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Java 源码阅读的第一步是Collection框架源码，这也是面试基础中的基础； 针对Collection的源码阅读写一个系列的文章，本文是第三篇ArrayDeque。 ---@pdai
JDK版本
JDK 1.8.0_110
概述总结
- ArrayDeque是可变长Array, 实现了Deque接口；Deque是"double ended queue", 表示双向的队列，英文读作"deck"；
- Deque 继承自 Queue接口，除了支持Queue的方法之外，还支持
insert,remove和examine操作，由于Deque是双向的，所以可以对队列的头和尾都进行操作，它同时也支持两组格式，一组是抛出异常的实现；另外一组是返回值的实现（没有则返回null）； - Java里有一个叫做Stack的类，却没有叫做Queue的类（它是个接口名字）。当需要使用栈时，Java已不推荐使用Stack，而是推荐使用更高效的ArrayDeque；既然Queue只是一个接口，当需要使用队列时也就首选ArrayDeque了（次选是LinkedList）。
- ArrayDeque底层通过数组实现，为了满足可以同时在数组两端插入或删除元素的需求，该数组还必须是循环的，即循环数组（circular array），也就是说数组的任何一点都可能被看作起点或者终点;
- ArrayDeque是非线程安全的（not thread-safe），当多个线程同时使用的时候，需要程序员手动同步；另外，该容器不允许放入
null元素。 - ArrayDeque也采用了快速失败的机制，通过记录modCount参数来实现。在面对并发的修改时，迭代器很快就会完全失败，而不是冒着在将来某个不确定时间发生任意不确定行为的风险;
- ArrayDeque底层的数据类型是Object[], Java泛型只是编译器提供的语法糖，所以这里的数组是一个Object数组，以便能够容纳任何类型的对象;
类关系图
ArrayDeque实现的接口和继承的类如下：
public class ArrayDeque extends AbstractCollection
implements Deque, Cloneable, Serializable
{
} 
Collection接口
Collection接口操作分为如下几类：
- Query Operations
- int size();
- boolean isEmpty();
- boolean contains(Object o);
- Iterator iterator();
- Object[] toArray();
- T[] toArray(T[] a);
- Modification Operations
- boolean add(E e);
- boolean remove(Object o);
- Bulk Operations
- boolean containsAll(Collection c);
- boolean addAll(Collection c);
- boolean removeAll(Collection c);
- default boolean removeIf(Predicate filter)
- boolean retainAll(Collection c);
- void clear();
- Comparison and hashing
- boolean equals(Object o);
- int hashCode();
- others
- default Spliterator spliterator() {}
- default Stream stream() {}
- default Stream parallelStream() {}
Queue
Queue接口继承自Collection接口，除了最基本的Collection的方法之外，它还支持额外的insertion, extraction和inspection操作。这里有两组格式，共6个方法，一组是抛出异常的实现；另外一组是返回值的实现（没有则返回null）。
| Throws exception | Returns special value | |
|---|---|---|
| Insert | add(e) |
offer(e) |
| Remove | remove() |
poll() |
| Examine | element() |
peek() |
Deque
Deque 继承自 Queue接口，除了支持Queue的方法之外，还支持insert, remove和examine操作，由于Deque是双向的，所以可以对队列的头和尾都进行操作，它同时也支持两组格式，一组是抛出异常的实现；另外一组是返回值的实现（没有则返回null）。共12个方法如下：
| First Element (Head) | Last Element (Tail) | ||
|---|---|---|---|
| Throws exception | Special value | Throws exception | |
| Insert | addFirst(e) |
offerFirst(e) |
addLast(e) |
| Remove | removeFirst() |
pollFirst() |
removeLast() |
| Examine | getFirst() |
peekFirst() |
getLast() |
当把Deque当做FIFO的queue来使用时，元素是从deque的尾部添加，从头部进行删除的； 所以deque的部分方法是和queue是等同的。具体如下：
| Queue Method | Equivalent Deque Method |
|---|---|
add(e) |
addLast(e) |
offer(e) |
offerLast(e) |
remove() |
removeFirst() |
poll() |
pollFirst() |
element() |
getFirst() |
peek() |
peekFirst() |
Deque的含义是“double ended queue”，即双端队列，它既可以当作栈使用，也可以当作队列使用。
下表列出了Deque与Queue相对应的接口：
| Queue Method | Equivalent Deque Method | 说明 |
|---|---|---|
add(e) |
addLast(e) |
向队尾插入元素，失败则抛出异常 |
offer(e) |
offerLast(e) |
向队尾插入元素，失败则返回false |
remove() |
removeFirst() |
获取并删除队首元素，失败则抛出异常 |
poll() |
pollFirst() |
获取并删除队首元素，失败则返回null |
element() |
getFirst() |
获取但不删除队首元素，失败则抛出异常 |
peek() |
peekFirst() |
获取但不删除队首元素，失败则返回null |
下表列出了Deque与Stack对应的接口：
| Stack Method | Equivalent Deque Method | 说明 |
|---|---|---|
push(e) |
addFirst(e) |
向栈顶插入元素，失败则抛出异常 |
| 无 | offerFirst(e) |
向栈顶插入元素，失败则返回false |
pop() |
removeFirst() |
获取并删除栈顶元素，失败则抛出异常 |
| 无 | pollFirst() |
获取并删除栈顶元素，失败则返回null |
peek() |
peekFirst() |
获取但不删除栈顶元素，失败则抛出异常 |
| 无 | peekFirst() |
获取但不删除栈顶元素，失败则返回null |
