public ArrayDeque() {
elements = new Object[16];//初始化大小，默认的为16
}
/**
* Constructs an empty array deque with an initial capacity
* sufficient to hold the specified number of elements.
*
* @param numElements lower bound on initial capacity of the deque
*/
public ArrayDeque(int numElements) {
allocateElements(numElements);// 分配空间
}
/**
* Constructs a deque containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator. (The first element returned by the collection's
* iterator becomes the first element, or <i>front</i> of the
* deque.)
*
* @param c the collection whose elements are to be placed into the deque
* @throws NullPointerException if the specified collection is null
*/
public ArrayDeque(Collection<? extends E> c) {
allocateElements(c.size());// 分配空间，然后添加
addAll(c);
}
// The main insertion and extraction methods are addFirst,
// addLast, pollFirst, pollLast. The other methods are defined in
// terms of these.
/**
* Inserts the specified element at the front of this deque.
*
* @param e the element to add
* @throws NullPointerException if the specified element is null
*/
public void addFirst(E e) {
if (e == null)
throw new NullPointerException();
//添加到head的前面，所以head-1，后面的与运算，不会出现数组越界，因为
//elements.length是2的n次方，这个可以参照之前的HashMap的分析
elements[head = (head - 1) & (elements.length - 1)] = e;
if (head == tail)//判断是否满了
doubleCapacity();
}
/**
* Inserts the specified element at the end of this deque.
*
* <p>This method is equivalent to {@link #add}.
*
* @param e the element to add
* @throws NullPointerException if the specified element is null
*/
public void addLast(E e) {
if (e == null)
throw new NullPointerException();
//添加到最后一个，这个方法和addFirst有很明显的不同，addFirst是添加的时候就要
//计算head的位置，因为原head位置是有值的，而addLast方法是存值之后在计算tail的，
//因为tail位置是没有存值的，他表示的是最右一个元素的下一个，是空，所以存值之后
//要计算tail的值
elements[tail] = e;
if ( (tail = (tail + 1) & (elements.length - 1)) == head)//判断是否满
doubleCapacity();
}
/**
* Inserts the specified element at the front of this deque.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Deque#offerFirst})
* @throws NullPointerException if the specified element is null
*/
public boolean offerFirst(E e) {//插入到第一个
addFirst(e);
return true;
}
/**
* Inserts the specified element at the end of this deque.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Deque#offerLast})
* @throws NullPointerException if the specified element is null
*/
public boolean offerLast(E e) {//插入到最后一个
addLast(e);
return true;
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeFirst() {//删除第一个
E x = pollFirst();
if (x == null)
throw new NoSuchElementException();
return x;
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeLast() {//删除最后一个
E x = pollLast();
if (x == null)
throw new NoSuchElementException();
return x;
}
public E pollFirst() {
final Object[] elements = this.elements;
final int h = head;
@SuppressWarnings("unchecked")
E result = (E) elements[h];
// Element is null if deque empty
if (result != null) {
elements[h] = null; // Must null out slot
// 删除第一个，这里的所有第一我们都认为是head所指的，不是数组的0位置，然后在
//计算head的值
head = (h + 1) & (elements.length - 1);
}
return result;
}
public E pollLast() {//删除最后一个
final Object[] elements = this.elements;
final int t = (tail - 1) & (elements.length - 1);
@SuppressWarnings("unchecked")
E result = (E) elements[t];
if (result != null) {
elements[t] = null;
tail = t;
}
return result;
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getFirst() {//返回第一个
@SuppressWarnings("unchecked")
E result = (E) elements[head];
if (result == null)
throw new NoSuchElementException();
return result;
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getLast() {// 返回最后一个
@SuppressWarnings("unchecked")
E result = (E) elements[(tail - 1) & (elements.length - 1)];
if (result == null)
throw new NoSuchElementException();
return result;
}
@SuppressWarnings("unchecked")
public E peekFirst() {
// elements[head] is null if deque empty
return (E) elements[head];
}
@SuppressWarnings("unchecked")
public E peekLast() {
return (E) elements[(tail - 1) & (elements.length - 1)];
}
/**
* Removes the first occurrence of the specified element in this
* deque (when traversing the deque from head to tail).
