ArrayDeque
特点：
1.由可调整大小的数组实现，无容量限制
2.线程不安全
3.当栈用比栈块，当队列用比LinkedList块。
4.不允许存储空元素。
5.支持两头添加元素。
public class ArrayDeque extends AbstractCollection
implements Deque, Cloneable, Serializable
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.
*/
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)).
*/
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;
注：1.发现关键字：transient，说明线程可见。如果只有单条操作，如，set，get，线程是安全的。总体上依然是线程不安全
2.最小初始化大小为2的整数倍
构造函数如下：默认elements，size为16。
public ArrayDeque() {
elements = 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 front 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 c) {
allocateElements(c.size());
addAll(c);
}
增
public boolean add(E e) {
addLast(e);
return true;
}
public void addLast(E e) {
if (e == null)
throw new NullPointerException();
elements[tail] = e;
if ( (tail = (tail + 1) & (elements.length - 1)) == head)
doubleCapacity();
}
public void addFirst(E e) {
if (e == null)
throw new NullPointerException();
elements[head = (head - 1) & (elements.length - 1)] = e;
if (head == tail)
doubleCapacity();
}
addLast将尾部的元素设置为新增的元素。
如果尾部和头部指针即将要重合，则将容量扩充两倍。
addFirst从头添加。
如果头部和尾部即将要重合，则将容量扩充两倍
删
public E remove() {
return removeFirst();
}
public E removeFirst() {
E x = pollFirst();
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;
}
pollFirst简单粗暴的将head移动到下一个位置，并将之前的元素置null。
删除指定位置,会导致数据整体前移或后移。
/**
* 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.
*
* 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 Object[] 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;
}
}
清空所有数据
/**
* 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);
}
}
改，无
查，遍历查询
a.用Iterator，这里是DeqIterator
参照一般的Iterator使用方法。
b.带删除查。while（!queue.isEmpty()）{queue.remove()}
public boolean isEmpty() {
return head == tail;
}
其他方法
/**
* Returns an array containing all of the elements in this deque
* in proper sequence (from first to last element).
*
* 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.
*
*
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()]);
}
其他设计思想：
如：
构造函数：elements = new Object[16];
addLast ：if ( (tail = (tail + 1) & (elements.length - 1)) == head)
doubleCapacity();
和addFirst：
elements[head = (head - 1) & (elements.length - 1)] = e;
if (head == tail)
doubleCapacity();
如果：
14&15==14
15&15==15
15&16==0
扩容：
/**
* Doubles 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 = a;
head = 0;
tail = n;
}