ArrayDeque.java

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Josh Bloch of Google Inc. and released to the public domain,
 * as explained at http://creativecommons.org/publicdomain/zero/1.0/.
 */

package java.util;

import java.io.Serializable;
import java.util.function.Consumer;
import java.util.function.Predicate;
import jdk.internal.misc.SharedSecrets;

/**
 * Resizable-array implementation of the {@link Deque} interface.  Array
 * deques have no capacity restrictions; they grow as necessary to support
 * usage.  They are not thread-safe; in the absence of external
 * synchronization, they do not support concurrent access by multiple threads.
 * Null elements are prohibited.  This class is likely to be faster than
 * {@link Stack} when used as a stack, and faster than {@link LinkedList}
 * when used as a queue.
 *
 * <p>Most {@code ArrayDeque} operations run in amortized constant time.
 * Exceptions include
 * {@link #remove(Object) remove},
 * {@link #removeFirstOccurrence removeFirstOccurrence},
 * {@link #removeLastOccurrence removeLastOccurrence},
 * {@link #contains contains},
 * {@link #iterator iterator.remove()},
 * and the bulk operations, all of which run in linear time.
 *
 * <p>The iterators returned by this class's {@link #iterator() iterator}
 * method are <em>fail-fast</em>: If the deque is modified at any time after
 * the iterator is created, in any way except through the iterator's own
 * {@code remove} method, the iterator will generally throw a {@link
 * ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the
 * future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>This class and its iterator implement all of the
 * <em>optional</em> methods of the {@link Collection} and {@link
 * Iterator} interfaces.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch and Doug Lea
 * @param <E> the type of elements held in this deque
 * @since   1.6
 */
/*
 * 顺序无界（队列容量支持扩容到Integer.MAX_VALUE）双向队列，非线程安全
 *
 * 实现了Cloneable接口，支持克隆操作
 */
public class ArrayDeque<E> extends AbstractCollection<E> implements Deque<E>, Cloneable, Serializable {
    
    /*
     * VMs excel at optimizing simple array loops where indices are
     * incrementing or decrementing over a valid slice, e.g.
     *
     * for (int i = start; i < end; i++) ... elements[i]
     *
     * Because in a circular array, elements are in general stored in
     * two disjoint such slices, we help the VM by writing unusual
     * nested loops for all traversals over the elements.  Having only
     * one hot inner loop body instead of two or three eases human
     * maintenance and encourages VM loop inlining into the caller.
     */
    
    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    
    /**
     * The array in which the elements of the deque are stored.
     * All array cells not holding deque elements are always null.
     * The array always has at least one null slot (at tail).
     */
    // 存储队列元素
    transient Object[] elements;
    
    /**
     * 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 0 <= head < elements.length equal to tail if
     * the deque is empty.
     */
    transient int head; // 队头游标
    
    /**
     * The index at which the next element would be added to the tail
     * of the deque (via addLast(E), add(E), or push(E));
     * elements[tail] is always null.
     */
    transient int tail; // 队尾游标
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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) {
        elements = new Object[(numElements<1) ? 1 : (numElements == Integer.MAX_VALUE) ? Integer.MAX_VALUE : numElements + 1];
    }
    
    /**
     * 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 <i>front</i> of the
     * deque.)
     *
     * @param c the collection whose elements are to be placed into the deque
     *
     * @throws NullPointerException if the specified collection is null
     */
    // 用指定容器中的元素初始化队列
    public ArrayDeque(Collection<? extends E> c) {
        this(c.size());
        // 取出容器中的元素，从队尾入队，非线程安全。队满时扩容
        copyElements(c);
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 入队 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Inserts the specified element at the front of this deque.
     *
     * @param e the element to add
     *
     * @return {@code true} (as specified by {@link Deque#offerFirst})
     *
     * @throws NullPointerException if the specified element is null
     */
    // 从队头入队，非线程安全。队满时扩容
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }
    
    /**
     * Inserts the specified element at the end of this deque.
     *
     * @param e the element to add
     *
     * @return {@code true} (as specified by {@link Deque#offerLast})
     *
     * @throws NullPointerException if the specified element is null
     */
    // 从队尾入队，非线程安全。队满时扩容
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }
    
    /**
     * Inserts the specified element at the end of this deque.
     *
     * <p>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
     */
    // 从队尾入队，非线程安全。队满时扩容
    public boolean offer(E e) {
        return offerLast(e);
    }
    
