JDK7集合框架源码阅读（七） ArrayDeque

基于版本jdk1.7.0_80

java.util.ArrayDeque

代码如下

/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
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* 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.*;

/**
* 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 <tt>ArrayDeque</tt> 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 <tt>iterator</tt> method are
* <i>fail-fast</i>: If the deque is modified at any time after the iterator
* is created, in any way except through the iterator's own <tt>remove</tt>
* 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 <tt>ConcurrentModificationException</tt> 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.
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @author  Josh Bloch and Doug Lea
* @since   1.6
* @param <E> the type of elements held in this collection
*/
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.
*/
private transient E[] elements;

/**
* 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.
*/
private 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)).
*/
private transient int tail;

/**
* The minimum capacity that we'll use for a newly created deque.
* Must be a power of 2.
*/
private static final int MIN_INITIAL_CAPACITY = 8;

// ******  Array allocation and resizing utilities ******

/**
* Allocate empty array to hold the given number of elements.
*
* @param numElements  the number of elements to hold
*/
private void allocateElements(int numElements) {
int initialCapacity = MIN_INITIAL_CAPACITY;
// Find the best power of two to hold elements.
// Tests "<=" because arrays aren't kept full.
if (numElements >= initialCapacity) {
initialCapacity = numElements;
initialCapacity |= (initialCapacity >>>  1);
initialCapacity |= (initialCapacity >>>  2);
initialCapacity |= (initialCapacity >>>  4);
initialCapacity |= (initialCapacity >>>  8);
initialCapacity |= (initialCapacity >>> 16);
initialCapacity++;

if (initialCapacity < 0)   // Too many elements, must back off
initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
}
elements = (E[]) new Object[initialCapacity];
}

/**
* Double the capacity of this deque.  Call only when full, i.e.,
* when head and tail have wrapped around to become equal.
*/
private void doubleCapacity() {
assert head == tail;
int p = head;
int n = elements.length;
int r = n - p; // number of elements to the right of p
int newCapacity = n << 1;
if (newCapacity < 0)
throw new IllegalStateException("Sorry, deque too big");
Object[] a = new Object[newCapacity];
System.arraycopy(elements, p, a, 0, r);
System.arraycopy(elements, 0, a, r, p);
elements = (E[])a;
head = 0;
tail = n;
}

/**
* Copies the elements from our element array into the specified array,
* in order (from first to last element in the deque).  It is assumed
* that the array is large enough to hold all elements in the deque.
*
* @return its argument
*/
private <T> T[] copyElements(T[] a) {
if (head < tail) {
System.arraycopy(elements, head, a, 0, size());
} else if (head > tail) {
int headPortionLen = elements.length - head;
System.arraycopy(elements, head, a, 0, headPortionLen);
System.arraycopy(elements, 0, a, headPortionLen, tail);
}
return a;
}

/**
* Constructs an empty array deque with an initial capacity
* sufficient to hold 16 elements.
*/
public ArrayDeque() {
elements = (E[]) new Object[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);
}

/**
* 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());
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();
elements[head = (head - 1) & (elements.length - 1)] = e;
if (head == tail)
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();
elements[tail] = e;
if ( (tail = (tail + 1) & (elements.length - 1)) == head)
doubleCapacity();
}

/**
* Inserts the specified element at the front of this deque.
*
* @param e the element to add
* @return <tt>true</tt> (as specified by {@link Deque#offerFirst})
* @throws NullPointerException if the specified element is null
*/
public boolean offerFirst(E e) {
addFirst(e);
return true;
}

/**
* Inserts the specified element at the end of this deque.
*
* @param e the element to add
* @return <tt>true</tt> (as specified by {@link Deque#offerLast})
* @throws NullPointerException if the specified element is null
*/
public boolean offerLast(E e) {
addLast(e);
return true;
}

/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeFirst() {
E x = pollFirst();
if (x == null)
throw new NoSuchElementException();
return x;
}

/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeLast() {
E x = pollLast();
if (x == null)
throw new NoSuchElementException();
return x;
}

public E pollFirst() {
int h = head;
E result = elements[h]; // Element is null if deque empty
if (result == null)
return null;
elements[h] = null;     // Must null out slot
head = (h + 1) & (elements.length - 1);
return result;
}

public E pollLast() {
int t = (tail - 1) & (elements.length - 1);
E result = elements[t];
if (result == null)
return null;
elements[t] = null;
tail = t;
return result;
}

/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getFirst() {
E x = elements[head];
if (x == null)
throw new NoSuchElementException();
return x;
}

