https://openjdk.java.net/ 
/*
 * 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.Deque;
import java.util.Iterator;
import java.util.NoSuchElementException;


A Deque that additionally supports blocking operations that wait for the deque to become non-empty when retrieving an element, and wait for space to become available in the deque when storing an element. 
BlockingDeque methods come in four forms, with different ways of handling operations that cannot be satisfied immediately, but may be satisfied at some point in the future: one throws an exception, the second returns a special value (either null or false, depending on the operation), the third blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up. These methods are summarized in the following table: 

Summary of BlockingDeque methods
 

    First Element (Head)
 

 

   
   Throws exception
   Special value
   Blocks
   Times out
 

 

   Insert
   addFirst(e)
   offerFirst(e)
   putFirst(e)
   offerFirst(e, time, unit)
 

 

   Remove
   removeFirst()
   pollFirst()
   takeFirst()
   pollFirst(time, unit)
 

 

   Examine
   getFirst()
   peekFirst()
   not applicable
   not applicable
 

 

    Last Element (Tail)
 

 

   
   Throws exception
   Special value
   Blocks
   Times out
 

 

   Insert
   addLast(e)
   offerLast(e)
   putLast(e)
   offerLast(e, time, unit)
 

 

   Remove
   removeLast()
   pollLast()
   takeLast()
   pollLast(time, unit)
 

 

   Examine
   getLast()
   peekLast()
   not applicable
   not applicable
 




Like any BlockingQueue, a BlockingDeque is thread safe, does not permit null elements, and may (or may not) be capacity-constrained. 
A BlockingDeque implementation may be used directly as a FIFO BlockingQueue. The methods inherited from the BlockingQueue interface are precisely equivalent to BlockingDeque methods as indicated in the following table: 

Comparison of BlockingQueue and BlockingDeque methods
 

   
    BlockingQueue Method
    Equivalent BlockingDeque Method
 

 

   Insert
   add(e)
   addLast(e)
 

 

   offer(e)
   offerLast(e)
 

 

   put(e)
   putLast(e)
 

 

   offer(e, time, unit)
   offerLast(e, time, unit)
 

 

   Remove
   remove()
   removeFirst()
 

 

   poll()
   pollFirst()
 

 

   take()
   takeFirst()
 

 

   poll(time, unit)
   pollFirst(time, unit)
 

 

   Examine
   element()
   getFirst()
 

 

   peek()
   peekFirst()
 




Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a BlockingDeque happen-before actions subsequent to the access or removal of that element from the BlockingDeque in another thread. 
This interface 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
/**
 * A {@link Deque} that additionally supports blocking operations that wait
 * for the deque to become non-empty when retrieving an element, and wait for
 * space to become available in the deque when storing an element.
 *
 * <p>{@code BlockingDeque} methods come in four forms, with different ways
 * of handling operations that cannot be satisfied immediately, but may be
 * satisfied at some point in the future:
 * one throws an exception, the second returns a special value (either
 * {@code null} or {@code false}, depending on the operation), the third
 * blocks the current thread indefinitely until the operation can succeed,
 * and the fourth blocks for only a given maximum time limit before giving
 * up.  These methods are summarized in the following table:
 *
 * <table class="plain">
 * <caption>Summary of BlockingDeque methods</caption>
 *  <tr>
 *    <th id="First" colspan="5"> First Element (Head)</th>
 *  </tr>
 *  <tr>
 *    <td></td>
 *    <th id="FThrow" style="font-weight:normal; font-style: italic">Throws exception</th>
 *    <th id="FValue" style="font-weight:normal; font-style: italic">Special value</th>
 *    <th id="FBlock" style="font-weight:normal; font-style: italic">Blocks</th>
 *    <th id="FTimes" style="font-weight:normal; font-style: italic">Times out</th>
 *  </tr>
 *  <tr>
 *    <th id="FInsert" style="text-align:left">Insert</th>
 *    <td headers="First FInsert FThrow">{@link #addFirst(Object) addFirst(e)}</td>
 *    <td headers="First FInsert FValue">{@link #offerFirst(Object) offerFirst(e)}</td>
 *    <td headers="First FInsert FBlock">{@link #putFirst(Object) putFirst(e)}</td>
 *    <td headers="First FInsert FTimes">{@link #offerFirst(Object, long, TimeUnit) offerFirst(e, time, unit)}</td>
 *  </tr>
 *  <tr>
 *    <th id="FRemove" style="text-align:left">Remove</th>
 *    <td headers="First FRemove FThrow">{@link #removeFirst() removeFirst()}</td>
 *    <td headers="First FRemove FValue">{@link #pollFirst() pollFirst()}</td>
 *    <td headers="First FRemove FBlock">{@link #takeFirst() takeFirst()}</td>
 *    <td headers="First FRemove FTimes">{@link #pollFirst(long, TimeUnit) pollFirst(time, unit)}</td>
 *  </tr>
 *  <tr>
 *    <th id="FExamine" style="text-align:left">Examine</th>
 *    <td headers="First FExamine FThrow">{@link #getFirst() getFirst()}</td>
 *    <td headers="First FExamine FValue">{@link #peekFirst() peekFirst()}</td>
 *    <td headers="First FExamine FBlock" style="font-style:italic">not applicable</td>
 *    <td headers="First FExamine FTimes" style="font-style:italic">not applicable</td>
 *  </tr>
 *  <tr>
 *    <th id="Last" colspan="5"> Last Element (Tail)</th>
 *  </tr>
 *  <tr>
 *    <td></td>
 *    <th id="LThrow" style="font-weight:normal; font-style: italic">Throws exception</th>
 *    <th id="LValue" style="font-weight:normal; font-style: italic">Special value</th>
 *    <th id="LBlock" style="font-weight:normal; font-style: italic">Blocks</th>
 *    <th id="LTimes" style="font-weight:normal; font-style: italic">Times out</th>
 *  </tr>
 *  <tr>
 *    <th id="LInsert" style="text-align:left">Insert</th>
 *    <td headers="Last LInsert LThrow">{@link #addLast(Object) addLast(e)}</td>
 *    <td headers="Last LInsert LValue">{@link #offerLast(Object) offerLast(e)}</td>
 *    <td headers="Last LInsert LBlock">{@link #putLast(Object) putLast(e)}</td>
 *    <td headers="Last LInsert LTimes">{@link #offerLast(Object, long, TimeUnit) offerLast(e, time, unit)}</td>
 *  </tr>
 *  <tr>
 *    <th id="LRemove" style="text-align:left">Remove</th>
 *    <td headers="Last LRemove LThrow">{@link #removeLast() removeLast()}</td>
 *    <td headers="Last LRemove LValue">{@link #pollLast() pollLast()}</td>
 *    <td headers="Last LRemove LBlock">{@link #takeLast() takeLast()}</td>
 *    <td headers="Last LRemove LTimes">{@link #pollLast(long, TimeUnit) pollLast(time, unit)}</td>
 *  </tr>
 *  <tr>
 *    <th id="LExamine" style="text-align:left">Examine</th>
 *    <td headers="Last LExamine LThrow">{@link #getLast() getLast()}</td>
 *    <td headers="Last LExamine LValue">{@link #peekLast() peekLast()}</td>
 *    <td headers="Last LExamine LBlock" style="font-style:italic">not applicable</td>
 *    <td headers="Last LExamine LTimes" style="font-style:italic">not applicable</td>
 *  </tr>
 * </table>
 *
 * <p>Like any {@link BlockingQueue}, a {@code BlockingDeque} is thread safe,
 * does not permit null elements, and may (or may not) be
 * capacity-constrained.
 *
 * <p>A {@code BlockingDeque} implementation may be used directly as a FIFO
 * {@code BlockingQueue}. The methods inherited from the
 * {@code BlockingQueue} interface are precisely equivalent to
 * {@code BlockingDeque} methods as indicated in the following table:
 *
 * <table class="plain">
 * <caption>Comparison of BlockingQueue and BlockingDeque methods</caption>
 *  <tr>
 *    <td></td>
 *    <th id="BQueue"> {@code BlockingQueue} Method</th>
 *    <th id="BDeque"> Equivalent {@code BlockingDeque} Method</th>
 *  </tr>
 *  <tr>
 *    <th id="Insert" rowspan="4" style="text-align:left; vertical-align:top">Insert</th>
 *    <th id="add" style="font-weight:normal; text-align:left">{@link #add(Object) add(e)}</th>
 *    <td headers="Insert BDeque add">{@link #addLast(Object) addLast(e)}</td>
 *  </tr>
 *  <tr>
 *    <th id="offer1" style="font-weight:normal; text-align:left">{@link #offer(Object) offer(e)}</th>
 *    <td headers="Insert BDeque offer1">{@link #offerLast(Object) offerLast(e)}</td>
 *  </tr>
 *  <tr>
 *    <th id="put" style="font-weight:normal; text-align:left">{@link #put(Object) put(e)}</th>
 *    <td headers="Insert BDeque put">{@link #putLast(Object) putLast(e)}</td>
 *  </tr>
 *  <tr>
 *    <th id="offer2" style="font-weight:normal; text-align:left">{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</th>
 *    <td headers="Insert BDeque offer2">{@link #offerLast(Object, long, TimeUnit) offerLast(e, time, unit)}</td>
 *  </tr>
 *  <tr>
 *    <th id="Remove" rowspan="4" style="text-align:left; vertical-align:top">Remove</th>
 *    <th id="remove" style="font-weight:normal; text-align:left">{@link #remove() remove()}</th>
 *    <td headers="Remove BDeque remove">{@link #removeFirst() removeFirst()}</td>
 *  </tr>
 *  <tr>
 *    <th id="poll1" style="font-weight:normal; text-align:left">{@link #poll() poll()}</th>
 *    <td headers="Remove BDeque poll1">{@link #pollFirst() pollFirst()}</td>
 *  </tr>
 *  <tr>
 *    <th id="take" style="font-weight:normal; text-align:left">{@link #take() take()}</th>
 *    <td headers="Remove BDeque take">{@link #takeFirst() takeFirst()}</td>
 *  </tr>
 *  <tr>
 *    <th id="poll2" style="font-weight:normal; text-align:left">{@link #poll(long, TimeUnit) poll(time, unit)}</th>
 *    <td headers="Remove BDeque poll2">{@link #pollFirst(long, TimeUnit) pollFirst(time, unit)}</td>
 *  </tr>
 *  <tr>
 *    <th id="Examine" rowspan="2" style="text-align:left; vertical-align:top">Examine</th>
 *    <th id="element" style="font-weight:normal; text-align:left">{@link #element() element()}</th>
 *    <td headers="Examine BDeque element">{@link #getFirst() getFirst()}</td>
 *  </tr>
 *  <tr>
 *    <th id="peek" style="font-weight:normal; text-align:left">{@link #peek() peek()}</th>
 *    <td headers="Examine BDeque peek">{@link #peekFirst() peekFirst()}</td>
 *  </tr>
 * </table>
 *
 * <p>Memory consistency effects: As with other concurrent
 * collections, actions in a thread prior to placing an object into a
 * {@code BlockingDeque}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions subsequent to the access or removal of that element from
 * the {@code BlockingDeque} in another thread.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/java.base/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 interface BlockingDeque<E> extends BlockingQueue<E>, Deque<E> {
    /*
     * We have "diamond" multiple interface inheritance here, and that
     * introduces ambiguities.  Methods might end up with different
     * specs depending on the branch chosen by javadoc.  Thus a lot of
     * methods specs here are copied from superinterfaces.
     */