    
    /**
     * Inserts the specified element at the front of this deque.
     *
     * @param e the element to add
     *
     * @throws NullPointerException if the specified element is null
     */
    // 从队头入队，非线程安全。队满时扩容
    public void addFirst(E e) {
        if(e == null) {
            throw new NullPointerException();
        }
        
        final Object[] es = elements;
        
        // 队头游标递减
        head = dec(head, es.length);
        
        // 从队头入队
        es[head] = e;
        
        // 队满后扩容
        if(head == tail) {
            grow(1);
        }
    }
    
    /**
     * Inserts the specified element at the end of this deque.
     *
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     *
     * @throws NullPointerException if the specified element is null
     */
    // 从队尾入队，非线程安全。队满时扩容
    public void addLast(E e) {
        if(e == null) {
            throw new NullPointerException();
        }
        
        final Object[] es = elements;
        
        // 从队尾入队
        es[tail] = e;
        
        // 队尾游标递增
        tail = inc(tail, es.length);
        
        // 队满后扩容
        if(head == tail) {
            grow(1);
        }
    }
    
    /**
     * Inserts the specified element at the end of this deque.
     *
     * <p>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
     */
    // 从队尾入队，非线程安全。队满时扩容
    public boolean add(E e) {
        addLast(e);
        return true;
    }
    
    
    /**
     * Adds all of the elements in the specified collection at the end
     * of this deque, as if by calling {@link #addLast} on each one,
     * in the order that they are returned by the collection's iterator.
     *
     * @param c the elements to be inserted into this deque
     *
     * @return {@code true} if this deque changed as a result of the call
     *
     * @throws NullPointerException if the specified collection or any
     *                              of its elements are null
     */
    // 取出容器中的元素，从队尾入队，非线程安全。队满时扩容
    public boolean addAll(Collection<? extends E> c) {
        final int s = size();
        
        final int needed = s + c.size() + 1 - elements.length;
        
        // 如果需要的空间大于现有空间，则需要扩容
        if(needed>0) {
            grow(needed);
        }
        
        // 取出容器中的元素，从队尾入队，非线程安全。队满时扩容
        copyElements(c);
        
        return size()>s;
    }
    
    /*▲ 入队 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 出队 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    // 从队头出队，线程安全。队空时返回null
    public E pollFirst() {
        final Object[] es = elements;
        final int h = head;
        
        // 取出队头元素
        E e = elementAt(es, h);
        
        // 如果队头元素不为空
        if(e != null) {
            // 置空队头
            es[h] = null;
            
            // 队头游标递增
            head = inc(h, es.length);
        }
        
        // 返回队头元素
        return e;
    }
    
    // 从队尾出队，线程安全。队空时返回null
    public E pollLast() {
        final Object[] es = elements;
        
        // 队尾游标递减
        final int t = dec(tail, es.length);
        
        // 取出队尾元素
        E e = elementAt(es, t);
        
        // 如果队尾元素不为空
        if(e != null) {
            tail = t;
            
            // 置空队尾
            es[tail] = null;
        }
        
        // 返回队尾元素
        return e;
    }
    
    /**
     * 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.
     *
     * <p>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
     */
    // 从队头出队，线程安全。队空时返回null
    public E poll() {
        return pollFirst();
    }
    
    
    /**
     * @throws NoSuchElementException {@inheritDoc}
     */
    // 从队头出队，线程安全。队空时抛异常
    public E removeFirst() {
        E e = pollFirst();
        
        if(e == null) {
            throw new NoSuchElementException();
        }
        
        return e;
    }
    
    /**
     * @throws NoSuchElementException {@inheritDoc}
     */
    // 从队尾出队，线程安全。队空时抛异常
    public E removeLast() {
        E e = pollLast();
        
        if(e == null) {
            throw new NoSuchElementException();
        }
        
        return e;
    }
    
    /**
     * 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.
     *
     * <p>This method is equivalent to {@link #removeFirst}.
     *
     * @return the head of the queue represented by this deque
     *
     * @throws NoSuchElementException {@inheritDoc}
     */
    // 从队头出队，线程安全。队空时抛异常
    public E remove() {
        return removeFirst();
    }
    