* If the deque does not contain the element, it is unchanged.
* More formally, removes the first element {@code e} such that
* {@code o.equals(e)} (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* @param o element to be removed from this deque, if present
* @return {@code true} if the deque contained the specified element
*/
public boolean removeFirstOccurrence(Object o) {//删除第一个出现的o
if (o != null) {
int mask = elements.length - 1;
int i = head;
for (Object x; (x = elements[i]) != null; i = (i + 1) & mask) {
if (o.equals(x)) {
delete(i);
return true;
}
}
}
return false;
}
/**
* Removes the last occurrence of the specified element in this
* deque (when traversing the deque from head to tail).
* If the deque does not contain the element, it is unchanged.
* More formally, removes the last element {@code e} such that
* {@code o.equals(e)} (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* @param o element to be removed from this deque, if present
* @return {@code true} if the deque contained the specified element
*/
public boolean removeLastOccurrence(Object o) {//删除最后一个出现的o
if (o != null) {
int mask = elements.length - 1;
int i = (tail - 1) & mask;
for (Object x; (x = elements[i]) != null; i = (i - 1) & mask) {
if (o.equals(x)) {
delete(i);
return true;
}
}
}
return false;
}
// *** Queue methods ***
/**
* Inserts the specified element at the end of this deque.
*
* <p>This method is equivalent to {@link #addLast}.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Collection#add})
* @throws NullPointerException if the specified element is null
*/
public boolean add(E e) {//添加
addLast(e);
return true;
}
/**
* Inserts the specified element at the end of this deque.
*
* <p>This method is equivalent to {@link #offerLast}.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Queue#offer})
* @throws NullPointerException if the specified element is null
*/
public boolean offer(E e) {//插入到最后
return offerLast(e);
}
/**
* Retrieves and removes the head of the queue represented by this deque.
*
* This method differs from {@link #poll poll} only in that it throws an
* exception if this deque is empty.
*
* <p>This method is equivalent to {@link #removeFirst}.
*
* @return the head of the queue represented by this deque
* @throws NoSuchElementException {@inheritDoc}
*/
public E remove() {
return removeFirst();
}
/**
* Retrieves and removes the head of the queue represented by this deque
* (in other words, the first element of this deque), or returns
* {@code null} if this deque is empty.
*
* <p>This method is equivalent to {@link #pollFirst}.
*
* @return the head of the queue represented by this deque, or
* {@code null} if this deque is empty
*/
public E poll() {
return pollFirst();
}
/**
* Retrieves, but does not remove, the head of the queue represented by
* this deque. This method differs from {@link #peek peek} only in
* that it throws an exception if this deque is empty.
*
* <p>This method is equivalent to {@link #getFirst}.
*
* @return the head of the queue represented by this deque
* @throws NoSuchElementException {@inheritDoc}
*/
public E element() {
return getFirst();
}
/**
* Retrieves, but does not remove, the head of the queue represented by
* this deque, or returns {@code null} if this deque is empty.
*
* <p>This method is equivalent to {@link #peekFirst}.
*
* @return the head of the queue represented by this deque, or
* {@code null} if this deque is empty
*/
public E peek() {
return peekFirst();
}
// *** Stack methods ***
/**
* Pushes an element onto the stack represented by this deque. In other
* words, inserts the element at the front of this deque.
*
* <p>This method is equivalent to {@link #addFirst}.
*
* @param e the element to push
* @throws NullPointerException if the specified element is null
*/
public void push(E e) {
addFirst(e);
}
/**
* Pops an element from the stack represented by this deque. In other
* words, removes and returns the first element of this deque.
*
* <p>This method is equivalent to {@link #removeFirst()}.
*
* @return the element at the front of this deque (which is the top
* of the stack represented by this deque)
* @throws NoSuchElementException {@inheritDoc}
*/
public E pop() {
return removeFirst();
}
private void checkInvariants() {
assert elements[tail] == null;
assert head == tail ? elements[head] == null :
(elements[head] != null &&
elements[(tail - 1) & (elements.length - 1)] != null);
assert elements[(head - 1) & (elements.length - 1)] == null;
}
/**
* Removes the element at the specified position in the elements array,
* adjusting head and tail as necessary. This can result in motion of
* elements backwards or forwards in the array.
