/*
 * 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 Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.Consumer;
import java.util.function.Predicate;

An optionally-bounded blocking deque based on linked nodes. 
The optional capacity bound constructor argument serves as a way to prevent excessive expansion. The capacity, if unspecified, is equal to Integer.MAX_VALUE. Linked nodes are dynamically created upon each insertion unless this would bring the deque above capacity. 
Most operations run in constant time (ignoring time spent blocking). Exceptions include remove, removeFirstOccurrence, removeLastOccurrence, 
contains, iterator.remove(), and the bulk operations, all of which run in linear time. 
This class and its iterator implement all of the optional methods of the Collection and Iterator interfaces. 
This class is a member of the

Java Collections Framework.
Author:
 Doug Lea
Type parameters:
<E> – the type of elements held in this deque
Since:
1.6/**
 * An optionally-bounded {@linkplain BlockingDeque blocking deque} based on
 * linked nodes.
 *
 * <p>The optional capacity bound constructor argument serves as a
 * way to prevent excessive expansion. The capacity, if unspecified,
 * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are
 * dynamically created upon each insertion unless this would bring the
 * deque above capacity.
 *
 * <p>Most operations run in constant time (ignoring time spent
 * blocking).  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>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/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @since 1.6
 * @author  Doug Lea
 * @param <E> the type of elements held in this deque
 */
public class LinkedBlockingDeque<E>
    extends AbstractQueue<E>
    implements BlockingDeque<E>, java.io.Serializable {

    /*
     * Implemented as a simple doubly-linked list protected by a
     * single lock and using conditions to manage blocking.
     *
     * To implement weakly consistent iterators, it appears we need to
     * keep all Nodes GC-reachable from a predecessor dequeued Node.
     * That would cause two problems:
     * - allow a rogue Iterator to cause unbounded memory retention
     * - cause cross-generational linking of old Nodes to new Nodes if
     *   a Node was tenured while live, which generational GCs have a
     *   hard time dealing with, causing repeated major collections.
     * However, only non-deleted Nodes need to be reachable from
     * dequeued Nodes, and reachability does not necessarily have to
     * be of the kind understood by the GC.  We use the trick of
     * linking a Node that has just been dequeued to itself.  Such a
     * self-link implicitly means to jump to "first" (for next links)
     * or "last" (for prev links).
     */

    /*
     * We have "diamond" multiple interface/abstract class inheritance
     * here, and that introduces ambiguities. Often we want the
     * BlockingDeque javadoc combined with the AbstractQueue
     * implementation, so a lot of method specs are duplicated here.
     */

    private static final long serialVersionUID = -387911632671998426L;

    
Doubly-linked list node class /** Doubly-linked list node class */
    static final class Node<E> {
        
The item, or null if this node has been removed.
/**
         * The item, or null if this node has been removed.
         */
        E item;

        
One of:
- the real predecessor Node
- this Node, meaning the predecessor is tail
- null, meaning there is no predecessor
/**
         * One of:
         * - the real predecessor Node
         * - this Node, meaning the predecessor is tail
         * - null, meaning there is no predecessor
         */
        Node<E> prev;

        
One of:
- the real successor Node
- this Node, meaning the successor is head
- null, meaning there is no successor
/**
         * One of:
         * - the real successor Node
         * - this Node, meaning the successor is head
         * - null, meaning there is no successor
         */
        Node<E> next;

        Node(E x) {
            item = x;
        }
    }

    
Pointer to first node.
Invariant: (first == null && last == null) ||
           (first.prev == null && first.item != null)
/**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    transient Node<E> first;

    
Pointer to last node.
Invariant: (first == null && last == null) ||
           (last.next == null && last.item != null)
/**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    transient Node<E> last;

    
Number of items in the deque /** Number of items in the deque */
    private transient int count;

    
Maximum number of items in the deque /** Maximum number of items in the deque */
    private final int capacity;

    
Main lock guarding all access /** Main lock guarding all access */
    final ReentrantLock lock = new ReentrantLock();

    
Condition for waiting takes /** Condition for waiting takes */
    private final Condition notEmpty = lock.newCondition();

    
Condition for waiting puts /** Condition for waiting puts */
    private final Condition notFull = lock.newCondition();

    
Creates a LinkedBlockingDeque with a capacity of Integer.MAX_VALUE. /**
     * Creates a {@code LinkedBlockingDeque} with a capacity of
     * {@link Integer#MAX_VALUE}.
     */
    public LinkedBlockingDeque() {
        this(Integer.MAX_VALUE);
    }