    
Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions, throwing an IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use offerFirst. 
Params:
  • e – the element to add
Throws:
/**
     * Inserts the specified element at the front of this deque if it is
     * possible to do so immediately without violating capacity restrictions,
     * throwing an {@code IllegalStateException} if no space is currently
     * available.  When using a capacity-restricted deque, it is generally
     * preferable to use {@link #offerFirst(Object) offerFirst}.
     *
     * @param e the element to add
     * @throws IllegalStateException {@inheritDoc}
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    void addFirst(E e);

    
Inserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions, throwing an IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use offerLast. 
Params:
  • e – the element to add
Throws:
/**
     * Inserts the specified element at the end of this deque if it is
     * possible to do so immediately without violating capacity restrictions,
     * throwing an {@code IllegalStateException} if no space is currently
     * available.  When using a capacity-restricted deque, it is generally
     * preferable to use {@link #offerLast(Object) offerLast}.
     *
     * @param e the element to add
     * @throws IllegalStateException {@inheritDoc}
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    void addLast(E e);

    
Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions, returning true upon success and false if no space is currently available. When using a capacity-restricted deque, this method is generally preferable to the addFirst method, which can fail to insert an element only by throwing an exception. 
Params:
  • e – the element to add
Throws:
/**
     * Inserts the specified element at the front of this deque if it is
     * possible to do so immediately without violating capacity restrictions,
     * returning {@code true} upon success and {@code false} if no space is
     * currently available.
     * When using a capacity-restricted deque, this method is generally
     * preferable to the {@link #addFirst(Object) addFirst} method, which can
     * fail to insert an element only by throwing an exception.
     *
     * @param e the element to add
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    boolean offerFirst(E e);

