Java容器解析系列(7) ArrayDeque 详解

ArrayDeque,从名字上就可以看出来,其是通过数组实现的双端队列,我们先来看其源码:

/**
有自动扩容机制;
不是线程安全的;
不允许添加null;
作为栈使用时比java.util.Stack快;
作为队列使用时比LinkedList快; 支持fast-fail;
 * @since 1.6
 */
public class ArrayDeque<E> extends AbstractCollection<E> implements Deque<E>, Cloneable, Serializable {

    // 元素数组;
    // 数组大小永远是2的n次方;
    // 保证所有的没有元素的位置,其值为null;
    // 关于这里的数组大小为什么要求是2的n次方，后面会具体解释
    private transient E[] elements;
    // 头指针和尾指针
    private transient int head;
    private transient int tail;

    // 最小容量
    private static final int MIN_INITIAL_CAPACITY = 8;

    // ****** Array allocation and resizing utilities ******

    // 找到<=指定元素的2的n次方的数作为队列容量大小,并分配数组空间
    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;
            // 模拟扩容,每次大小都是翻倍
            initialCapacity |= (initialCapacity >>> 1);
            initialCapacity |= (initialCapacity >>> 2);
            initialCapacity |= (initialCapacity >>> 4);
            initialCapacity |= (initialCapacity >>> 8);
            initialCapacity |= (initialCapacity >>> 16);
            initialCapacity++;
            // 越界了
            if (initialCapacity < 0) // Too many elements, must back off
                initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
        }
        elements = (E[]) new Object[initialCapacity];
    }

    // 将队列的容量翻倍,只在队列满时(也就是head == tail成立的时候)调用;
    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 = (E[]) a;
        head = 0;
        tail = n;
    }
    // 复制本地元素到指定数组中，供toArray()调用
    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;
    }

    // 默认大小为16
    public ArrayDeque() {
        elements = (E[]) new Object[16];
    }
    // 指定队列容量，实际队列的容量可能不是指定的数，因为队列容量必须为2的n次方
    public ArrayDeque(int numElements) {
        allocateElements(numElements);
    }

    public ArrayDeque(Collection<? extends E> c) {
        allocateElements(c.size());
        addAll(c);
    }

    // 重要的是addFirst(),addLast(),pollFirst(),pollLast()这4个方法,其他的方法都是基于这4个方法

    // 添加新元素到头部，头指针-1
    // 时间复杂度:O(1)
    public void addFirst(E e) {
        // 不允许添加null
        if (e == null)
            throw new NullPointerException();
        // 因为 elements.length 为 2 的n次方,表达式(head - 1) & (elements.length - 1) 与 (head - 1) % elments.length相等，且前者效率比后者高(位运算效率比取模高)
        // 这里的elements.length - 1也可以成称为掩码(mask)
        elements[head = (head - 1) & (elements.length - 1)] = e;
        // 如果添加元素导致队列满了，扩容
        if (head == tail)
            doubleCapacity();
    }
    // 添加新元素到尾部，尾指针+1
    // 时间复杂度:O(1)
    public void addLast(E e) {
        // 不允许添加null
        if (e == null)
            throw new NullPointerException();
        elements[tail] = e;
        // 因为 elements.length 为 2 的n次方,表达式(tail + 1) & (elements.length - 1) 与 (head + 1) % elments.length相等
        if ((tail = (tail + 1) & (elements.length - 1)) == head)
            // 如果添加元素导致队列满了，扩容
            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;
    }
    // 移除队头
    // 时间复杂度:O(1)
    public E pollFirst() {
        int h = head;
        E result = elements[h]; // Element is null if deque empty
        // 证明队列是空的(ArrayDeque保证所有的没有元素的位置,其值为null)
        if (result == null)
            return null;
        // 保证没有元素的地方值为null,且保证GC能正常回收
        elements[h] = null; // Must null out slot
        // 头指针右移
        head = (h + 1) & (elements.length - 1);
        return result;
    }
    // 移除队尾
    // 时间复杂度:O(1)
    public E pollLast() {
        int t = (tail - 1) & (elements.length - 1);
        E result = elements[t];
        // 队列是空的
        if (result == null)
            return null;
        elements[t] = null;
        // 尾指针左移
        tail = t;
        return result;
    }

    public E getFirst() {
        E x = elements[head];
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }

    public E getLast() {
        E x = elements[(tail - 1) & (elements.length - 1)];
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }

    public E peekFirst() {
        return elements[head]; // elements[head] is null if deque empty
    }

    public E peekLast() {
        return elements[(tail - 1) & (elements.length - 1)];
    }