    
Creates a LinkedBlockingDeque with the given (fixed) capacity. 
Params:
capacity – the capacity of this deque
Throws:
IllegalArgumentException – if capacity is less than 1/**
     * Creates a {@code LinkedBlockingDeque} with the given (fixed) capacity.
     *
     * @param capacity the capacity of this deque
     * @throws IllegalArgumentException if {@code capacity} is less than 1
     */
    public LinkedBlockingDeque(int capacity) {
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
    }

    
Creates a LinkedBlockingDeque with a capacity of Integer.MAX_VALUE, initially containing the elements of the given collection, added in traversal order of the collection's iterator. 
Params:
c – the collection of elements to initially contain
Throws:
NullPointerException – if the specified collection or any
        of its elements are null/**
     * Creates a {@code LinkedBlockingDeque} with a capacity of
     * {@link Integer#MAX_VALUE}, initially containing the elements of
     * the given collection, added in traversal order of the
     * collection's iterator.
     *
     * @param c the collection of elements to initially contain
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public LinkedBlockingDeque(Collection<? extends E> c) {
        this(Integer.MAX_VALUE);
        addAll(c);
    }


    // Basic linking and unlinking operations, called only while holding lock

    
Links node as first element, or returns false if full.
/**
     * Links node as first element, or returns false if full.
     */
    private boolean linkFirst(Node<E> node) {
        // assert lock.isHeldByCurrentThread();
        if (count >= capacity)
            return false;
        Node<E> f = first;
        node.next = f;
        first = node;
        if (last == null)
            last = node;
        else
            f.prev = node;
        ++count;
        notEmpty.signal();
        return true;
    }

    
Links node as last element, or returns false if full.
/**
     * Links node as last element, or returns false if full.
     */
    private boolean linkLast(Node<E> node) {
        // assert lock.isHeldByCurrentThread();
        if (count >= capacity)
            return false;
        Node<E> l = last;
        node.prev = l;
        last = node;
        if (first == null)
            first = node;
        else
            l.next = node;
        ++count;
        notEmpty.signal();
        return true;
    }

    
Removes and returns first element, or null if empty.
/**
     * Removes and returns first element, or null if empty.
     */
    private E unlinkFirst() {
        // assert lock.isHeldByCurrentThread();
        Node<E> f = first;
        if (f == null)
            return null;
        Node<E> n = f.next;
        E item = f.item;
        f.item = null;
        f.next = f; // help GC
        first = n;
        if (n == null)
            last = null;
        else
            n.prev = null;
        --count;
        notFull.signal();
        return item;
    }

    
Removes and returns last element, or null if empty.
/**
     * Removes and returns last element, or null if empty.
     */
    private E unlinkLast() {
        // assert lock.isHeldByCurrentThread();
        Node<E> l = last;
        if (l == null)
            return null;
        Node<E> p = l.prev;
        E item = l.item;
        l.item = null;
        l.prev = l; // help GC
        last = p;
        if (p == null)
            first = null;
        else
            p.next = null;
        --count;
        notFull.signal();
        return item;
    }

    
Unlinks x.
/**
     * Unlinks x.
     */
    void unlink(Node<E> x) {
        // assert lock.isHeldByCurrentThread();
        // assert x.item != null;
        Node<E> p = x.prev;
        Node<E> n = x.next;
        if (p == null) {
            unlinkFirst();
        } else if (n == null) {
            unlinkLast();
        } else {
            p.next = n;
            n.prev = p;
            x.item = null;
            // Don't mess with x's links.  They may still be in use by
            // an iterator.
            --count;
            notFull.signal();
        }
    }

    // BlockingDeque methods

    
Throws:
IllegalStateException – if this deque is full
NullPointerException – {@inheritDoc}/**
     * @throws IllegalStateException if this deque is full
     * @throws NullPointerException {@inheritDoc}
     */
    public void addFirst(E e) {
        if (!offerFirst(e))
            throw new IllegalStateException("Deque full");
    }

    
Throws:
IllegalStateException – if this deque is full
NullPointerException –  {@inheritDoc}/**
     * @throws IllegalStateException if this deque is full
     * @throws NullPointerException  {@inheritDoc}
     */
    public void addLast(E e) {
        if (!offerLast(e))
            throw new IllegalStateException("Deque full");
    }

    
Throws:
NullPointerException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offerFirst(E e) {
        if (e == null) throw new NullPointerException();
        Node<E> node = new Node<E>(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return linkFirst(node);
        } finally {
            lock.unlock();
        }
    }

    
Throws:
NullPointerException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offerLast(E e) {
        if (e == null) throw new NullPointerException();
        Node<E> node = new Node<E>(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return linkLast(node);
        } finally {
            lock.unlock();
        }
    }

