ArrayDeque类源码解析

ArrayDeque
1.数组实现双向队列
2.没有实现同步方法，线程不安全，效率较高
3.比LinkedList效率高
4.实现了栈，队列，作为栈使用时候效率比Stack高，作为队列时候比LinkedList效率高

通过数组实现双端队列，注意实现的还是循环队列

所在包

package java.util;
import java.io.*;

继承AbstractCollection
实现Deque、Cloneable、Serializable

public class ArrayDeque<E> extends AbstractCollection<E>
                           implements Deque<E>, Cloneable, Serializable{
// 内部代码下面讲解
}

四个变量

    /**
     * 数组作为存储结构
     */
    private transient E[] elements;

    /**
     * 头节点下标
     */
    private transient int head;

    /**
     * 尾节点下标
     */
    private transient int tail;

    /**
     * 创建队列最小容量，容量必须是2的x次方
     */
    private static final int MIN_INITIAL_CAPACITY = 8;

分配空间

    /**
     * 分配空间
     *
     * @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 = (E[]) new Object[initialCapacity];
    }

满的时候，进行扩容，这里需要移动元素

    /**
     * head == tail 时候表示队列满了，需要扩充
     * Double 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; // 元素个数
        int newCapacity = n << 1; // 新的空间，左移一位，新增加空间是原始空间的大小
        if (newCapacity < 0)
            throw new IllegalStateException("Sorry, deque too big");
        Object[] a = new Object[newCapacity];
        System.arraycopy(elements, p, a, 0, r); // 移动元素
        System.arraycopy(elements, 0, a, r, p);
        elements = (E[])a;
        head = 0;
        tail = n;
    }

队列元素复制到数组

    /**
     * 队列元素复制到数组
     *
     * @return its argument
     */
    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;
    }

三个构造器

    /**
     * 构造函数用于创建一个空数组双端队列容纳16个元素的初始容量。
     */
    public ArrayDeque() {
        elements = (E[]) new Object[16];
    }

    /**
     * 构造函数用于创建一个空数组与双端队列的初始容量足以容纳指定的元素数
     *
     * @param numElements  lower bound on initial capacity of the deque
     */
    public ArrayDeque(int numElements) {
        allocateElements(numElements);
    }

    /**
     * 构造函数用于创建一个包含指定集合的元素的双端队列。
     *
     * @param c the collection whose elements are to be placed into the deque
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayDeque(Collection<? extends E> c) {
        allocateElements(c.size());
        addAll(c);
    }

入队列的操作

队头插入
先计算插入位置，再插入
插入位置：
head = (head-1)&（length-1）
等价于：（head-1）%（length-1）

    /**
     * 队头插入元素
     *
     * @param e the element to add
     * @throws NullPointerException if the specified element is null
     */
    public void addFirst(E e) {
        if (e == null)
            throw new NullPointerException();
        elements[head = (head - 1) & (elements.length - 1)] = e; // 计算头插入位置
        if (head == tail) // 队列满了，需要扩容
            doubleCapacity();
    }

队尾插入
先插入，再计算出下一个可插入位置
下一个插入位置：last = (last+1)&(length-1)

    /**
     * 队尾插入元素
     *
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     * @throws NullPointerException if the specified element is null
     */
    public void addLast(E e) {
        if (e == null)
            throw new NullPointerException();
        elements[tail] = e;
        if ( (tail = (tail + 1) & (elements.length - 1)) == head) // 计算下一个插入位置，并判断是否满
            doubleCapacity();
    }

队头插入元素

    /**
     * 队头插入元素
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Deque#offerFirst})
     * @throws NullPointerException if the specified element is null
     */
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

队尾插入元素

    /**
     * 队尾插入元素
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Deque#offerLast})
     * @throws NullPointerException if the specified element is null
     */
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

删除的相关操作
删除队头元素

    /**
     * 删除队头元素 
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E removeFirst() {
        E x = pollFirst();
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }

删除队尾元素

    /**
     * 删除队尾元素
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E removeLast() {
        E x = pollLast();
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }

删除队头元素
直接删除，更新队头位置
head = (head+1)&(length-1)

    /**
     * 删除队头元素
     */
    public E pollFirst() {
        int h = head;
        E result = 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); // 移动head 
        return result;
    }

删除队尾元素
先找到位置，在删除元素
tail = （tail -1）&(length-1)

    /**
     * 删除队尾元素
     */
    public E pollLast() {
        int t = (tail - 1) & (elements.length - 1);// 移动tail 
        E result = elements[t];
        if (result == null)
            return null;
        elements[t] = null;
        tail = t;
        return result;
    }

查看操作
查看队头元素
直接查看

    /**
     * 查看队头元素
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E getFirst() {
        E x = elements[head];
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }

