Queue实现方式（ArrayDeque和LinkedList）ArrayDeque：循环数组elements保存元素
transient Object[] elements;
transient int head;
transient int tail;
构造
public ArrayDeque() {
elements = new Object[16];
}
public ArrayDeque(int numElements) {
allocateElements(numElements);
}
调整数组长度：
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 = new Object[initialCapacity];
}解析：根据特定参数设置初始数组的大小。中间移位和或操作是将initialCapacity左边最高位的1复制到右边，再加1变成2的幂次方。
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 = a;
head = 0;
tail = n;
}解析：这个数组长度扩展，当元素个数增长到等于数组大小时（head==tail）,此时将数组大小扩充一倍，再使用arraycopy将元素复制到新的数组。arraycopy（src，srcPos，dest，destPos，length），src指原数组，srcPos指数组复制的开始位置，dest指目的数组，destPos指目的数组的起始位置，length指要复制的长度。
添加
public void addFirst(E e) {
if (e == null)
throw new NullPointerException();
elements[head = (head - 1) & (elements.length - 1)] = e;
if (head == tail)
doubleCapacity();
}
public void addLast(E e) {
if (e == null)
throw new NullPointerException();
elements[tail] = e;
if ( (tail = (tail + 1) & (elements.length - 1)) == head)
doubleCapacity();
}
public boolean offerFirst(E e) {
addFirst(e);
return true;
}
public boolean offerLast(E e) {
addLast(e);
return true;
}
添加是将新的元素添加到队头或者队尾，add和offer的功能实现是一样的。因为已经初始化了数组大小，出头元素时head往后移动，如尾元素时tail往后移动，当移动到数组最后时，会将head或者tail循环到数组的前面。所以
head = (head - 1) & (elements.length - 1)的功能相当于
head = (head - 1) %（element.length - 1）由于tail指向的是队尾的后一位，所以先入队尾，再
(tail = (tail + 1) & (elements.length - 1)功能相当于
tail = (tail + 1) % (elements.length - 1)当tail==head时，说明元素已满，所以要扩充数组大小，并使用arraycopy拷贝数组。
移除
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);
return result;
}
public E pollLast() {
int t = (tail - 1) & (elements.length - 1);
@SuppressWarnings("unchecked")
E result = (E) elements[t];
if (result == null)
return null;
elements[t] = null;
tail = t;
return result;
}
解析：poll和remove都是移除元素操作，实现方法一样。出队头元素，将头元素为null，将head移动到下一个位置；
head = (h + 1) & (elements.length - 1);
出队尾元素，将最后一个元素为null，tail指向前一个位置。
t = (tail - 1) & (elements.length - 1);
获取元素
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)];
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)];
}
解析：peek和get的方法实现一样。主要是获取头元素和尾元素的数组下标。
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