    
Throws:
NullPointerException – {@inheritDoc}
InterruptedException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     * @throws InterruptedException {@inheritDoc}
     */
    public void putFirst(E e) throws InterruptedException {
        if (e == null) throw new NullPointerException();
        Node<E> node = new Node<E>(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            while (!linkFirst(node))
                notFull.await();
        } finally {
            lock.unlock();
        }
    }

    
Throws:
NullPointerException – {@inheritDoc}
InterruptedException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     * @throws InterruptedException {@inheritDoc}
     */
    public void putLast(E e) throws InterruptedException {
        if (e == null) throw new NullPointerException();
        Node<E> node = new Node<E>(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            while (!linkLast(node))
                notFull.await();
        } finally {
            lock.unlock();
        }
    }

    
Throws:
NullPointerException – {@inheritDoc}
InterruptedException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     * @throws InterruptedException {@inheritDoc}
     */
    public boolean offerFirst(E e, long timeout, TimeUnit unit)
        throws InterruptedException {
        if (e == null) throw new NullPointerException();
        Node<E> node = new Node<E>(e);
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (!linkFirst(node)) {
                if (nanos <= 0L)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
            return true;
        } finally {
            lock.unlock();
        }
    }

    
Throws:
NullPointerException – {@inheritDoc}
InterruptedException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     * @throws InterruptedException {@inheritDoc}
     */
    public boolean offerLast(E e, long timeout, TimeUnit unit)
        throws InterruptedException {
        if (e == null) throw new NullPointerException();
        Node<E> node = new Node<E>(e);
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (!linkLast(node)) {
                if (nanos <= 0L)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
            return true;
        } finally {
            lock.unlock();
        }
    }

    
Throws:
NoSuchElementException – {@inheritDoc}/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E removeFirst() {
        E x = pollFirst();
        if (x == null) throw new NoSuchElementException();
        return x;
    }

    
Throws:
NoSuchElementException – {@inheritDoc}/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E removeLast() {
        E x = pollLast();
        if (x == null) throw new NoSuchElementException();
        return x;
    }

    public E pollFirst() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return unlinkFirst();
        } finally {
            lock.unlock();
        }
    }

    public E pollLast() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return unlinkLast();
        } finally {
            lock.unlock();
        }
    }

    public E takeFirst() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            E x;
            while ( (x = unlinkFirst()) == null)
                notEmpty.await();
            return x;
        } finally {
            lock.unlock();
        }
    }

    public E takeLast() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            E x;
            while ( (x = unlinkLast()) == null)
                notEmpty.await();
            return x;
        } finally {
            lock.unlock();
        }
    }

    public E pollFirst(long timeout, TimeUnit unit)
        throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            E x;
            while ( (x = unlinkFirst()) == null) {
                if (nanos <= 0L)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
            return x;
        } finally {
            lock.unlock();
        }
    }

    public E pollLast(long timeout, TimeUnit unit)
        throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            E x;
            while ( (x = unlinkLast()) == null) {
                if (nanos <= 0L)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
            return x;
        } finally {
            lock.unlock();
        }
    }

    
Throws:
NoSuchElementException – {@inheritDoc}/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E getFirst() {
        E x = peekFirst();
        if (x == null) throw new NoSuchElementException();
        return x;
    }

    
Throws:
NoSuchElementException – {@inheritDoc}/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E getLast() {
        E x = peekLast();
        if (x == null) throw new NoSuchElementException();
        return x;
    }

    public E peekFirst() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (first == null) ? null : first.item;
        } finally {
            lock.unlock();
        }
    }

    public E peekLast() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (last == null) ? null : last.item;
        } finally {
            lock.unlock();
        }
    }

    public boolean removeFirstOccurrence(Object o) {
        if (o == null) return false;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (Node<E> p = first; p != null; p = p.next) {
                if (o.equals(p.item)) {
                    unlink(p);
                    return true;
                }
            }
            return false;
        } finally {
            lock.unlock();
        }
    }

    public boolean removeLastOccurrence(Object o) {
        if (o == null) return false;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (Node<E> p = last; p != null; p = p.prev) {
                if (o.equals(p.item)) {
                    unlink(p);
                    return true;
                }
            }
            return false;
        } finally {
            lock.unlock();
        }
    }

    // BlockingQueue methods

    
Inserts the specified element at the end of this deque unless it would violate capacity restrictions. When using a capacity-restricted deque, it is generally preferable to use method offer. 
This method is equivalent to addLast. 
Throws:
IllegalStateException – if this deque is full
NullPointerException – if the specified element is null/**
     * Inserts the specified element at the end of this deque unless it would
     * violate capacity restrictions.  When using a capacity-restricted deque,
     * it is generally preferable to use method {@link #offer(Object) offer}.
     *
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @throws IllegalStateException if this deque is full
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        addLast(e);
        return true;
    }