查看队尾元素
需要计算队尾位置：tail = (tail-1)&(length -1)

    /**
     * 查看队尾元素
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E getLast() {
        E x = elements[(tail - 1) & (elements.length - 1)];
        if (x == null)
            throw new NoSuchElementException();
        return x;
    }

查看队头元素
直接查看

    // 查看队头元素
    public E peekFirst() {
        return elements[head]; // elements[head] is null if deque empty
    }

查看队尾元素

    // 查看队尾元素 
    public E peekLast() {
        return elements[(tail - 1) & (elements.length - 1)];
    }

删除操作

    /**
     * 删除第一次出现的元素 o 
     *
     * @param o element to be removed from this deque, if present
     * @return <tt>true</tt> if the deque contained the specified element
     */
    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;
    }

    /**
     * 删除最后一次出现的元素 
     *
     * @param o element to be removed from this deque, if present
     * @return <tt>true</tt> if the deque contained the specified element
     */
    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;
    }

有关队列的方法

    /**
     * 插入元素 入队
     *
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        addLast(e);
        return true;
    }

    /**
     * 插入元素  入队
     *
     * <p>This method is equivalent to {@link #offerLast}.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Queue#offer})
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        return offerLast(e);
    }

    /**
     * 删除队头元素 
     * <p>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();
    }

    /**
     * 出队
     *
     * <p>This method is equivalent to {@link #pollFirst}.
     *
     * @return the head of the queue represented by this deque, or
     *         <tt>null</tt> if this deque is empty
     */
    public E poll() {
        return pollFirst();
    }

    /**
     * 查看队头元素 
     *
     * <p>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();
    }

    /**
     * 查看队头元素 
     *
     * @return the head of the queue represented by this deque, or
     *         <tt>null</tt> if this deque is empty
     */
    public E peek() {
        return peekFirst();
    }

有关栈的方法

    /**
     * 入栈
     * <p>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);
    }

    /**
     * 出栈
     * <p>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();
    }

具体删除操作，没有看懂

    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;
    }

    /**
     * 删除i 位置元素 
     *
     * <p>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 E[] elements = this.elements;
        final int mask = elements.length - 1;
        final int h = head;
        final int t = tail;
        final int front = (i - h) & mask; // 前驱
        final int back  = (t - i) & mask; // 后继 

        // 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;
        }
    }

集合方法

 /**
     * 元素个数 
     * @return the number of elements in this deque
     */
    public int size() {
        return (tail - head) & (elements.length - 1);
    }

    /**
     * 是否空 
     * @return <tt>true</tt> if this deque contains no elements
     */
    public boolean isEmpty() {
        return head == tail;
    }

有关迭代器操作

    /**
     * 返回一个迭代器 
     * @return an iterator over the elements in this deque
     */
    public Iterator<E> iterator() {
        return new DeqIterator();
    }

    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }
    // head 到 tail 遍历
    private class DeqIterator implements Iterator<E> {
        /**
         * head 
         */
        private int cursor = head;

        /**
         * tail 
         */
        private int fence = tail;

        /**
         * 最近调用的元素索引
         * Reset to -1 if element is deleted by a call to remove.
         */
        private int lastRet = -1;

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

        public E next() { // 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() { // 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;
        }
    }
    // tail 到 head 遍历 
    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;
        }
    }

包含，清空、复制到数组

    /**
     * 是否包含 o 
     *
     * @param o object to be checked for containment in this deque
     * @return <tt>true</tt> 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;
        E x;
        while ( (x = elements[i]) != null) {
            if (o.equals(x))
                return true;
            i = (i + 1) & mask;
        }
        return false;
    }

    /**
     * 删除元素 o 
     *
     * @param o element to be removed from this deque, if present
     * @return <tt>true</tt> if this deque contained the specified element
     */
    public boolean remove(Object o) {
        return removeFirstOccurrence(o);
    }

    /**
     *清空
     */
    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);
        }
    }

    /**
     * 转换成数组 
     *
     * @return an array containing all of the elements in this deque
     */
    public Object[] toArray() {
        return copyElements(new Object[size()]);
    }

    /**
     * 队列元素更新数组元素
     * @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
     */
    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方法

    /**
     * Returns a copy of this deque.
     *
     * @return a copy of this deque
     */
    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();
        }
    }

    /**
     * Appease the serialization gods.
     */
    private static final long serialVersionUID = 2340985798034038923L;

    /**
     * Serialize this deque.
     *
     * @serialData The current size (<tt>int</tt>) of the deque,
     * followed by all of its elements (each an object reference) in
     * first-to-last order.
     */
    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]);
    }

    /**
     * Deserialize this deque.
     */
    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();
    }