    
    /**
     * Removes the first occurrence of the specified element in this
     * deque (when traversing the deque from head to tail).
     * 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).
     *
     * @param o element to be removed from this deque, if present
     *
     * @return {@code true} if the deque contained the specified element
     */
    // 从前往后遍历队列，移除首个包含指定元素的结点
    public boolean removeFirstOccurrence(Object o) {
        if(o != null) {
            final Object[] es = elements;
            for(int i = head, end = tail, to = (i<=end) ? end : es.length; ; i = 0, to = end) {
                for(; i<to; i++) {
                    if(o.equals(es[i])) {
                        // 删除索引i处的元素
                        delete(i);
                        return true;
                    }
                }
                if(to == end) {
                    break;
                }
            }
        }
        return false;
    }
    
    /**
     * Removes the last occurrence of the specified element in this
     * deque (when traversing the deque from head to tail).
     * If the deque does not contain the element, it is unchanged.
     * More formally, removes the last 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).
     *
     * @param o element to be removed from this deque, if present
     *
     * @return {@code true} if the deque contained the specified element
     */
    // 从后往前遍历队列，移除首个包含指定元素的结点
    public boolean removeLastOccurrence(Object o) {
        if(o != null) {
            final Object[] es = elements;
            for(int i = tail, end = head, to = (i >= end) ? end : 0; ; i = es.length, to = end) {
                for(i--; i>to - 1; i--)
                    if(o.equals(es[i])) {
                        // 删除索引i处的元素
                        delete(i);
                        return true;
                    }
                if(to == end)
                    break;
            }
        }
        return false;
    }
    
    /**
     * 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).
     *
     * <p>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
     */
    // 从前往后遍历队列，移除首个包含指定元素的结点
    public boolean remove(Object o) {
        return removeFirstOccurrence(o);
    }
    
    
    /**
     * @throws NullPointerException {@inheritDoc}
     */
    // 移除所有满足过滤条件的元素，不阻塞（非线程安全）
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        return bulkRemove(filter);
    }
    
    
    /**
     * @throws NullPointerException {@inheritDoc}
     */
    // (匹配则移除)移除队列中所有与给定容器中的元素匹配的元素，不阻塞（非线程安全）
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return bulkRemove(e -> c.contains(e));
    }
    
    /**
     * @throws NullPointerException {@inheritDoc}
     */
    // (不匹配则移除)移除队列中所有与给定容器中的元素不匹配的元素，不阻塞（非线程安全）
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return bulkRemove(e -> !c.contains(e));
    }
    
    
    /**
     * Removes all of the elements from this deque.
     * The deque will be empty after this call returns.
     */
    // 清空当前容器中所有元素
    public void clear() {
        circularClear(elements, head, tail);
        head = tail = 0;
    }
    
    /*▲ 出队 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 栈式操作 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Pushes an element onto the stack represented by this deque.  In other
     * words, inserts the element at the front of this deque.
     *
     * <p>This method is equivalent to {@link #addFirst}.
     *
     * @param e the element to push
     *
     * @throws NullPointerException if the specified element is null
     */
    // 从队头入队，非线程安全。队满时扩容（栈式操作）
    public void push(E e) {
        addFirst(e);
    }
    
    /**
     * Pops an element from the stack represented by this deque.  In other
     * words, removes and returns the first element of this deque.
     *
     * <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}
     */
    // 从队头出队，线程安全。队空时抛异常（栈式操作）
    public E pop() {
        return removeFirst();
    }
    
    /*▲ 栈式操作 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 取值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    
    /**
     * 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.
     *
     * <p>This method is equivalent to {@link #getFirst}.
     *
     * @return the head of the queue represented by this deque
     *
     * @throws NoSuchElementException {@inheritDoc}
     */
    // 获取队头元素，非线程安全。队空时抛出异常
    public E element() {
        return getFirst();
    }
    
    /**
     * @throws NoSuchElementException {@inheritDoc}
     */
    // 获取队头元素，非线程安全。队空时抛出异常
    public E getFirst() {
        E e = elementAt(elements, head);
        if(e == null) {
            throw new NoSuchElementException();
        }
        return e;
    }
    
    /**
     * Retrieves, but does not remove, the head of the queue represented by
     * this deque, or returns {@code null} if this deque is empty.
     *
     * <p>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
     */
    // 获取队头元素，非线程安全。队空时返回null
    public E peek() {
        return peekFirst();
    }
    