    
Throws:
NullPointerException – if the specified element is null/**
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        return offerLast(e);
    }

    
Throws:
NullPointerException – {@inheritDoc}
InterruptedException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     * @throws InterruptedException {@inheritDoc}
     */
    public void put(E e) throws InterruptedException {
        putLast(e);
    }

    
Throws:
NullPointerException – {@inheritDoc}
InterruptedException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     * @throws InterruptedException {@inheritDoc}
     */
    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {
        return offerLast(e, timeout, unit);
    }

    
Retrieves and removes the head of the queue represented by this deque. This method differs from poll() only in that it throws an exception if this deque is empty. 
This method is equivalent to removeFirst. 
Throws:
NoSuchElementException – if this deque is empty
Returns:
the head of the queue represented by this deque/**
     * 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() removeFirst}.
     *
     * @return the head of the queue represented by this deque
     * @throws NoSuchElementException if this deque is empty
     */
    public E remove() {
        return removeFirst();
    }

    public E poll() {
        return pollFirst();
    }

    public E take() throws InterruptedException {
        return takeFirst();
    }

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        return pollFirst(timeout, unit);
    }

    
Retrieves, but does not remove, the head of the queue represented by this deque. This method differs from peek() only in that it throws an exception if this deque is empty. 
This method is equivalent to getFirst. 
Throws:
NoSuchElementException – if this deque is empty
Returns:
the head of the queue represented by this deque/**
     * 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() getFirst}.
     *
     * @return the head of the queue represented by this deque
     * @throws NoSuchElementException if this deque is empty
     */
    public E element() {
        return getFirst();
    }

    public E peek() {
        return peekFirst();
    }

    
Returns the number of additional elements that this deque can ideally (in the absence of memory or resource constraints) accept without blocking. This is always equal to the initial capacity of this deque less the current size of this deque. 
Note that you cannot always tell if an attempt to insert an element will succeed by inspecting remainingCapacity because it may be the case that another thread is about to insert or remove an element. /**
     * Returns the number of additional elements that this deque can ideally
     * (in the absence of memory or resource constraints) accept without
     * blocking. This is always equal to the initial capacity of this deque
     * less the current {@code size} of this deque.
     *
     * <p>Note that you <em>cannot</em> always tell if an attempt to insert
     * an element will succeed by inspecting {@code remainingCapacity}
     * because it may be the case that another thread is about to
     * insert or remove an element.
     */
    public int remainingCapacity() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return capacity - count;
        } finally {
            lock.unlock();
        }
    }

    
Throws:
UnsupportedOperationException – {@inheritDoc}
ClassCastException –            {@inheritDoc}
NullPointerException –          {@inheritDoc}
IllegalArgumentException –      {@inheritDoc}/**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c) {
        return drainTo(c, Integer.MAX_VALUE);
    }

    
Throws:
UnsupportedOperationException – {@inheritDoc}
ClassCastException –            {@inheritDoc}
NullPointerException –          {@inheritDoc}
IllegalArgumentException –      {@inheritDoc}/**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c, int maxElements) {
        Objects.requireNonNull(c);
        if (c == this)
            throw new IllegalArgumentException();
        if (maxElements <= 0)
            return 0;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = Math.min(maxElements, count);
            for (int i = 0; i < n; i++) {
                c.add(first.item);   // In this order, in case add() throws.
                unlinkFirst();
            }
            return n;
        } finally {
            lock.unlock();
        }
    }

    // Stack methods

    
Throws:
IllegalStateException – if this deque is full
NullPointerException – {@inheritDoc}/**
     * @throws IllegalStateException if this deque is full
     * @throws NullPointerException {@inheritDoc}
     */
    public void push(E e) {
        addFirst(e);
    }

    
Throws:
NoSuchElementException – {@inheritDoc}/**
     * @throws NoSuchElementException {@inheritDoc}
     */
    public E pop() {
        return removeFirst();
    }

    // Collection methods

    
Removes the first occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first element e such that o.equals(e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call). 
This method is equivalent to removeFirstOccurrence. 
Params:
o – element to be removed from this deque, if present
Returns:
true if this deque changed as a result of the call/**
     * Removes the first occurrence 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) removeFirstOccurrence}.
     *
     * @param o element to be removed from this deque, if present
     * @return {@code true} if this deque changed as a result of the call
     */
    public boolean remove(Object o) {
        return removeFirstOccurrence(o);
    }