上面两个表共定义了Deque的12个接口。添加，删除，取值都有两套接口，它们功能相同，区别是对失败情况的处理不同。一套接口遇到失败就会抛出异常，另一套遇到失败会返回特殊值（false或null）。除非某种实现对容量有限制，大多数情况下，添加操作是不会失败的。虽然Deque的接口有12个之多，但无非就是对容器的两端进行操作，或添加，或删除，或查看。明白了这一点讲解起来就会非常简单。
类的实现
底层数据结构
从名字可以看出ArrayDeque底层通过数组实现，为了满足可以同时在数组两端插入或删除元素的需求，该数组还必须是循环的，即循环数组（circular array），也就是说数组的任何一点都可能被看作起点或者终点。ArrayDeque是非线程安全的（not thread-safe），当多个线程同时使用的时候，需要程序员手动同步；另外，该容器不允许放入null元素。
/**
* 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.
*/
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;head指向首端第一个有效元素，tail指向尾端第一个可以插入元素的空位。因为是循环数组，所以head不一定总等于0，tail也不一定总是比head大。
基础数据结构方法
下面再说说扩容函数doubleCapacity()，其逻辑是申请一个更大的数组（原数组的两倍），然后将原数组复制过去。
/**
* Allocates 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 = new Object[initialCapacity];
}
/**
* 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; // head右边元素的个数
int newCapacity = n << 1;//原空间的2倍
if (newCapacity < 0)
throw new IllegalStateException("Sorry, deque too big");
Object[] a = new Object[newCapacity];
System.arraycopy(elements, p, a, 0, r);//复制右半部分，对应上图中绿色部分
System.arraycopy(elements, 0, a, r, p);//复制左半部分，对应上图中灰色部分
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[] 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;
} 构造函数
/**
* Constructs an empty array deque with an initial capacity
* sufficient to hold 16 elements.
*/
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);
}基本的插入和取出方法
主要都是基于如下四个方法addFirst(), addLast(), pollFirst(), pollLast()
addFirst()
addFirst(E e)的作用是在Deque的首端插入元素，也就是在head的前面插入元素，在空间足够且下标没有越界的情况下，只需要将elements[--head] = e即可。
实际需要考虑：1.空间是否够用，以及2.下标是否越界的问题。
//addFirst(E e)
public void addFirst(E e) {
if (e == null)//不允许放入null
throw new NullPointerException();
elements[head = (head - 1) & (elements.length - 1)] = e;//2.下标是否越界
if (head == tail)//1.空间是否够用
doubleCapacity();//扩容
}上述代码我们看到，空间问题是在插入之后解决的，因为tail总是指向下一个可插入的空位，也就意味着elements数组至少有一个空位，所以插入元素的时候不用考虑空间问题。
下标越界的处理解决起来非常简单，head = (head - 1) & (elements.length - 1)就可以了，这段代码相当于取余，同时解决了head为负值的情况。因为elements.length必需是2的指数倍，elements - 1就是二进制低位全1，跟head - 1相与之后就起到了取模的作用，如果head - 1为负数（其实只可能是-1），则相当于对其取相对于elements.length的补码。
addLast()
addLast(E e)的作用是在Deque的尾端插入元素，也就是在tail的位置插入元素，由于tail总是指向下一个可以插入的空位，因此只需要elements[tail] = e;即可。插入完成后再检查空间，如果空间已经用光，则调用doubleCapacity()进行扩容。
public void addLast(E e) {
if (e == null)//不允许放入null
throw new NullPointerException();
elements[tail] = e;//赋值
if ( (tail = (tail + 1) & (elements.length - 1)) == head)//下标越界处理
doubleCapacity();//扩容
}pollFirst()
pollFirst()的作用是删除并返回Deque首端元素，也即是head位置处的元素。如果容器不空，只需要直接返回elements[head]即可，当然还需要处理下标的问题。由于ArrayDeque中不允许放入null，当elements[head] == null时，意味着容器为空。
public E pollFirst() {
E result = elements[head];
if (result == null)//null值意味着deque为空
return null;
elements[h] = null;//let GC work
head = (head + 1) & (elements.length - 1);//下标越界处理
return result;
}pollLast()
pollLast()的作用是删除并返回Deque尾端元素，也即是tail位置前面的那个元素。
public E pollLast() {
int t = (tail - 1) & (elements.length - 1);//tail的上一个位置是最后一个元素
E result = elements[t];
if (result == null)//null值意味着deque为空
return null;
elements[t] = null;//let GC work
tail = t;
return result;
}peekFirst()
peekFirst()的作用是返回但不删除Deque首端元素，也即是head位置处的元素，直接返回elements[head]即可。
public E peekFirst() {
return elements[head]; // elements[head] is null if deque empty
}peekLast()
peekLast()的作用是返回但不删除Deque尾端元素，也即是tail位置前面的那个元素。
public E peekLast() {
return elements[(tail - 1) & (elements.length - 1)];
}队列（Queue）方法
add(E e)
/**
* Inserts the specified element at the end of this deque.