*
* <p>This method is called delete rather than remove to emphasize
* that its semantics differ from those of {@link List#remove(int)}.
*
* @return true if elements moved backwards
*/
//删除数组中下标为i的元素，之前所有的操作都是操作head和tail的，但这个不一样
//他会引起head或tail的改变，
boolean delete(int i) {
checkInvariants();
final Object[] elements = this.elements;
final int mask = elements.length - 1;
final int h = head;
final int t = tail;
final int front = (i - h) & mask;//i距离head的位置
final int back = (t - i) & mask;//i距离tail的位置
// Invariant: head <= i < tail mod circularity
if (front >= ((t - h) & mask))//保证i的位置元素是存在的
throw new ConcurrentModificationException();
// Optimize for least element motion
if (front < back) {//前面的少，从前往后挪，最后head要加1
if (h <= i) {
System.arraycopy(elements, h, elements, h + 1, front);
} else { // Wrap around
System.arraycopy(elements, 0, elements, 1, i);
elements[0] = elements[mask];
System.arraycopy(elements, h, elements, h + 1, mask - h);
}
elements[h] = null;
head = (h + 1) & mask;
return false;
} else {//后面少，从后往前挪，后面的tail要减1
if (i < t) { // Copy the null tail as well
System.arraycopy(elements, i + 1, elements, i, back);
tail = t - 1;
} else { // Wrap around
System.arraycopy(elements, i + 1, elements, i, mask - i);
elements[mask] = elements[0];
System.arraycopy(elements, 1, elements, 0, t);
tail = (t - 1) & mask;
}
return true;
}
}
// *** Collection Methods ***
/**
* Returns the number of elements in this deque.
*
* @return the number of elements in this deque
*/
public int size() {//元素的size
return (tail - head) & (elements.length - 1);
}
/**
* Returns {@code true} if this deque contains no elements.
*
* @return {@code true} if this deque contains no elements
*/
//是否为空，在上面添加元素的时候也可能head==tail，当添加元素之后head==tail的时候
//就认为是满了，然后扩容，重新调整head和tail的值
public boolean isEmpty() {
return head == tail;
}
/**
* Returns an iterator over the elements in this deque. The elements
* will be ordered from first (head) to last (tail). This is the same
* order that elements would be dequeued (via successive calls to
* {@link #remove} or popped (via successive calls to {@link #pop}).
*
* @return an iterator over the elements in this deque
*/
public Iterator<E> iterator() {
return new DeqIterator();
}
public Iterator<E> descendingIterator() {//迭代器
return new DescendingIterator();
}
private class DeqIterator implements Iterator<E> {
/**
* Index of element to be returned by subsequent call to next.
*/
private int cursor = head;
/**
* Tail recorded at construction (also in remove), to stop
* iterator and also to check for comodification.
*/
private int fence = tail;
/**
* Index of element returned by most recent call to next.
* Reset to -1 if element is deleted by a call to remove.
*/
private int lastRet = -1;
public boolean hasNext() {
return cursor != fence;
}
public E next() {
if (cursor == fence)
throw new NoSuchElementException();
@SuppressWarnings("unchecked")
E result = (E) elements[cursor];
// This check doesn't catch all possible comodifications,
// but does catch the ones that corrupt traversal
if (tail != fence || result == null)
throw new ConcurrentModificationException();
lastRet = cursor;
cursor = (cursor + 1) & (elements.length - 1);
return result;
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
if (delete(lastRet)) { // if left-shifted, undo increment in next()
cursor = (cursor - 1) & (elements.length - 1);
fence = tail;
}
lastRet = -1;
}
@Override
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
Object[] a = elements;
int m = a.length - 1, f = fence, i = cursor;
cursor = f;
while (i != f) {
@SuppressWarnings("unchecked") E e = (E)a[i];
i = (i + 1) & m;
// Android-note: This uses a different heuristic for detecting
// concurrent modification exceptions than next(). As such, this is a less
// precise test.
if (e == null)
throw new ConcurrentModificationException();
action.accept(e);
}
}
}
/**
* This class is nearly a mirror-image of DeqIterator, using tail
* instead of head for initial cursor, and head instead of tail
* for fence.