    
Returns the number of elements in this deque.
Returns:
the number of elements in this deque/**
     * Returns the number of elements in this deque.
     *
     * @return the number of elements in this deque
     */
    public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }

    
Returns true if this deque contains the specified element. More formally, returns true if and only if this deque contains at least one element e such that o.equals(e). 
Params:
o – object to be checked for containment in this deque
Returns:
true if this deque contains the specified element/**
     * Returns {@code true} if this deque contains the specified element.
     * More formally, returns {@code true} if and only if this deque contains
     * at least one element {@code e} such that {@code o.equals(e)}.
     *
     * @param o object to be checked for containment in this deque
     * @return {@code true} if this deque contains the specified element
     */
    public boolean contains(Object o) {
        if (o == null) return false;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (Node<E> p = first; p != null; p = p.next)
                if (o.equals(p.item))
                    return true;
            return false;
        } finally {
            lock.unlock();
        }
    }

    
Appends all of the elements in the specified collection to the end of this deque, in the order that they are returned by the specified collection's iterator. Attempts to addAll of a deque to itself result in IllegalArgumentException. 
Params:
c – the elements to be inserted into this deque
Throws:
NullPointerException – if the specified collection or any
        of its elements are null
IllegalArgumentException – if the collection is this deque
IllegalStateException – if this deque is full
See Also:
add(Object)
Returns:
true if this deque changed as a result of the call/**
     * Appends all of the elements in the specified collection to the end of
     * this deque, in the order that they are returned by the specified
     * collection's iterator.  Attempts to {@code addAll} of a deque to
     * itself result in {@code IllegalArgumentException}.
     *
     * @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
     * @throws IllegalArgumentException if the collection is this deque
     * @throws IllegalStateException if this deque is full
     * @see #add(Object)
     */
    public boolean addAll(Collection<? extends E> c) {
        if (c == this)
            // As historically specified in AbstractQueue#addAll
            throw new IllegalArgumentException();

        // Copy c into a private chain of Nodes
        Node<E> beg = null, end = null;
        int n = 0;
        for (E e : c) {
            Objects.requireNonNull(e);
            n++;
            Node<E> newNode = new Node<E>(e);
            if (beg == null)
                beg = end = newNode;
            else {
                end.next = newNode;
                newNode.prev = end;
                end = newNode;
            }
        }
        if (beg == null)
            return false;

        // Atomically append the chain at the end
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (count + n <= capacity) {
                beg.prev = last;
                if (first == null)
                    first = beg;
                else
                    last.next = beg;
                last = end;
                count += n;
                notEmpty.signalAll();
                return true;
            }
        } finally {
            lock.unlock();
        }
        // Fall back to historic non-atomic implementation, failing
        // with IllegalStateException when the capacity is exceeded.
        return super.addAll(c);
    }

    
Returns an array containing all of the elements in this deque, in
proper sequence (from first to last element).
The returned array will be "safe" in that no references to it are
maintained by this deque.  (In other words, this method must allocate
a new array).  The caller is thus free to modify the returned array.
This method acts as bridge between array-based and collection-based
APIs.
Returns:
an array containing all of the elements in this deque/**
     * 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
     */
    @SuppressWarnings("unchecked")
    public Object[] toArray() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] a = new Object[count];
            int k = 0;
            for (Node<E> p = first; p != null; p = p.next)
                a[k++] = p.item;
            return a;
        } finally {
            lock.unlock();
        }
    }

    
Returns an array containing all of the elements in this deque, in
proper sequence; the runtime type of the returned array is that of
the specified array.  If the deque fits in the specified array, it
is returned therein.  Otherwise, a new array is allocated with the
runtime type of the specified array and the size of this deque.
If this deque fits in the specified array with room to spare (i.e., the array has more elements than this deque), the element in the array immediately following the end of the deque is set to null. 
Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs. 
Suppose 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 String: 
 String[] y = x.toArray(new String[0]);
 Note that toArray(new Object[0]) is identical in function to toArray(). 
Params:
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
Throws:
ArrayStoreException – if the runtime type of the specified array
        is not a supertype of the runtime type of every element in
        this deque
NullPointerException – if the specified array is null
Returns:
an array containing all of the elements in this deque/**
     * Returns an array containing all of the elements in this deque, in
     * proper sequence; 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 ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (a.length < count)
                a = (T[])java.lang.reflect.Array.newInstance
                    (a.getClass().getComponentType(), count);

            int k = 0;
            for (Node<E> p = first; p != null; p = p.next)
                a[k++] = (T)p.item;
            if (a.length > k)
                a[k] = null;
            return a;
        } finally {
            lock.unlock();
        }
    }

    public String toString() {
        return Helpers.collectionToString(this);
    }