    // 时间复杂度O(n)
    public boolean removeFirstOccurrence(Object o) {
        if (o == null)
            return false;
        int mask = elements.length - 1;
        int i = head;
        E x;
        while ((x = elements[i]) != null) {
            if (o.equals(x)) {
                delete(i);
                return true;
            }
            i = (i + 1) & mask;
        }
        return false;
    }
    // 时间复杂度O(n)
    public boolean removeLastOccurrence(Object o) {
        if (o == null)
            return false;
        int mask = elements.length - 1;
        int i = (tail - 1) & mask;
        E x;
        while ((x = elements[i]) != null) {
            if (o.equals(x)) {
                delete(i);
                return true;
            }
            i = (i - 1) & mask;
        }
        return false;
    }

    // *** Queue methods ***

    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();
    }

    // *** Stack methods ***

    public void push(E e) {
        addFirst(e);
    }

    public E pop() {
        return removeFirst();
    }

    private void checkInvariants() {
        assert elements[tail] == null;
        assert head == tail ? elements[head] == null
        : (elements[head] != null && elements[(tail - 1) & (elements.length - 1)] != null);
        assert elements[(head - 1) & (elements.length - 1)] == null;
    }

    private boolean delete(int i) {
        checkInvariants();
        final E[] 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;
        }
    }

    // *** Collection Methods ***

    public int size() {
        return (tail - head) & (elements.length - 1);
    }

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

    public Iterator<E> iterator() {
        return new DeqIterator();
    }

    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    private class DeqIterator implements Iterator<E> {
        private int cursor = head;
        private int fence = tail;
        private int lastRet = -1;

        public boolean hasNext() {
            return cursor != fence;
        }

        public E next() {
            if (cursor == fence)
                throw new NoSuchElementException();
            E result = elements[cursor];
            // This check doesn't catch all possible comodifications,
            // but does catch the ones that corrupt traversal
            if (tail != fence || result == null)
                throw new ConcurrentModificationException();
            lastRet = cursor;
            cursor = (cursor + 1) & (elements.length - 1);
            return result;
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            if (delete(lastRet)) { // if left-shifted, undo increment in next()
                cursor = (cursor - 1) & (elements.length - 1);
                fence = tail;
            }
            lastRet = -1;
        }
    }
    // 反向迭代器，犹如godv的那支箭。。。
    private class DescendingIterator implements Iterator<E> {
        /*
         * This class is nearly a mirror-image of DeqIterator, using tail instead of
         * head for initial cursor, and head instead of tail for fence.
         */
        private int cursor = tail;
        private int fence = head;
        private int lastRet = -1;

        public boolean hasNext() {
            return cursor != fence;
        }

        public E next() {
            if (cursor == fence)
                throw new NoSuchElementException();
            cursor = (cursor - 1) & (elements.length - 1);
            E result = elements[cursor];
            if (head != fence || result == null)
                throw new ConcurrentModificationException();
            lastRet = cursor;
            return result;
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            if (!delete(lastRet)) {
                cursor = (cursor + 1) & (elements.length - 1);
                fence = head;
            }
            lastRet = -1;
        }
    }

    public boolean contains(Object o) {
        if (o == null)
            return false;
        int mask = elements.length - 1;
        int i = head;
        E x;
        while ((x = elements[i]) != null) {
            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() {
        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 methods ***

    public ArrayDeque<E> clone() {
        try {
            ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
            result.elements = Arrays.copyOf(elements, elements.length);
            return result;

        } catch (CloneNotSupportedException e) {
            throw new AssertionError();
        }
    }

    private static final long serialVersionUID = 2340985798034038923L;

    private void writeObject(ObjectOutputStream s) throws IOException {
        s.defaultWriteObject();

        // Write out size
        s.writeInt(size());

        // Write out elements in order.
        int mask = elements.length - 1;
        for (int i = head; i != tail; i = (i + 1) & mask)
            s.writeObject(elements[i]);
    }

    private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
        s.defaultReadObject();

        // Read in size and allocate array
        int size = s.readInt();
        allocateElements(size);
        head = 0;
        tail = size;

        // Read in all elements in the proper order.
        for (int i = 0; i < size; i++)
            elements[i] = (E) s.readObject();
    }
}

从源码可以很容易的看出来:ArrayDeque本质为数组实现的循环队列,关于循环队列，请参考博客:循环队列(顺序队列)

增删的主要实现方法为addFirst(),addLast(),pollFirst(),pollLast()这4个方法,其他的方法都是调用这4个方法来实现功能，且其时间复杂度均为O(1)

ArrayDeque中的数组大小必须为2的n次方，这一点的解释可以参考上述源码中addFirst()和addLast()的注释，已经说得很清楚了