ArrayDeque类--数组实现双端队列

package java.util;

import java.io.Serializable;
import java.util.function.Consumer;

import sun.misc.SharedSecrets;

//Deque的实现类，基于数组实现双端队列
//可变数组，没有容量限制，线程不安全，不支持null值
public class ArrayDeque<E> extends AbstractCollection<E>
        implements Deque<E>, Cloneable, Serializable {
    transient Object[] elements; // 数组队列，非私有，便于内部类访问
    transient int head;//队列头
    transient int tail;//队列尾
    private static final int MIN_INITIAL_CAPACITY = 8;//队列的最小容量

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

    private static int calculateSize(int numElements){
        int initialCapacity = MIN_INITIAL_CAPACITY;
        //这一段代码是将从最高位1开始，将后面全部变成1，也就是找到一个刚好比initialCapacity的2^n数
        if(numElements >= initialCapacity){
            initialCapacity = numElements;
            initialCapacity |= (initialCapacity >>> 1);//>>>无符号右移，高位补0
            initialCapacity |= (initialCapacity >>> 2);
            initialCapacity |= (initialCapacity >>> 4);
            initialCapacity |= (initialCapacity >>> 8);
            initialCapacity |= (initialCapacity >>> 16);
            initialCapacity++;

            if(initialCapacity < 0)   // Too many elements, must back off，符号位为1
                initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
        }
        return initialCapacity;
    }

    //初始化队列(数组)
    private void allocateElements(int numElements){
        elements = new Object[calculateSize(numElements)];
    }

    //扩展队列，队列容量变为2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];
        //分两次将原数据复制到数组a中
        System.arraycopy(elements, p, a, 0, r); //native方法
        System.arraycopy(elements, 0, a, r, p);
        elements = a;
        head = 0;
        tail = n;
    }

    //将数组数据从element复制到数组a
    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;
    }

    //双端队列的无参构造，没有指定队列容量
    public ArrayDeque(){
        elements = new Object[16];
    }

    //指定了队列容量的构造方法
    public ArrayDeque(int numElements){
        allocateElements(numElements);
    }

    //根据一个容器来构造双端队列，并容器所有数据添加到双端队列中
    public ArrayDeque(Collection<? extends E> c){
        allocateElements(c.size()); //容量是刚好比c的2^n
        addAll(c);
    }

    //在双端队列前面插入数据
    public void addFirst(E e){
        if(e == null)
            throw new NullPointerException(); //不能插入null
        elements[head = (head - 1) & (elements.length - 1)] = e;//element-1是比length小的全1的数，保证head-1不会超过length
        if(head == tail)
            doubleCapacity();
    }

    //在双端后面插入数据
    public void addLast(E e){
        if(e == null)
            throw new NullPointerException();
        elements[tail] = e;
        if((tail = (tail + 1) & (elements.length - 1)) == head)
            doubleCapacity();
    }

    //这里的offer同add
    public boolean offerFirst(E e){
        addFirst(e);
        return true;
    }

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

    //从队列头部移除一个元素，会抛异常
    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;
    }

    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;
    }


    public E getFirst(){
        @SuppressWarnings("unchecked")
        E result = (E) elements[head];
        if(result == null)
            throw new NoSuchElementException();
        return result;
    }

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

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

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

    //从头到尾，移除第一个出现的对象o,如果没有此对象o,返回false
    public boolean removeFirstOccurrence(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)){
                delete(i); //删除队列中的这个元素
                return true;
            }
            i = (i + 1) & mask;
        }
        return false;
    }

    public boolean removeLastOccurrence(Object o){
        if(o == null)
            return false;
        int mask = elements.length - 1;
        int i = (tail - 1) & mask;
        Object 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();
    }

    //断言检查
    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;
    }

    //删除队列的元素
    private boolean delete(int i){
        checkInvariants(); //断言
        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)) //检查是否有并发操作
            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();
            @SuppressWarnings("unchecked")
            E result = (E) 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;
        }

        public void forEachRemaining(Consumer<? super E> action){
            Objects.requireNonNull(action);
            Object[] a = elements;
            int m = a.length - 1, f = fence, i = cursor;
            cursor = f;
            while (i != f) {
                @SuppressWarnings("unchecked") E e = (E) a[i];
                i = (i + 1) & m;
                if(e == null)
                    throw new ConcurrentModificationException();
                action.accept(e);
            }
        }
    }

    private class DescendingIterator implements Iterator<E> {
        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);
            @SuppressWarnings("unchecked")
            E result = (E) 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;
        Object 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()]);
    }

    @SuppressWarnings("unchecked")
    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 {
            @SuppressWarnings("unchecked")
            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;

    //把deque写入到流中
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.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]); }
    }

    //从流中读数据到deque
    private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException{
        s.defaultReadObject();

        // Read in size and allocate array
        int size = s.readInt();
        int capacity = calculateSize(size);
        SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);
        allocateElements(size);
        head = 0;
        tail = size;

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

    public Spliterator<E> spliterator(){
        return new DeqSpliterator<E>(this, -1, -1);
    }

    static final class DeqSpliterator<E> implements Spliterator<E> {
        private final ArrayDeque<E> deq;
        private int fence;  // -1 until first use
        private int index;  // current index, modified on traverse/split

        /**
         * Creates new spliterator covering the given array and range
         */
        DeqSpliterator(ArrayDeque<E> deq, int origin, int fence){
            this.deq = deq;
            this.index = origin;
            this.fence = fence;
        }

        private int getFence(){ // force initialization
            int t;
            if((t = fence) < 0){
                t = fence = deq.tail;
                index = deq.head;
            }
            return t;
        }

        public DeqSpliterator<E> trySplit(){
            int t = getFence(), h = index, n = deq.elements.length;
            if(h != t && ((h + 1) & (n - 1)) != t){
                if(h > t)
                    t += n;
                int m = ((h + t) >>> 1) & (n - 1);
                return new DeqSpliterator<>(deq, h, index = m);
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super E> consumer){
            if(consumer == null)
                throw new NullPointerException();
            Object[] a = deq.elements;
            int m = a.length - 1, f = getFence(), i = index;
            index = f;
            while (i != f) {
                @SuppressWarnings("unchecked") E e = (E) a[i];
                i = (i + 1) & m;
                if(e == null)
                    throw new ConcurrentModificationException();
                consumer.accept(e);
            }
        }

        public boolean tryAdvance(Consumer<? super E> consumer){
            if(consumer == null)
                throw new NullPointerException();
            Object[] a = deq.elements;
            int m = a.length - 1, f = getFence(), i = index;
            if(i != fence){
                @SuppressWarnings("unchecked") E e = (E) a[i];
                index = (i + 1) & m;
                if(e == null)
                    throw new ConcurrentModificationException();
                consumer.accept(e);
                return true;
            }
            return false;
        }

        public long estimateSize(){
            int n = getFence() - index;
            if(n < 0)
                n += deq.elements.length;
            return (long) n;
        }

        @Override
        public int characteristics(){
            return Spliterator.ORDERED | Spliterator.SIZED |
                   Spliterator.NONNULL | Spliterator.SUBSIZED;
        }
    }

}