ArrayDeque 是 Deque 接口的变长数组实现，它没有容量限制，会根据需要来扩容。它不是线程安全的，缺乏外部的同步机制，导致它不支持多线程的并发访问。Null 对象是不许存在的，这个类很可能比 Stack 实现的栈和 LinkedList 实现的队列更快。
其绝大多数操作运行在常数时间，除了一些需要线性时间执行的方法：remove、removeFirstOccurrence、removeLastOccurrence、contains、iterator.remove()和批量操作。
它继承了 AbstractCollection，实现了Deque、Cloneable 和 Serializable。
public class ArrayDeque<E> extends AbstractCollection<E>
implements Deque<E>, Cloneable, Serializable// deque存放数据的数组，它的容量就是数组的长度，总是2的幂次方。数组不允许被充满，除非暂时性的用add方法导致数组充满，这里就要立刻扩容，去避免head和tail变得相同。同时，这里也保证数组中没有元素的位置始终为空。
transient Object[] elements;
transient int head; // deque的头部，remove和pop的位置，deque为空时和tail相等
transient int tail; // deque的尾部,addLast、add和push的位置
private static final int MIN_INITIAL_CAPACITY = 8; // 新建deque的最小容量，必须为2的幂次方public ArrayDeque() {
elements = new Object[16]; // 默认长度16
}
public ArrayDeque(int numElements) {
allocateElements(numElements);
}
// 类似HashMap的方法分配数组长度，2的幂次方且刚比指定的长度大
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); // 按位或，有一个为1就为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 = new Object[initialCapacity];
}
public ArrayDeque(Collection<? extends E> c) {
allocateElements(c.size());
addAll(c); // AbstractCollection中的方法
}数组分配和大小调整的方法：
private void doubleCapacity() { // 当新增数据导致头尾重合时就要double数组容量
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); // 从头开始到最后移到a最前
System.arraycopy(elements, 0, a, r, p); // 从最前到头移到上述的后面
elements = a; // 头尾分别是0、n
head = 0;
tail = n;
}
// 将原来数组的元素拷贝到新数组，有序的（从头到尾）
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;
}主要的插入和取出方法，包括 addFirst 、addLast、 pollFirst 、pollLast，其它的方法都是根据它们再定义的：
public void addFirst(E e) {
if (e == null)
throw new NullPointerException();
elements[head = (head - 1) & (elements.length - 1)] = e; // 取余，如果之前head为0，这里head为最后一位
if (head == tail)
doubleCapacity();
}
public void addLast(E e) {
if (e == null)
throw new NullPointerException();
elements[tail] = e; // 先赋tail值
if ( (tail = (tail + 1) & (elements.length - 1)) == head) // 移动tail位置
doubleCapacity();
}
public boolean offerFirst(E e) {
addFirst(e);
return true;
}
public boolean offerLast(E e) {
addLast(e);
return true;
}
public E removeFirst() {
E x = pollFirst();
if (x == null)
throw new NoSuchElementException();
return x;
}
public E removeLast() {
E x = pollLast();
if (x == null)
throw new NoSuchElementException();
return x;
}
public E pollFirst() {
int h = head;
@SuppressWarnings("unchecked")
E result = (E) 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); // head向后移动一位取余
return result;
}
public E pollLast() {
int t = (tail - 1) & (elements.length - 1); // taild向前移动一位取余
@SuppressWarnings("unchecked")
E result = (E) elements[t];
if (result == null) // 空队列
return null;
elements[t] = null; // 最后一个数置空
tail = t;
return result;
}
public E getFirst() {
@SuppressWarnings("unchecked")
E result = (E) elements[head];
if (result == null)
throw new NoSuchElementException();
return result;
}
public E getLast() {
@SuppressWarnings("unchecked")
E result = (E) elements[(tail - 1) & (elements.length - 1)]; // 位置是tail前一位取余
if (result == null)
throw new NoSuchElementException();
return result;
}
public E peekFirst() {
// elements[head] is null if deque empty
return (E) elements[head];
}
public E peekLast() {
return (E) elements[(tail - 1) & (elements.length - 1)];
}
public boolean removeFirstOccurrence(Object o) {
if (o == null)
return false;
int mask = elements.length - 1;
int i = head;
Object x;
while ( (x = elements[i]) != null) {
if (o.equals(x)) {
delete(i);
return true;
}
i = (i + 1) & mask; // 从头到尾找
}
return false;
}
public boolean removeLastOccurrence(Object o) {
if (o == null)
return false;
int mask = elements.length - 1;
int i = (tail - 1) & mask;
Object x;
while ( (x = elements[i]) != null) {
if (o.equals(x)) {
delete(i);
return true;
}
i = (i - 1) & mask; // 从尾到头找
}
return false;
}队列方法：
public boolean add(E e) {
addLast(e);
return true;
}
public boolean offer(E e) {
return offerLast(e);
}
public E remove() {
return removeFirst();
}
public E poll() {
return pollFirst();
}
public E element() {
return getFirst();
}
public E peek() {
return peekFirst();
}栈方法：
public void push(E e) {
addFirst(e);
}
public E pop() {
return removeFirst();
}删除特定索引的元素：
private 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; // 相对于head的距离
final int back = (t - i) & mask; // 相对于tail的距离
// Invariant: head <= i < tail mod circularity
if (front >= ((t - h) & mask)) // i不在数据范围内
throw new ConcurrentModificationException();
// Optimize for least element motion
if (front < back) { // 比较靠前就head后移一位
if (h <= i) {
System.arraycopy(elements, h, elements, h + 1, front);
} else { // Wrap around
// ...i...tail...head...
System.arraycopy(elements, 0, elements, 1, i); // 前i个后移一位将第一个覆盖
elements[0] = elements[mask]; // 最后一位到最前
System.arraycopy(elements, h, elements, h + 1, mask - h); // head到最后第二位后移一位
}
elements[h] = null;
head = (h + 1) & mask; // 原head置空，新head为后一位
return false;
} else { // 比较靠后就tail前移一位
if (i < t) { // Copy the null tail as well
System.arraycopy(elements, i + 1, elements, i, back);
tail = t - 1;
} else { // Wrap around
// ...tail...head...i...
System.arraycopy(elements, i + 1, elements, i, mask - i); // i+1到最后前移一位
elements[mask] = elements[0]; // 最前一位到最后
System.arraycopy(elements, 1, elements, 0, t); // 1到tail前移一位
tail = (t - 1) & mask; // tial前移一位
}
return true;
}
}
private void checkInvariants() {
assert elements[tail] == null; // tail为空
assert head == tail ? elements[head] == null : (elements[head] != null && elements[(tail - 1) & (elements.length - 1)] != null); // deque为空或者最前最后的元素均存在
assert elements[(head - 1) & (elements.length - 1)] == null; // head前一位为空
}集合方法：
public int size() {
return (tail - head) & (elements.length - 1); // 取余
}
public boolean isEmpty() {
return head == tail;
}
public boolean contains(Object o) {
if (o == null)
return false;
int mask = elements.length - 1;
int i = head;
Object x;
while ( (x = elements[i]) != null) { // 从头到尾查
if (o.equals(x))
return true;
i = (i + 1) & mask;
}
return false;
}
public boolean remove(Object o) {
return removeFirstOccurrence(o);
}
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);
}
}
public Object[] toArray() { // 转成数组，从头到尾
return copyElements(new Object[size()]);
}
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方法：
public ArrayDeque<E> clone() {
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();
}
}