    // 获取队头元素，非线程安全。队空时返回null
    public E peekFirst() {
        return elementAt(elements, head);
    }
    
    
    /**
     * @throws NoSuchElementException {@inheritDoc}
     */
    // 获取队尾元素，非线程安全。队空时抛出异常
    public E getLast() {
        final Object[] es = elements;
        E e = elementAt(es, dec(tail, es.length));
        if(e == null) {
            throw new NoSuchElementException();
        }
        return e;
    }
    
    // 获取队尾元素，非线程安全。队空时返回null
    public E peekLast() {
        final Object[] es;
        return elementAt(es = elements, dec(tail, es.length));
    }
    
    /*▲ 取值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 包含查询 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    // 判断队列中是否包含元素o
    public boolean contains(Object o) {
        if(o != null) {
            final Object[] es = elements;
            for(int i = head, end = tail, to = (i<=end) ? end : es.length; ; i = 0, to = end) {
                for(; i<to; i++) {
                    if(o.equals(es[i])) {
                        return true;
                    }
                }
                if(to == end) {
                    break;
                }
            }
        }
        return false;
    }
    
    /*▲ 包含查询 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 视图 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns an array containing all of the elements in this deque
     * in proper sequence (from first to last element).
     *
     * <p>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.
     *
     * <p>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 toArray(Object[].class);
    }
    
    /**
     * 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.
     *
     * <p>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}.
     *
     * <p>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.
     *
     * <p>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}:
     *
     * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
     *
     * 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> T[] toArray(T[] a) {
        final int size = size();
        
        if(size>a.length) {
            return toArray((Class<T[]>) a.getClass());
        }
        
        final Object[] es = elements;
        
        int i = head, j = 0, len = Math.min(size, es.length - i);
        
        while(true) {
            System.arraycopy(es, i, a, j, len);
            if((j += len) == size) {
                break;
            }
            
            i = 0;
            len = tail;
        }
        
        if(size<a.length) {
            a[size] = null;
        }
        
        return a;
    }
    
    /*▲ 视图 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 迭代 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * @throws NullPointerException {@inheritDoc}
     */
    // 遍历所有元素，并执行相应的择取操作
    public void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        final Object[] es = elements;
        for(int i = head, end = tail, to = (i<=end) ? end : es.length; ; i = 0, to = end) {
            for(; i<to; i++) {
                action.accept(elementAt(es, i));
            }
            
            if(to == end) {
                if(end != tail) {
                    throw new ConcurrentModificationException();
                }
                break;
            }
        }
    }
    
    
    /**
     * 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
     */
    // 返回当前队列的顺序迭代器
    public Iterator<E> iterator() {
        return new DeqIterator();
    }
    
    // 返回当前队列的逆序迭代器
    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }
    
    /**
     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
     * deque.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
     * {@link Spliterator#SUBSIZED}, {@link Spliterator#ORDERED}, and
     * {@link Spliterator#NONNULL}.  Overriding implementations should document
     * the reporting of additional characteristic values.
     *
     * @return a {@code Spliterator} over the elements in this deque
     *
     * @since 1.8
     */
    // 返回描述此队列中元素的Spliterator
    public Spliterator<E> spliterator() {
        return new DeqSpliterator();
    }
    
    /*▲ 迭代 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the number of elements in this deque.
     *
     * @return the number of elements in this deque
     */
    // 返回队列中元素数量
    public int size() {
        return sub(tail, head, elements.length);
    }
    
    /**
     * Returns {@code true} if this deque contains no elements.
     *
     * @return {@code true} if this deque contains no elements
     */
    // 判断队列是否为空
    public boolean isEmpty() {
        return head == tail;
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 序列化 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    private static final long serialVersionUID = 2340985798034038923L;
    
    
    /**
     * Saves this deque to a stream (that is, serializes it).
     *
     * @param s the stream
     *
     * @throws java.io.IOException if an I/O error occurs
     * @serialData The current size ({@code int}) of the deque,
     * followed by all of its elements (each an object reference) in
     * first-to-last order.
     */
    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
        s.defaultWriteObject();
        
        // Write out size
        s.writeInt(size());
        
        // Write out elements in order.
        final Object[] es = elements;
        for(int i = head, end = tail, to = (i<=end) ? end : es.length; ; i = 0, to = end) {
            for(; i<to; i++)
                s.writeObject(es[i]);
            if(to == end)
                break;
        }
    }
    