*
* This method is equivalent to {@link #addLast}.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Collection#add})
* @throws NullPointerException if the specified element is null
*/
public boolean add(E e) {
addLast(e);
return true;
}
offer(E e)
/**
* Inserts the specified element at the end of this deque.
*
* This method is equivalent to {@link #offerLast}.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Queue#offer})
* @throws NullPointerException if the specified element is null
*/
public boolean offer(E e) {
return offerLast(e);
}
remove()
/**
* 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.
*
* 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();
}
poll()
/**
* Retrieves and removes the head of the queue represented by this deque
* (in other words, the first element of this deque), or returns
* {@code null} if this deque is empty.
*
* This method is equivalent to {@link #pollFirst}.
*
* @return the head of the queue represented by this deque, or
* {@code null} if this deque is empty
*/
public E poll() {
return pollFirst();
}
element()
/**
* 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.
*
* 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();
}
peek()
/**
* Retrieves, but does not remove, the head of the queue represented by
* this deque, or returns {@code null} if this deque is empty.
*
* This method is equivalent to {@link #peekFirst}.
*
* @return the head of the queue represented by this deque, or
* {@code null} if this deque is empty
*/
public E peek() {
return peekFirst();
}
栈（Stack）方法
push(E e)
/**
* Pushes an element onto the stack represented by this deque. In other
* words, inserts the element at the front of this deque.
*
* 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);
}
pop()
/**
* Pops an element from the stack represented by this deque. In other
* words, removes and returns the first element of this deque.
*
* 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();
}
集合（Collection）方法
size()
/**
* 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);
}isEmpty()
/**
* Returns {@code true} if this deque contains no elements.
*
* @return {@code true} if this deque contains no elements
*/
public boolean isEmpty() {
return head == tail;
}iterator()
/**
* 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 iterator() {
return new DeqIterator();
}
public Iterator descendingIterator() {
return new DescendingIterator();
} contains(Object o)
/**
* Returns {@code true} if this deque contains the specified element.
* More formally, returns {@code true} if and only if this deque contains
* at least one element {@code e} such that {@code o.equals(e)}.
*
* @param o object to be checked for containment in this deque
* @return {@code true} 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;
Object x;
while ( (x = elements[i]) != null) {
if (o.equals(x))
return true;
i = (i + 1) & mask;
}
return false;
}remove(Object o)
/**
* 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 {@code e} such that
* {@code o.equals(e)} (if such an element exists).
* Returns {@code true} if this deque contained the specified element
* (or equivalently, if this deque changed as a result of the call).
*
* This method is equivalent to {@link #removeFirstOccurrence(Object)}.
*
* @param o element to be removed from this deque, if present
* @return {@code true} if this deque contained the specified element
*/
public boolean remove(Object o) {
return removeFirstOccurrence(o);
}
clear()
/**
* 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);
}
}toArray()
/**
* 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()]);
}
toArray(T[] a)
/**
* 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.
*
* 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
* {@code null}.
*
*
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.
*
*
Suppose {@code x} is a deque known to contain only strings.
* The following code can be used to dump the deque into a newly
* allocated array of {@code String}:
*
*
{@code String[] y = x.toArray(new String[0]);}
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @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
*/
@SuppressWarnings("unchecked")
public 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;
}