/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getLast() {
E x = elements[(tail - 1) & (elements.length - 1)];
if (x == null)
throw new NoSuchElementException();
return x;
}

public E peekFirst() {
return elements[head]; // elements[head] is null if deque empty
}

public E peekLast() {
return 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 <tt>e</tt> such that
* <tt>o.equals(e)</tt> (if such an element exists).
* Returns <tt>true</tt> 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 <tt>true</tt> if the deque contained the specified element
*/
public boolean removeFirstOccurrence(Object o) {
if (o == null)
return false;
int mask = elements.length - 1;
int i = head;
E x;
while ( (x = elements[i]) != null) {
if (o.equals(x)) {
delete(i);
return true;
}
i = (i + 1) & mask;
}
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 <tt>e</tt> such that
* <tt>o.equals(e)</tt> (if such an element exists).
* Returns <tt>true</tt> 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 <tt>true</tt> if the deque contained the specified element
*/
public boolean removeLastOccurrence(Object o) {
if (o == null)
return false;
int mask = elements.length - 1;
int i = (tail - 1) & mask;
E x;
while ( (x = elements[i]) != null) {
if (o.equals(x)) {
delete(i);
return true;
}
i = (i - 1) & mask;
}
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 <tt>true</tt> (as specified by {@link Collection#add})
* @throws NullPointerException if the specified element is null
*/
public boolean add(E e) {
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 <tt>true</tt> (as specified by {@link Queue#offer})
* @throws NullPointerException if the specified element is null
*/
public boolean offer(E e) {
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
* <tt>null</tt> if this deque is empty.
*
* <p>This method is equivalent to {@link #pollFirst}.
*
* @return the head of the queue represented by this deque, or
*         <tt>null</tt> 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 <tt>null</tt> if this deque is empty.
*
* <p>This method is equivalent to {@link #peekFirst}.
*
* @return the head of the queue represented by this deque, or
*         <tt>null</tt> 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
*/
private boolean delete(int i) {
checkInvariants();
final E[] elements = this.elements;
final int mask = elements.length - 1;
final int h = head;
final int t = tail;
final int front = (i - h) & mask;
final int back  = (t - i) & mask;

// Invariant: head <= i < tail mod circularity
if (front >= ((t - h) & mask))
throw new ConcurrentModificationException();

// Optimize for least element motion
if (front < back) {
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 {
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() {
return (tail - head) & (elements.length - 1);
}

/**
* Returns <tt>true</tt> if this deque contains no elements.
*
* @return <tt>true</tt> if this deque contains no elements
*/
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() {
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();
E result = 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;
}
}

private class DescendingIterator implements Iterator<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 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);
E result = 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 <tt>true</tt> if this deque contains the specified element.
* More formally, returns <tt>true</tt> if and only if this deque contains
* at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
*
* @param o object to be checked for containment in this deque
* @return <tt>true</tt> if this deque contains the specified element
*/
public boolean contains(Object o) {
if (o == null)
return false;
int mask = elements.length - 1;
int i = head;
E x;
while ( (x = elements[i]) != null) {
if (o.equals(x))
return true;
i = (i + 1) & mask;
}
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 <tt>e</tt> such that
* <tt>o.equals(e)</tt> (if such an element exists).
* Returns <tt>true</tt> 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}.
*
* @param o element to be removed from this deque, if present
* @return <tt>true</tt> 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() {
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() {
return copyElements(new Object[size()]);
}

/**
* 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
* <tt>null</tt>.
*
* <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 <tt>x</tt> is a deque known to contain only strings.
* The following code can be used to dump the deque into a newly
* allocated array of <tt>String</tt>:
*
* <pre>
*     String[] y = x.toArray(new String[0]);</pre>
*
* Note that <tt>toArray(new Object[0])</tt> is identical in function to
* <tt>toArray()</tt>.
*
* @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
*/
public <T> T[] toArray(T[] a) {
int size = size();
if (a.length < size)
a = (T[])java.lang.reflect.Array.newInstance(
a.getClass().getComponentType(), size);
copyElements(a);
if (a.length > size)
a[size] = null;
return a;
}

// *** Object methods ***

/**
* Returns a copy of this deque.
*
* @return a copy of this deque
*/
public ArrayDeque<E> clone() {
try {
ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
result.elements = Arrays.copyOf(elements, elements.length);
return result;

} catch (CloneNotSupportedException e) {
throw new AssertionError();
}
}

/**
* Appease the serialization gods.
*/
private static final long serialVersionUID = 2340985798034038923L;

/**
* Serialize this deque.
*
* @serialData The current size (<tt>int</tt>) of the deque,
* followed by all of its elements (each an object reference) in
* first-to-last order.
*/
private void writeObject(ObjectOutputStream s) throws IOException {
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]);
}