    
Atomically removes all of the elements from this deque.
The deque will be empty after this call returns.
/**
     * Atomically removes all of the elements from this deque.
     * The deque will be empty after this call returns.
     */
    public void clear() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (Node<E> f = first; f != null; ) {
                f.item = null;
                Node<E> n = f.next;
                f.prev = null;
                f.next = null;
                f = n;
            }
            first = last = null;
            count = 0;
            notFull.signalAll();
        } finally {
            lock.unlock();
        }
    }

    
Used for any element traversal that is not entirely under lock.
Such traversals must handle both:
- dequeued nodes (p.next == p)
- (possibly multiple) interior removed nodes (p.item == null)
/**
     * Used for any element traversal that is not entirely under lock.
     * Such traversals must handle both:
     * - dequeued nodes (p.next == p)
     * - (possibly multiple) interior removed nodes (p.item == null)
     */
    Node<E> succ(Node<E> p) {
        if (p == (p = p.next))
            p = first;
        return p;
    }

    
Returns an iterator over the elements in this deque in proper sequence.
The elements will be returned in order from first (head) to last (tail).
The returned iterator is
weakly consistent.
Returns:
an iterator over the elements in this deque in proper sequence/**
     * Returns an iterator over the elements in this deque in proper sequence.
     * The elements will be returned in order from first (head) to last (tail).
     *
     * <p>The returned iterator is
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * @return an iterator over the elements in this deque in proper sequence
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    
Returns an iterator over the elements in this deque in reverse
sequential order.  The elements will be returned in order from
last (tail) to first (head).
The returned iterator is
weakly consistent.
Returns:
an iterator over the elements in this deque in reverse order/**
     * Returns an iterator over the elements in this deque in reverse
     * sequential order.  The elements will be returned in order from
     * last (tail) to first (head).
     *
     * <p>The returned iterator is
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * @return an iterator over the elements in this deque in reverse order
     */
    public Iterator<E> descendingIterator() {
        return new DescendingItr();
    }

    
Base class for LinkedBlockingDeque iterators.
/**
     * Base class for LinkedBlockingDeque iterators.
     */
    private abstract class AbstractItr implements Iterator<E> {
        
The next node to return in next().
/**
         * The next node to return in next().
         */
        Node<E> next;

        
nextItem holds on to item fields because once we claim that
an element exists in hasNext(), we must return item read
under lock even if it was in the process of being removed
when hasNext() was called.
/**
         * nextItem holds on to item fields because once we claim that
         * an element exists in hasNext(), we must return item read
         * under lock even if it was in the process of being removed
         * when hasNext() was called.
         */
        E nextItem;

        
Node returned by most recent call to next. Needed by remove.
Reset to null if this element is deleted by a call to remove.
/**
         * Node returned by most recent call to next. Needed by remove.
         * Reset to null if this element is deleted by a call to remove.
         */
        private Node<E> lastRet;

        abstract Node<E> firstNode();
        abstract Node<E> nextNode(Node<E> n);

        private Node<E> succ(Node<E> p) {
            if (p == (p = nextNode(p)))
                p = firstNode();
            return p;
        }

        AbstractItr() {
            // set to initial position
            final ReentrantLock lock = LinkedBlockingDeque.this.lock;
            lock.lock();
            try {
                if ((next = firstNode()) != null)
                    nextItem = next.item;
            } finally {
                lock.unlock();
            }
        }

        public boolean hasNext() {
            return next != null;
        }

        public E next() {
            Node<E> p;
            if ((p = next) == null)
                throw new NoSuchElementException();
            lastRet = p;
            E x = nextItem;
            final ReentrantLock lock = LinkedBlockingDeque.this.lock;
            lock.lock();
            try {
                E e = null;
                for (p = nextNode(p); p != null && (e = p.item) == null; )
                    p = succ(p);
                next = p;
                nextItem = e;
            } finally {
                lock.unlock();
            }
            return x;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            // A variant of forEachFrom
            Objects.requireNonNull(action);
            Node<E> p;
            if ((p = next) == null) return;
            lastRet = p;
            next = null;
            final ReentrantLock lock = LinkedBlockingDeque.this.lock;
            final int batchSize = 64;
            Object[] es = null;
            int n, len = 1;
            do {
                lock.lock();
                try {
                    if (es == null) {
                        p = nextNode(p);
                        for (Node<E> q = p; q != null; q = succ(q))
                            if (q.item != null && ++len == batchSize)
                                break;
                        es = new Object[len];
                        es[0] = nextItem;
                        nextItem = null;
                        n = 1;
                    } else
                        n = 0;
                    for (; p != null && n < len; p = succ(p))
                        if ((es[n] = p.item) != null) {
                            lastRet = p;
                            n++;
                        }
                } finally {
                    lock.unlock();
                }
                for (int i = 0; i < n; i++) {
                    @SuppressWarnings("unchecked") E e = (E) es[i];
                    action.accept(e);
                }
            } while (n > 0 && p != null);
        }