    
Inserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions, returning true upon success and false if no space is currently available. When using a capacity-restricted deque, this method is generally preferable to the addLast method, which can fail to insert an element only by throwing an exception. 
Params:
  • e – the element to add
Throws:
/**
     * Inserts the specified element at the end of this deque if it is
     * possible to do so immediately without violating capacity restrictions,
     * returning {@code true} upon success and {@code false} if no space is
     * currently available.
     * When using a capacity-restricted deque, this method is generally
     * preferable to the {@link #addLast(Object) addLast} method, which can
     * fail to insert an element only by throwing an exception.
     *
     * @param e the element to add
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    boolean offerLast(E e);

    
Inserts the specified element at the front of this deque,
waiting if necessary for space to become available.

Params:
  • e – the element to add
Throws:
/**
     * Inserts the specified element at the front of this deque,
     * waiting if necessary for space to become available.
     *
     * @param e the element to add
     * @throws InterruptedException if interrupted while waiting
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    void putFirst(E e) throws InterruptedException;

    
Inserts the specified element at the end of this deque,
waiting if necessary for space to become available.

Params:
  • e – the element to add
Throws:
/**
     * Inserts the specified element at the end of this deque,
     * waiting if necessary for space to become available.
     *
     * @param e the element to add
     * @throws InterruptedException if interrupted while waiting
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    void putLast(E e) throws InterruptedException;

    
Inserts the specified element at the front of this deque,
waiting up to the specified wait time if necessary for space to
become available.

Params:
  • e – the element to add
  • timeout – how long to wait before giving up, in units of unit
  • unit – a TimeUnit determining how to interpret the timeout parameter
Throws:
Returns:true if successful, or false if the specified waiting time elapses before space is available
/**
     * Inserts the specified element at the front of this deque,
     * waiting up to the specified wait time if necessary for space to
     * become available.
     *
     * @param e the element to add
     * @param timeout how long to wait before giving up, in units of
     *        {@code unit}
     * @param unit a {@code TimeUnit} determining how to interpret the
     *        {@code timeout} parameter
     * @return {@code true} if successful, or {@code false} if
     *         the specified waiting time elapses before space is available
     * @throws InterruptedException if interrupted while waiting
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    boolean offerFirst(E e, long timeout, TimeUnit unit)
        throws InterruptedException;

    
Inserts the specified element at the end of this deque,
waiting up to the specified wait time if necessary for space to
become available.

Params:
  • e – the element to add
  • timeout – how long to wait before giving up, in units of unit
  • unit – a TimeUnit determining how to interpret the timeout parameter
Throws:
Returns:true if successful, or false if the specified waiting time elapses before space is available
/**
     * Inserts the specified element at the end of this deque,
     * waiting up to the specified wait time if necessary for space to
     * become available.
     *
     * @param e the element to add
     * @param timeout how long to wait before giving up, in units of
     *        {@code unit}
     * @param unit a {@code TimeUnit} determining how to interpret the
     *        {@code timeout} parameter
     * @return {@code true} if successful, or {@code false} if
     *         the specified waiting time elapses before space is available
     * @throws InterruptedException if interrupted while waiting
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    boolean offerLast(E e, long timeout, TimeUnit unit)
        throws InterruptedException;

    
Retrieves and removes the first element of this deque, waiting
if necessary until an element becomes available.

Throws:
Returns:the head of this deque
/**
     * Retrieves and removes the first element of this deque, waiting
     * if necessary until an element becomes available.
     *
     * @return the head of this deque
     * @throws InterruptedException if interrupted while waiting
     */
    E takeFirst() throws InterruptedException;

    
Retrieves and removes the last element of this deque, waiting
if necessary until an element becomes available.

Throws:
Returns:the tail of this deque
/**
     * Retrieves and removes the last element of this deque, waiting
     * if necessary until an element becomes available.
     *
     * @return the tail of this deque
     * @throws InterruptedException if interrupted while waiting
     */
    E takeLast() throws InterruptedException;

    
Retrieves and removes the first element of this deque, waiting
up to the specified wait time if necessary for an element to
become available.