    /**
     * Reconstitutes this deque from a stream (that is, deserializes it).
     *
     * @param s the stream
     *
     * @throws ClassNotFoundException if the class of a serialized object
     *                                could not be found
     * @throws java.io.IOException    if an I/O error occurs
     */
    private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        
        // Read in size and allocate array
        int size = s.readInt();
        SharedSecrets.getJavaObjectInputStreamAccess().checkArray(s, Object[].class, size + 1);
        elements = new Object[size + 1];
        this.tail = size;
        
        // Read in all elements in the proper order.
        for(int i = 0; i<size; i++)
            elements[i] = s.readObject();
    }
    
    /*▲ 序列化 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Returns a copy of this deque.
     *
     * @return a copy of this deque
     */
    // 克隆
    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();
        }
    }
    
    
    
    // 取出容器中的元素，从队尾入队，非线程安全。队满时扩容
    private void copyElements(Collection<? extends E> c) {
        for(E e : c) {
            addLast(e);
        }
    }
    
    /**
     * Removes the element at the specified position in the elements array.
     * This can result in forward or backwards motion of array elements.
     * We optimize for least element motion.
     *
     * <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 near tail moved backwards
     */
    // 删除索引i处的元素（通过优化，尽量移动较少的元素）
    boolean delete(int i) {
        final Object[] es = elements;
        final int capacity = es.length;
        final int h = head;
        final int t = tail;
        
        // number of elements before to-be-deleted elt
        final int front = sub(i, h, capacity);
        
        // number of elements after to-be-deleted elt
        final int back = sub(t, i, capacity) - 1;
        
        if(front<back) {
            // move front elements forwards
            if(h<=i) {
                System.arraycopy(es, h, es, h + 1, front);
            } else { // Wrap around
                System.arraycopy(es, 0, es, 1, i);
                es[0] = es[capacity - 1];
                System.arraycopy(es, h, es, h + 1, front - (i + 1));
            }
            es[h] = null;
            head = inc(h, capacity);
            return false;
        } else {
            // move back elements backwards
            tail = dec(t, capacity);
            if(i<=tail) {
                System.arraycopy(es, i + 1, es, i, back);
            } else { // Wrap around
                System.arraycopy(es, i + 1, es, i, capacity - (i + 1));
                es[capacity - 1] = es[0];
                System.arraycopy(es, 1, es, 0, t - 1);
            }
            es[tail] = null;
            return true;
        }
    }
    
    /** Implementation of bulk remove methods. */
    // 批量移除元素，即满足过滤条件的元素将被移除，不阻塞（非线程安全）
    private boolean bulkRemove(Predicate<? super E> filter) {
        final Object[] es = elements;
        
        // Optimize for initial run of survivors
        for(int i = head, end = tail, to = (i<=end) ? end : es.length; ; i = 0, to = end) {
            for(; i<to; i++) {
                if(filter.test(elementAt(es, i))) {
                    return bulkRemoveModified(filter, i);
                }
            }
            
            if(to == end) {
                if(end != tail) {
                    throw new ConcurrentModificationException();
                }
                break;
            }
        }
        return false;
    }
    
    /**
     * Helper for bulkRemove, in case of at least one deletion.
     * Tolerate predicates that reentrantly access the collection for
     * read (but writers still get CME), so traverse once to find
     * elements to delete, a second pass to physically expunge.
     *
     * @param beg valid index of first element to be deleted
     */
    // 清空，即移除所有元素，不阻塞（非线程安全）
    private boolean bulkRemoveModified(Predicate<? super E> filter, final int beg) {
        final Object[] es = elements;
        final int capacity = es.length;
        final int end = tail;
        final long[] deathRow = nBits(sub(end, beg, capacity));
        
        deathRow[0] = 1L;   // set bit 0
        for(int i = beg + 1, to = (i<=end) ? end : es.length, k = beg; ; i = 0, to = end, k -= capacity) {
            for(; i<to; i++) {
                if(filter.test(elementAt(es, i))) {
                    setBit(deathRow, i - k);
                }
            }
            
            if(to == end) {
                break;
            }
        }
        
        // a two-finger traversal, with hare i reading, tortoise w writing
        int w = beg;
        for(int i = beg + 1, to = (i<=end) ? end : es.length, k = beg; ; w = 0) { // w rejoins i on second leg
            // In this loop, i and w are on the same leg, with i > w
            for(; i<to; i++) {
                if(isClear(deathRow, i - k)) {
                    es[w++] = es[i];
                }
            }
            