*/
private class DescendingIterator implements Iterator<E> {
private int cursor = tail;
private int fence = head;
private int lastRet = -1;
public boolean hasNext() {
return cursor != fence;
}
public E next() {
if (cursor == fence)
throw new NoSuchElementException();
cursor = (cursor - 1) & (elements.length - 1);
@SuppressWarnings("unchecked")
E result = (E) elements[cursor];
if (head != fence || result == null)
throw new ConcurrentModificationException();
lastRet = cursor;
return result;
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
if (!delete(lastRet)) {
cursor = (cursor + 1) & (elements.length - 1);
fence = head;
}
lastRet = -1;
}
}
/**
* Returns {@code true} if this deque contains the specified element.
* More formally, returns {@code true} if and only if this deque contains
* at least one element {@code e} such that {@code o.equals(e)}.
*
* @param o object to be checked for containment in this deque
* @return {@code true} if this deque contains the specified element
*/
public boolean contains(Object o) {
if (o != null) {
int mask = elements.length - 1;
int i = head;
for (Object x; (x = elements[i]) != null; i = (i + 1) & mask) {
if (o.equals(x))
return true;
}
}
return false;
}
/**
* Removes a single instance of the specified element from this deque.
* If the deque does not contain the element, it is unchanged.
* More formally, removes the first element {@code e} such that
* {@code o.equals(e)} (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
*
* @param o element to be removed from this deque, if present
* @return {@code true} if this deque contained the specified element
*/
public boolean remove(Object o) {
return removeFirstOccurrence(o);
}
/**
* Removes all of the elements from this deque.
* The deque will be empty after this call returns.
*/
public void clear() {// 清空
int h = head;
int t = tail;
if (h != t) { // clear all cells
head = tail = 0;
int i = h;
int mask = elements.length - 1;
do {
elements[i] = null;
i = (i + 1) & mask;
} while (i != t);
}
}
/**
* Returns an array containing all of the elements in this deque
* 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 deque. (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 deque
*/
public Object[] toArray() {//转化为数组
final int head = this.head;
final int tail = this.tail;
boolean wrap = (tail < head);//head有可能小于tail，尤其在addFirst方法中
int end = wrap ? tail + elements.length : tail;
//如果head小于tail，直接copy从head到tail即可，如果head大于tail，再从head到tail
//肯定不合适，tail需要加上数组的大小，但这里始终有一点困惑，tail + elements.length
// 可能会大于数组的长度，有可能出现数组越界异常，但是看了Arrays.copyOfRange的源码之后
//发现我多虑了
/**
public static <T,U> T[] copyOfRange(U[] original, int from, int to, Class<? extends T[]> newType) {
int newLength = to - from;
if (newLength < 0)
throw new IllegalArgumentException(from + " > " + to);
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, from, copy, 0,
Math.min(original.length - from, newLength));//这里是关键，所以不会出现越界异常
return copy;
}
*/
Object[] a = Arrays.copyOfRange(elements, head, end);
//如果wrap为真，那么上面copy的只是从head到数组的最后，那么数组前面到tail的那部分还没copy，所以这里copy
//前面的剩余部分
if (wrap)
System.arraycopy(elements, 0, a, elements.length - head, tail);
return a;
}
/**
* Returns an array containing all of the elements in this deque in
* proper sequence (from first to last element); the runtime type of the
* returned array is that of the specified array. If the deque 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 deque.
*
* <p>If this deque fits in the specified array with room to spare
* (i.e., the array has more elements than this deque), the element in
* the array immediately following the end of the deque is set to
* {@code null}.
*
* <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 deque known to contain only strings.