Params:
  • timeout – how long to wait before giving up, in units of unit
  • unit – a TimeUnit determining how to interpret the timeout parameter
Throws:
Returns:the head of this deque, or null if the specified waiting time elapses before an element is available
/**
     * Retrieves and removes the first element of this deque, waiting
     * up to the specified wait time if necessary for an element to
     * become available.
     *
     * @param timeout how long to wait before giving up, in units of
     *        {@code unit}
     * @param unit a {@code TimeUnit} determining how to interpret the
     *        {@code timeout} parameter
     * @return the head of this deque, or {@code null} if the specified
     *         waiting time elapses before an element is available
     * @throws InterruptedException if interrupted while waiting
     */
    E pollFirst(long timeout, TimeUnit unit)
        throws InterruptedException;

    
Retrieves and removes the last element of this deque, waiting
up to the specified wait time if necessary for an element to
become available.

Params:
  • timeout – how long to wait before giving up, in units of unit
  • unit – a TimeUnit determining how to interpret the timeout parameter
Throws:
Returns:the tail of this deque, or null if the specified waiting time elapses before an element is available
/**
     * Retrieves and removes the last element of this deque, waiting
     * up to the specified wait time if necessary for an element to
     * become available.
     *
     * @param timeout how long to wait before giving up, in units of
     *        {@code unit}
     * @param unit a {@code TimeUnit} determining how to interpret the
     *        {@code timeout} parameter
     * @return the tail of this deque, or {@code null} if the specified
     *         waiting time elapses before an element is available
     * @throws InterruptedException if interrupted while waiting
     */
    E pollLast(long timeout, TimeUnit unit)
        throws InterruptedException;

    
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). 
Params:
  • o – element to be removed from this deque, if present
Throws:
Returns:true if an element was removed as a result of this 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).
     *
     * @param o element to be removed from this deque, if present
     * @return {@code true} if an element was removed as a result of this call
     * @throws ClassCastException if the class of the specified element
     *         is incompatible with this deque
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    boolean removeFirstOccurrence(Object o);

    
Removes the last occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the last 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). 
Params:
  • o – element to be removed from this deque, if present
Throws:
Returns:true if an element was removed as a result of this call
/**
     * Removes the last occurrence of the specified element from this deque.
     * 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 an element was removed as a result of this call
     * @throws ClassCastException if the class of the specified element
     *         is incompatible with this deque
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    boolean removeLastOccurrence(Object o);

    // *** BlockingQueue methods ***

    
Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque) if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use offer. 
This method is equivalent to addLast. 
Params:
  • e – the element to add
Throws:
/**
     * Inserts the specified element into the queue represented by this deque
     * (in other words, at the tail of this deque) if it is possible to do so
     * immediately without violating capacity restrictions, returning
     * {@code true} upon success and throwing an
     * {@code IllegalStateException} if no space is currently available.
     * When using a capacity-restricted deque, it is generally preferable to
     * use {@link #offer(Object) offer}.
     *
     * <p>This method is equivalent to {@link #addLast(Object) addLast}.
     *
     * @param e the element to add
     * @throws IllegalStateException {@inheritDoc}
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    boolean add(E e);

    
Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque) if it is possible to do so immediately without violating capacity restrictions, returning true upon success and false if no space is currently available. When using a capacity-restricted deque, this method is generally preferable to the add method, which can fail to insert an element only by throwing an exception. 
This method is equivalent to offerLast. 
Params:
  • e – the element to add
Throws:
/**
     * Inserts the specified element into the queue represented by this deque
     * (in other words, at the tail of this deque) if it is possible to do so
     * immediately without violating capacity restrictions, returning
     * {@code true} upon success and {@code false} if no space is currently
     * available.  When using a capacity-restricted deque, this method is
     * generally preferable to the {@link #add} method, which can fail to
     * insert an element only by throwing an exception.
     *
     * <p>This method is equivalent to {@link #offerLast(Object) offerLast}.
     *
     * @param e the element to add
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    boolean offer(E e);

    
Inserts the specified element into the queue represented by this deque
(in other words, at the tail of this deque), waiting if necessary for
space to become available.

This method is equivalent to putLast. 
Params:
  • e – the element to add
Throws:
/**
     * Inserts the specified element into the queue represented by this deque
     * (in other words, at the tail of this deque), waiting if necessary for
     * space to become available.
     *
     * <p>This method is equivalent to {@link #putLast(Object) putLast}.
     *
     * @param e the element to add
     * @throws InterruptedException {@inheritDoc}
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    void put(E e) throws InterruptedException;

    
Inserts the specified element into the queue represented by this deque
(in other words, at the tail of this deque), waiting up to the
specified wait time if necessary for space to become available.