/**
* Deserialize this deque.
*/
private void readObject(ObjectInputStream s)
throws IOException, ClassNotFoundException {
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] = (E)s.readObject();
}
}

View Code

1. 接口分析

继承于AbstractCollection

Deque，Cloneable，java.io.Serializable接口

2. 实现原理

循环数组存放元素，定义head与tail指针

head：队列中第一个元素指向的位置，或者说调用pop方法，队列将要被弹出元素的位置

tail：调用addLast方法，队列下一个元素将要被插入的位置

两种情况下head==tail，1. 队列为空时，2. 队列塞满时的瞬间（马上会调用扩容函数，这样head又不等于tail了）

所以只要在插入元素后检测head==tail是否成立，即可知道队列是否已满，如果成立，需要调用扩容函数

至于判定队列是否为空，只要检测head==null是否成立即可

3. 底层数组的大小必须是2的n次幂

主要原因是为了后续计算方便，底层数组如果长度为2的n次幂，很多操作可以用位运算解决，不然得用取模，相对较慢

但是这里有一些黑魔法

ArrayDeque有一个带int参数的构造函数，可以用于设置底层数组的长度，如果传入的长度不为2的n次幂，那么会向上取整到一个最接近的2的n次幂，然后新建一个对应长度的数组，对应代码如下：

private void allocateElements(int numElements) {
int initialCapacity = MIN_INITIAL_CAPACITY;
// Find the best power of two to hold elements.
// Tests "<=" because arrays aren't kept full.
if (numElements >= initialCapacity) {
initialCapacity = numElements;
initialCapacity |= (initialCapacity >>>  1);
initialCapacity |= (initialCapacity >>>  2);
initialCapacity |= (initialCapacity >>>  4);
initialCapacity |= (initialCapacity >>>  8);
initialCapacity |= (initialCapacity >>> 16);
initialCapacity++;

if (initialCapacity < 0)   // Too many elements, must back off
initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
}
elements = (E[]) new Object[initialCapacity];
}

这一段代码非常有趣，我试着描述一下它的工作原理

假设我们传入的numElements为1024，将它转成二进制的话，就是0100,0000,0000，最高位有一个连续的1

在第一次位运算中，最高位的一个1会向左移动一位并复制，也就是得到了0110,0000,0000，现在我们高位有两个连续的1了

在第二次位运算中，最高位的两个1会向左移动两位并复制，也就是得到了0111,1000,0000，现在我们高位有四个连续的1了

。。。

连续操作几次之后，最高位的一个1，会将后面的bit全部覆盖，也就是得到0111,1111,1111

现在只要再自加1，就能得到比numElements大的最近的2的n次幂了

4. add与poll操作

public void addLast(E e) {
if (e == null)//队列中不能加入null元素，否则会引起poll函数的错误判断
throw new NullPointerException();
elements[tail] = e;
if ( (tail = (tail + 1) & (elements.length - 1)) == head)//tail向后移动，如果越界则归0。插入元素后如果head==tail，那么说明底层数组已满
doubleCapacity();//扩容
}

public E pollFirst() {
int h = head;
E result = elements[h]; // Element is null if deque empty
if (result == null)//如果head指向的元素为null，那么队列为空
return null;
elements[h] = null;     // Must null out slot
head = (h + 1) & (elements.length - 1);//head向后移动，如果越界则归0
return result;
}

这个代码是写得非常好的，我自认写不出这么简洁的代码

5. 扩容

private void doubleCapacity() {
assert head == tail;
int p = head;
int n = elements.length;
int r = n - p; // number of elements to the right of p
int newCapacity = n << 1;
if (newCapacity < 0)
throw new IllegalStateException("Sorry, deque too big");
Object[] a = new Object[newCapacity];
System.arraycopy(elements, p, a, 0, r);//[head,elements.length)的半段
System.arraycopy(elements, 0, a, r, p);//[0,head)的半段
elements = (E[])a;
head = 0;//重置指针
tail = n;
}

6. 不变量检测

private void checkInvariants() {
assert elements[tail] == null;//tail指针指向的位置必须为null，虽然在队列满的瞬间tail指向的元素不为null，但是马上会进行扩容操作，然后就又为null了
assert head == tail ? elements[head] == null ://如果head==tail，那么队列必然为空，head指针指向的元素也必须为null
(elements[head] != null &&//队列不为空，那么head指向的元素也不为null
elements[(tail - 1) & (elements.length - 1)] != null);//tail指针的前一个元素也必须不为null
assert elements[(head - 1) & (elements.length - 1)] == null;//head指针的前一个元素必须为null
}