        public void remove() {
            Node<E> n = lastRet;
            if (n == null)
                throw new IllegalStateException();
            lastRet = null;
            final ReentrantLock lock = LinkedBlockingDeque.this.lock;
            lock.lock();
            try {
                if (n.item != null)
                    unlink(n);
            } finally {
                lock.unlock();
            }
        }
    }

    
Forward iterator /** Forward iterator */
    private class Itr extends AbstractItr {
        Itr() {}                        // prevent access constructor creation
        Node<E> firstNode() { return first; }
        Node<E> nextNode(Node<E> n) { return n.next; }
    }

    
Descending iterator /** Descending iterator */
    private class DescendingItr extends AbstractItr {
        DescendingItr() {}              // prevent access constructor creation
        Node<E> firstNode() { return last; }
        Node<E> nextNode(Node<E> n) { return n.prev; }
    }

    
A customized variant of Spliterators.IteratorSpliterator.
Keep this class in sync with (very similar) LBQSpliterator.
/**
     * A customized variant of Spliterators.IteratorSpliterator.
     * Keep this class in sync with (very similar) LBQSpliterator.
     */
    private final class LBDSpliterator implements Spliterator<E> {
        static final int MAX_BATCH = 1 << 25;  // max batch array size;
        Node<E> current;    // current node; null until initialized
        int batch;          // batch size for splits
        boolean exhausted;  // true when no more nodes
        long est = size();  // size estimate

        LBDSpliterator() {}

        public long estimateSize() { return est; }

        public Spliterator<E> trySplit() {
            Node<E> h;
            if (!exhausted &&
                ((h = current) != null || (h = first) != null)
                && h.next != null) {
                int n = batch = Math.min(batch + 1, MAX_BATCH);
                Object[] a = new Object[n];
                final ReentrantLock lock = LinkedBlockingDeque.this.lock;
                int i = 0;
                Node<E> p = current;
                lock.lock();
                try {
                    if (p != null || (p = first) != null)
                        for (; p != null && i < n; p = succ(p))
                            if ((a[i] = p.item) != null)
                                i++;
                } finally {
                    lock.unlock();
                }
                if ((current = p) == null) {
                    est = 0L;
                    exhausted = true;
                }
                else if ((est -= i) < 0L)
                    est = 0L;
                if (i > 0)
                    return Spliterators.spliterator
                        (a, 0, i, (Spliterator.ORDERED |
                                   Spliterator.NONNULL |
                                   Spliterator.CONCURRENT));
            }
            return null;
        }

        public boolean tryAdvance(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            if (!exhausted) {
                E e = null;
                final ReentrantLock lock = LinkedBlockingDeque.this.lock;
                lock.lock();
                try {
                    Node<E> p;
                    if ((p = current) != null || (p = first) != null)
                        do {
                            e = p.item;
                            p = succ(p);
                        } while (e == null && p != null);
                    if ((current = p) == null)
                        exhausted = true;
                } finally {
                    lock.unlock();
                }
                if (e != null) {
                    action.accept(e);
                    return true;
                }
            }
            return false;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            if (!exhausted) {
                exhausted = true;
                Node<E> p = current;
                current = null;
                forEachFrom(action, p);
            }
        }

        public int characteristics() {
            return (Spliterator.ORDERED |
                    Spliterator.NONNULL |
                    Spliterator.CONCURRENT);
        }
    }

    
Returns a Spliterator over the elements in this deque. 
The returned spliterator is
weakly consistent.
The Spliterator reports Spliterator.CONCURRENT, Spliterator.ORDERED, and Spliterator.NONNULL. 
Implementation Note:
 The Spliterator implements trySplit to permit limited parallelism.
Returns:
a Spliterator over the elements in this deque
Since:
1.8/**
     * Returns a {@link Spliterator} over the elements in this deque.
     *
     * <p>The returned spliterator is
     * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
     * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
     *
     * @implNote
     * The {@code Spliterator} implements {@code trySplit} to permit limited
     * parallelism.
     *
     * @return a {@code Spliterator} over the elements in this deque
     * @since 1.8
     */
    public Spliterator<E> spliterator() {
        return new LBDSpliterator();
    }