This method is equivalent to offerLast. 
Params:
  • e – the element to add
Throws:
Returns:true if the element was added to this deque, else false
/**
     * Inserts the specified element into the queue represented by this deque
     * (in other words, at the tail of this deque), waiting up to the
     * specified wait time if necessary for space to become available.
     *
     * <p>This method is equivalent to
     * {@link #offerLast(Object,long,TimeUnit) offerLast}.
     *
     * @param e the element to add
     * @return {@code true} if the element was added to this deque, else
     *         {@code false}
     * @throws InterruptedException {@inheritDoc}
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this deque
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException;

    
Retrieves and removes the head of the queue represented by this deque (in other words, the first element of 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:
Returns:the head of the queue represented by this deque
/**
     * Retrieves and removes the head of the queue represented by this deque
     * (in other words, the first element of 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
     */
    E remove();

    
Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returns null if this deque is empty. 
This method is equivalent to Deque.pollFirst(). 
Returns:the head of this deque, or null if this deque is empty
/**
     * 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 this deque, or {@code null} if this deque is empty
     */
    E poll();

    
Retrieves and removes the head of the queue represented by this deque
(in other words, the first element of this deque), waiting if
necessary until an element becomes available.

This method is equivalent to takeFirst. 
Throws:
Returns:the head of this deque
/**
     * Retrieves and removes the head of the queue represented by this deque
     * (in other words, the first element of this deque), waiting if
     * necessary until an element becomes available.
     *
     * <p>This method is equivalent to {@link #takeFirst() takeFirst}.
     *
     * @return the head of this deque
     * @throws InterruptedException if interrupted while waiting
     */
    E take() throws InterruptedException;

    
Retrieves and removes the head of the queue represented by this deque
(in other words, the first element of this deque), waiting up to the
specified wait time if necessary for an element to become available.

This method is equivalent to pollFirst. 
Throws:
Returns:the head of this deque, or null if the specified waiting time elapses before an element is available
/**
     * Retrieves and removes the head of the queue represented by this deque
     * (in other words, the first element of this deque), waiting up to the
     * specified wait time if necessary for an element to become available.
     *
     * <p>This method is equivalent to
     * {@link #pollFirst(long,TimeUnit) pollFirst}.
     *
     * @return the head of this deque, or {@code null} if the
     *         specified waiting time elapses before an element is available
     * @throws InterruptedException if interrupted while waiting
     */
    E poll(long timeout, TimeUnit unit)
        throws InterruptedException;

    
Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of 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:
Returns:the head of this deque
/**
     * Retrieves, but does not remove, the head of the queue represented by
     * this deque (in other words, the first element of 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 this deque
     * @throws NoSuchElementException if this deque is empty
     */
    E element();

    
Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returns null if this deque is empty. 
This method is equivalent to peekFirst. 
Returns:the head of this deque, or null if this deque is empty
/**
     * Retrieves, but does not remove, 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 #peekFirst() peekFirst}.
     *
     * @return the head of this deque, or {@code null} if this deque is empty
     */
    E peek();

    
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
Throws:
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
     * @throws ClassCastException if the class of the specified element
     *         is incompatible with this deque
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    boolean remove(Object o);

    
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
Throws:
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
     * @throws ClassCastException if the class of the specified element
     *         is incompatible with this deque
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    boolean contains(Object 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
     */
    int size();

    
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).

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).
     *
     * @return an iterator over the elements in this deque in proper sequence
     */
    Iterator<E> iterator();

    // *** Stack methods ***

    
Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing an IllegalStateException if no space is currently available. 
This method is equivalent to addFirst. 
Throws:
/**
     * Pushes an element onto the stack represented by this deque (in other
     * words, at the head of this deque) if it is possible to do so
     * immediately without violating capacity restrictions, throwing an
     * {@code IllegalStateException} if no space is currently available.
     *
     * <p>This method is equivalent to {@link #addFirst(Object) addFirst}.
     *
     * @throws IllegalStateException {@inheritDoc}
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException if the specified element is null
     * @throws IllegalArgumentException {@inheritDoc}
     */
    void push(E e);
}