            if(to == end) {
                break;
            }
            
            // In this loop, w is on the first leg, i on the second
            for(i = 0, to = end, k -= capacity; i<to && w<capacity; i++) {
                if(isClear(deathRow, i - k)) {
                    es[w++] = es[i];
                }
            }
            
            if(i >= to) {
                if(w == capacity) {
                    w = 0; // "corner" case
                }
                break;
            }
        }
        
        if(end != tail) {
            throw new ConcurrentModificationException();
        }
        
        circularClear(es, tail = w, end);
        
        return true;
    }
    
    /**
     * Nulls out slots starting at array index i, upto index end.
     * Condition i == end means "empty" - nothing to do.
     */
    private static void circularClear(Object[] es, int i, int end) {
        // assert 0 <= i && i < es.length;
        // assert 0 <= end && end < es.length;
        for(int to = (i<=end) ? end : es.length; ; i = 0, to = end) {
            for(; i<to; i++) {
                es[i] = null;
            }
            
            if(to == end) {
                break;
            }
        }
    }
    
    private <T> T[] toArray(Class<T[]> klazz) {
        final Object[] es = elements;
        
        final T[] a;
        
        final int head = this.head;
        final int tail = this.tail;
        final int end;
        
        if((end = tail + ((head<=tail) ? 0 : es.length)) >= 0) {
            /* Uses null extension feature of copyOfRange */
            // 截取es中[head, head+end)范围内的Object值，并强转为klazz类型的值，然后返回
            a = Arrays.copyOfRange(es, head, end, klazz);
            
            // 溢出
        } else {
            // integer overflow!
            a = Arrays.copyOfRange(es, 0, end - head, klazz);
            System.arraycopy(es, head, a, 0, es.length - head);
        }
        
        if(end != tail) {
            System.arraycopy(es, 0, a, es.length - head, tail);
        }
        
        return a;
    }
    
    /**
     * Increases the capacity of this deque by at least the given amount.
     *
     * @param needed the required minimum extra capacity; must be positive
     */
    // 扩容
    private void grow(int needed) {
        // overflow-conscious code
        final int oldCapacity = elements.length;
        
        // Double capacity if small; else grow by 50%
        int jump = (oldCapacity<64) ? (oldCapacity + 2) : (oldCapacity >> 1);
        
        int newCapacity;
        
        if(jump<needed || (newCapacity = (oldCapacity + jump)) - MAX_ARRAY_SIZE>0) {
            newCapacity = newCapacity(needed, jump);
        }
        
        final Object[] es = elements = Arrays.copyOf(elements, newCapacity);
        
        // Exceptionally, here tail == head needs to be disambiguated
        if(tail<head || (tail == head && es[head] != null)) {
            // wrap around; slide first leg forward to end of array
            int newSpace = newCapacity - oldCapacity;
            System.arraycopy(es, head, es, head + newSpace, oldCapacity - head);
            for(int i = head, to = (head += newSpace); i<to; i++) {
                es[i] = null;
            }
        }
    }
    
    /** Capacity calculation for edge conditions, especially overflow. */
    private int newCapacity(int needed, int jump) {
        final int oldCapacity = elements.length, minCapacity;
        
        if((minCapacity = oldCapacity + needed) - MAX_ARRAY_SIZE>0) {
            if(minCapacity<0) {
                throw new IllegalStateException("Sorry, deque too big");
            }
            return Integer.MAX_VALUE;
        }
        
        if(needed>jump) {
            return minCapacity;
        }
        
        return (oldCapacity + jump - MAX_ARRAY_SIZE<0) ? oldCapacity + jump : MAX_ARRAY_SIZE;
    }
    
    /**
     * Returns element at array index i.
     * This is a slight abuse of generics, accepted by javac.
     */
    // 返回es[i]
    @SuppressWarnings("unchecked")
    static final <E> E elementAt(Object[] es, int i) {
        return (E) es[i];
    }
    