* The following code can be used to dump the deque into a newly
* allocated array of {@code String}:
*
* <pre> {@code 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 deque 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 all of the elements in this deque
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in
* this deque
* @throws NullPointerException if the specified array is null
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {//把元素提取到数组中然后在返回
final int head = this.head;
final int tail = this.tail;
boolean wrap = (tail < head);//又是head在后面的情况
int size = (tail - head) + (wrap ? elements.length : 0);//大小
//copy数组的偏移量，因为有可能先提取后部分然后在提取前部分
int firstLeg = size - (wrap ? tail : 0);
int len = a.length;
if (size > len) {
a = (T[]) Arrays.copyOfRange(elements, head, head + size,
a.getClass());
} else {
System.arraycopy(elements, head, a, 0, firstLeg);
if (size < len)
a[size] = null;
}
if (wrap)
System.arraycopy(elements, 0, a, firstLeg, tail);
return a;
}
// *** Object methods ***
/**
* Returns a copy of this deque.
*
* @return a copy of this deque
*/
public ArrayDeque<E> clone() {//克隆
try {
@SuppressWarnings("unchecked")
ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
result.elements = Arrays.copyOf(elements, elements.length);
return result;
} catch (CloneNotSupportedException e) {
throw new AssertionError();
}
}
private static final long serialVersionUID = 2340985798034038923L;
/**
* Saves this deque to a stream (that is, serializes it).
*
* @param s the stream
* @throws java.io.IOException if an I/O error occurs
* @serialData The current size ({@code int}) of the deque,
* followed by all of its elements (each an object reference) in
* first-to-last order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {//序列化写
s.defaultWriteObject();
// Write out size
s.writeInt(size());
// Write out elements in order.
int mask = elements.length - 1;
for (int i = head; i != tail; i = (i + 1) & mask)
s.writeObject(elements[i]);
}
/**
* Reconstitutes this deque from a stream (that is, deserializes it).
* @param s the stream
* @throws ClassNotFoundException if the class of a serialized object
* could not be found
* @throws java.io.IOException if an I/O error occurs
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {//序列化读
s.defaultReadObject();
// Read in size and allocate array
int size = s.readInt();
allocateElements(size);
head = 0;
tail = size;
// Read in all elements in the proper order.
for (int i = 0; i < size; i++)
elements[i] = 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
* deque.
*
* <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
* {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
* {@link Spliterator#NONNULL}. Overriding implementations should document
* the reporting of additional characteristic values.
*
* @return a {@code Spliterator} over the elements in this deque
* @since 1.8
*/
public Spliterator<E> spliterator() {
return new DeqSpliterator<E>(this, -1, -1);
}
static final class DeqSpliterator<E> implements Spliterator<E> {
private final ArrayDeque<E> deq;
private int fence; // -1 until first use
private int index; // current index, modified on traverse/split
/** Creates new spliterator covering the given array and range. */
DeqSpliterator(ArrayDeque<E> deq, int origin, int fence) {
this.deq = deq;
this.index = origin;
this.fence = fence;
}
private int getFence() { // force initialization
int t;
if ((t = fence) < 0) {
t = fence = deq.tail;
index = deq.head;
}
return t;
}
public DeqSpliterator<E> trySplit() {
int t = getFence(), h = index, n = deq.elements.length;
if (h != t && ((h + 1) & (n - 1)) != t) {
if (h > t)
t += n;
int m = ((h + t) >>> 1) & (n - 1);
return new DeqSpliterator<E>(deq, h, index = m);
}
return null;
}
public void forEachRemaining(Consumer<? super E> consumer) {
if (consumer == null)
throw new NullPointerException();
Object[] a = deq.elements;
int m = a.length - 1, f = getFence(), i = index;
index = f;
while (i != f) {
@SuppressWarnings("unchecked") E e = (E)a[i];
i = (i + 1) & m;
if (e == null)
throw new ConcurrentModificationException();
consumer.accept(e);
}
}
public boolean tryAdvance(Consumer<? super E> consumer) {
if (consumer == null)
throw new NullPointerException();
Object[] a = deq.elements;
int m = a.length - 1, f = getFence(), i = index;
if (i != f) {
@SuppressWarnings("unchecked") E e = (E)a[i];
index = (i + 1) & m;
if (e == null)
throw new ConcurrentModificationException();
consumer.accept(e);
return true;
}
return false;
}
public long estimateSize() {
int n = getFence() - index;
if (n < 0)
n += deq.elements.length;
return (long) n;
}
@Override
public int characteristics() {
return Spliterator.ORDERED | Spliterator.SIZED |
Spliterator.NONNULL | Spliterator.SUBSIZED;
}
}
}