Java源码阅读之ArrayDeque

Summary：

public class ArrayDeque<E> extends AbstractCollection<E> implements Deque<E>, Cloneable, Serializabletail
对象中有一个Obejct[]数组；该数组的容量为2的幂
tail指向的位置不存储数据；head指向的位置是队列中第一个数据;
数组可以扩容，扩容后大小为原数组长度的2倍
数组中存储的数据有(tail-head)&(elements.length-1)
数据添加的位置是tail；之后tail=（tail+1）&(elements.length-1)
数据移出的位置是head；之后head=（head+1）&(elements.length-1)

Fields：

transient Object[] elements;
transient int head;
transient int tail;
private static final int MIN_INITIAL_CAPACITY = 8;

Constructor：

public ArrayDeque() {
elements = new Object[16];
}
public ArrayDeque(int numElements) {
allocateElements(numElements);
}
//将转换的数改成2的幂；这样能将取余运算替换为逻辑与运算
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];
}

size()

//注意：这里很巧妙！！
public int size() {
return (tail - head) & (elements.length - 1);
}

isEmpty()

public boolean isEmpty() {
return head == tail;
}

addXX():

添加元素默认将其添加到队列尾端

public boolean add(E e) {
addLast(e);
return true;
}
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();
}

removeXX():

移除元素默认移除第一个;队列空抛异常

public E remove() {
return removeFirst();
}
public E removeFirst() {
E x = pollFirst();
if (x == null)
throw new NoSuchElementException();
return x;
}

pollXX():

移除第一个元素，队列空则返回null

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;
}

扩容：

//扩容扩两倍
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;
}