    /**
     * A version of elementAt that checks for null elements.
     * This check doesn't catch all possible comodifications,
     * but does catch ones that corrupt traversal.
     */
    // 返回es[i]，为null时抛异常
    static final <E> E nonNullElementAt(Object[] es, int i) {
        @SuppressWarnings("unchecked")
        E e = (E) es[i];
        if(e == null) {
            throw new ConcurrentModificationException();
        }
        return e;
    }
    
    /**
     * Circularly increments i, mod modulus.
     * Precondition and postcondition: 0 <= i < modulus.
     */
    // 递增i，会轮转
    static final int inc(int i, int modulus) {
        if(++i >= modulus) {
            i = 0;
        }
        return i;
    }
    
    /**
     * Circularly decrements i, mod modulus.
     * Precondition and postcondition: 0 <= i < modulus.
     */
    // 递减i，会轮转
    static final int dec(int i, int modulus) {
        if(--i<0) {
            i = modulus - 1;
        }
        return i;
    }
    
    /**
     * Circularly adds the given distance to index i, mod modulus.
     * Precondition: 0 <= i < modulus, 0 <= distance <= modulus.
     *
     * @return index 0 <= i < modulus
     */
    // 将i增distance，会轮转
    static final int inc(int i, int distance, int modulus) {
        i += distance;
        
        if(i - modulus >= 0) {
            i -= modulus;
        }
        
        return i;
    }
    
    /**
     * Subtracts j from i, mod modulus.
     * Index i must be logically ahead of index j.
     * Precondition: 0 <= i < modulus, 0 <= j < modulus.
     *
     * @return the "circular distance" from j to i; corner case i == j
     * is disambiguated to "empty", returning 0.
     */
    // 将i减distance，会轮转（可用于计算i到j之间的元素个数）
    static final int sub(int i, int j, int modulus) {
        i -= j;
        
        if(i<0) {
            i += modulus;
        }
        
        return i;
    }
    
    private static long[] nBits(int n) {
        return new long[((n - 1) >> 6) + 1];
    }
    
    private static void setBit(long[] bits, int i) {
        bits[i >> 6] |= 1L << i;
    }
    
    private static boolean isClear(long[] bits, int i) {
        return (bits[i >> 6] & (1L << i)) == 0;
    }
    
    /** debugging */
    void checkInvariants() {
        // Use head and tail fields with empty slot at tail strategy.
        // head == tail disambiguates to "empty".
        try {
            int capacity = elements.length;
            // assert 0 <= head && head < capacity;
            // assert 0 <= tail && tail < capacity;
            // assert capacity > 0;
            // assert size() < capacity;
            // assert head == tail || elements[head] != null;
            // assert elements[tail] == null;
            // assert head == tail || elements[dec(tail, capacity)] != null;
        } catch(Throwable t) {
            System.err.printf("head=%d tail=%d capacity=%d%n", head, tail, elements.length);
            System.err.printf("elements=%s%n", Arrays.toString(elements));
            throw t;
        }
    }
    
    
    
    
    
    
    // 顺序迭代器
    private class DeqIterator implements Iterator<E> {
        /** Index of element to be returned by subsequent call to next. */
        int cursor;             // 当前将要从优先队列中访问的元素的索引
        
        /**
         * Index of element returned by most recent call to next.
         * Reset to -1 if element is deleted by a call to remove.
         */
        int lastRet = -1;       // 上次从优先队列中访问过的元素的索引
        
        /** Number of elements yet to be returned. */
        int remaining = size(); // 未遍历元素数量
        
        DeqIterator() {
            cursor = head;
        }
        
        public final boolean hasNext() {
            return remaining>0;
        }
        
        public E next() {
            if(remaining<=0) {
                throw new NoSuchElementException();
            }
            
            final Object[] es = elements;
            
            E e = nonNullElementAt(es, cursor);
            
            // 游标递增
            cursor = inc(lastRet=cursor, es.length);
            
            // 未遍历元素数量
            remaining--;
            
            return e;
        }
        
        // 移除刚刚遍历的那个元素
        public final void remove() {
            if(lastRet<0) {
                throw new IllegalStateException();
            }
            
            // 删除索引lastRet处的元素
            postDelete(delete(lastRet));
            
            lastRet = -1;
        }
        
        // 如果成功删除了元素，需要递减当前游标，以回到刚刚遍历的位置
        void postDelete(boolean leftShifted) {
            if(leftShifted) {
                cursor = dec(cursor, elements.length);
            }
        }
        