    
Throws:
NullPointerException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     */
    public void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        forEachFrom(action, null);
    }

    
Runs action on each element found during a traversal starting at p.
If p is null, traversal starts at head.
/**
     * Runs action on each element found during a traversal starting at p.
     * If p is null, traversal starts at head.
     */
    void forEachFrom(Consumer<? super E> action, Node<E> p) {
        // Extract batches of elements while holding the lock; then
        // run the action on the elements while not
        final ReentrantLock lock = this.lock;
        final int batchSize = 64;       // max number of elements per batch
        Object[] es = null;             // container for batch of elements
        int n, len = 0;
        do {
            lock.lock();
            try {
                if (es == null) {
                    if (p == null) p = first;
                    for (Node<E> q = p; q != null; q = succ(q))
                        if (q.item != null && ++len == batchSize)
                            break;
                    es = new Object[len];
                }
                for (n = 0; p != null && n < len; p = succ(p))
                    if ((es[n] = p.item) != null)
                        n++;
            } finally {
                lock.unlock();
            }
            for (int i = 0; i < n; i++) {
                @SuppressWarnings("unchecked") E e = (E) es[i];
                action.accept(e);
            }
        } while (n > 0 && p != null);
    }

    
Throws:
NullPointerException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        return bulkRemove(filter);
    }

    
Throws:
NullPointerException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return bulkRemove(e -> c.contains(e));
    }

    
Throws:
NullPointerException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return bulkRemove(e -> !c.contains(e));
    }

    
Implementation of bulk remove methods. /** Implementation of bulk remove methods. */
    @SuppressWarnings("unchecked")
    private boolean bulkRemove(Predicate<? super E> filter) {
        boolean removed = false;
        Node<E> p = null;
        final ReentrantLock lock = this.lock;
        Node<E>[] nodes = null;
        int n, len = 0;
        do {
            // 1. Extract batch of up to 64 elements while holding the lock.
            long deathRow = 0;          // "bitset" of size 64
            lock.lock();
            try {
                if (nodes == null) {
                    if (p == null) p = first;
                    for (Node<E> q = p; q != null; q = succ(q))
                        if (q.item != null && ++len == 64)
                            break;
                    nodes = (Node<E>[]) new Node<?>[len];
                }
                for (n = 0; p != null && n < len; p = succ(p))
                    nodes[n++] = p;
            } finally {
                lock.unlock();
            }

            // 2. Run the filter on the elements while lock is free.
            for (int i = 0; i < n; i++) {
                final E e;
                if ((e = nodes[i].item) != null && filter.test(e))
                    deathRow |= 1L << i;
            }

            // 3. Remove any filtered elements while holding the lock.
            if (deathRow != 0) {
                lock.lock();
                try {
                    for (int i = 0; i < n; i++) {
                        final Node<E> q;
                        if ((deathRow & (1L << i)) != 0L
                            && (q = nodes[i]).item != null) {
                            unlink(q);
                            removed = true;
                        }
                    }
                } finally {
                    lock.unlock();
                }
            }
        } while (n > 0 && p != null);
        return removed;
    }

    
Saves this deque to a stream (that is, serializes it).
Params:
s – the stream
Throws:
IOException – if an I/O error occurs
@serialData
The capacity (int), followed by elements (each an Object) in the proper order, followed by a null/**
     * 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 capacity (int), followed by elements (each an
     * {@code Object}) in the proper order, followed by a null
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            // Write out capacity and any hidden stuff
            s.defaultWriteObject();
            // Write out all elements in the proper order.
            for (Node<E> p = first; p != null; p = p.next)
                s.writeObject(p.item);
            // Use trailing null as sentinel
            s.writeObject(null);
        } finally {
            lock.unlock();
        }
    }

    
Reconstitutes this deque from a stream (that is, deserializes it).
Params:
s – the stream
Throws:
ClassNotFoundException – if the class of a serialized object
        could not be found
IOException – if an I/O error occurs/**
     * 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();
        count = 0;
        first = null;
        last = null;
        // Read in all elements and place in queue
        for (;;) {
            @SuppressWarnings("unchecked") E item = (E)s.readObject();
            if (item == null)
                break;
            add(item);
        }
    }

    void checkInvariants() {
        // assert lock.isHeldByCurrentThread();
        // Nodes may get self-linked or lose their item, but only
        // after being unlinked and becoming unreachable from first.
        for (Node<E> p = first; p != null; p = p.next) {
            // assert p.next != p;
            // assert p.item != null;
        }
    }

}