        // 遍历剩余元素，并对其执行相应的择取操作
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            
            int r = remaining;
            if(r<=0) {
                return;
            }
            
            remaining = 0;
            
            final Object[] es = elements;
            if(es[cursor] == null || sub(tail, cursor, es.length) != r) {
                throw new ConcurrentModificationException();
            }
            
            for(int i = cursor, end = tail, to = (i<=end) ? end : es.length; ; i = 0, to = end) {
                for(; i<to; i++) {
                    action.accept(elementAt(es, i));
                }
                
                if(to == end) {
                    if(end != tail) {
                        throw new ConcurrentModificationException();
                    }
                    lastRet = dec(end, es.length);
                    break;
                }
            }
        }
        
    }
    
    // 逆序迭代器
    private class DescendingIterator extends DeqIterator {
        DescendingIterator() {
            cursor = dec(tail, elements.length);
        }
        
        public final E next() {
            if(remaining<=0) {
                throw new NoSuchElementException();
            }
            final Object[] es = elements;
            E e = nonNullElementAt(es, cursor);
            cursor = dec(lastRet = cursor, es.length);
            remaining--;
            return e;
        }
        
        // 遍历剩余元素，并对其执行相应的择取操作
        public final void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            int r = remaining;
            if(r<=0) {
                return;
            }
            
            remaining = 0;
            
            final Object[] es = elements;
            if(es[cursor] == null || sub(cursor, head, es.length) + 1 != r) {
                throw new ConcurrentModificationException();
            }
            
            for(int i = cursor, end = head, to = (i >= end) ? end : 0; ; i = es.length - 1, to = end) {
                // hotspot generates faster code than for: i >= to !
                for(; i>to - 1; i--) {
                    action.accept(elementAt(es, i));
                }
                
                if(to == end) {
                    if(end != head) {
                        throw new ConcurrentModificationException();
                    }
                    lastRet = end;
                    break;
                }
            }
        }
        
        // 如果成功删除了元素，需要递增当前游标，以回到刚刚遍历的位置
        void postDelete(boolean leftShifted) {
            if(!leftShifted) {
                cursor = inc(cursor, elements.length);
            }
        }
    }
    
    // 描述此队列中元素的Spliterator
    final class DeqSpliterator implements Spliterator<E> {
        private int fence;      // -1 until first use
        private int cursor;     // current index, modified on traverse/split
        
        /** Constructs late-binding spliterator over all elements. */
        DeqSpliterator() {
            this.fence = -1;
        }
        
        /** Constructs spliterator over the given range. */
        DeqSpliterator(int origin, int fence) {
            // assert 0 <= origin && origin < elements.length;
            // assert 0 <= fence && fence < elements.length;
            this.cursor = origin;
            this.fence = fence;
        }
        
        public DeqSpliterator trySplit() {
            final Object[] es = elements;
            final int i, n;
            return ((n = sub(getFence(), i = cursor, es.length) >> 1)<=0) ? null : new DeqSpliterator(i, cursor = inc(i, n, es.length));
        }
        
        public void forEachRemaining(Consumer<? super E> action) {
            if(action == null)
                throw new NullPointerException();
            final int end = getFence(), cursor = this.cursor;
            final Object[] es = elements;
            if(cursor != end) {
                this.cursor = end;
                // null check at both ends of range is sufficient
                if(es[cursor] == null || es[dec(end, es.length)] == null)
                    throw new ConcurrentModificationException();
                for(int i = cursor, to = (i<=end) ? end : es.length; ; i = 0, to = end) {
                    for(; i<to; i++)
                        action.accept(elementAt(es, i));
                    if(to == end)
                        break;
                }
            }
        }
        
        public boolean tryAdvance(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            final Object[] es = elements;
            if(fence<0) {
                fence = tail;
                cursor = head;
            } // late-binding
            final int i;
            if((i = cursor) == fence)
                return false;
            E e = nonNullElementAt(es, i);
            cursor = inc(i, es.length);
            action.accept(e);
            return true;
        }
        
        public long estimateSize() {
            return sub(getFence(), cursor, elements.length);
        }
        
        public int characteristics() {
            return Spliterator.NONNULL | Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
        
        /** Ensures late-binding initialization; then returns fence. */
        private int getFence() { // force initialization
            int t;
            if((t = fence)<0) {
                t = fence = tail;
                cursor = head;
            }
            return t;
        }
    }
}