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 * 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
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 * 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 and Josh Bloch 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;


A linear collection that supports element insertion and removal at
both ends.  The name deque is short for "double ended queue" and is usually pronounced "deck". Most Deque implementations place no fixed limits on the number of elements they may contain, but this interface supports capacity-restricted deques as well as those with no fixed size limit. 
This interface defines methods to access the elements at both ends of the deque. Methods are provided to insert, remove, and examine the element. Each of these methods exists in two forms: one throws an exception if the operation fails, the other returns a special value (either null or false, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restricted Deque implementations; in most implementations, insert operations cannot fail. 
The twelve methods described above are summarized in the
following table:


Summary of Deque methods
 
 

   
    First Element (Head)
    Last Element (Tail)
 

 

   Throws exception
   Special value
   Throws exception
   Special value
 

 
 
 

   Insert
   addFirst(e)
   offerFirst(e)
   addLast(e)
   offerLast(e)
 

 

   Remove
   removeFirst()
   pollFirst()
   removeLast()
   pollLast()
 

 

   Examine
   getFirst()
   peekFirst()
   getLast()
   peekLast()
 

 



This interface extends the Queue interface. When a deque is used as a queue, FIFO (First-In-First-Out) behavior results. Elements are added at the end of the deque and removed from the beginning. The methods inherited from the Queue interface are precisely equivalent to Deque methods as indicated in the following table: 

Comparison of Queue and Deque methods
 
 

    Queue Method
    Equivalent Deque Method
 

 
 
 

   add(e)
   addLast(e)
 

 

   offer(e)
   offerLast(e)
 

 

   remove()
   removeFirst()
 

 

   poll()
   pollFirst()
 

 

   element()
   getFirst()
 

 

   peek()
   peekFirst()
 

 



Deques can also be used as LIFO (Last-In-First-Out) stacks. This interface should be used in preference to the legacy Stack class. When a deque is used as a stack, elements are pushed and popped from the beginning of the deque. Stack methods are equivalent to Deque methods as indicated in the table below: 

Comparison of Stack and Deque methods
 
 

    Stack Method
    Equivalent Deque Method
 

 
 
 

   push(e)
   addFirst(e)
 

 

   pop()
   removeFirst()
 

 

   peek()
   getFirst()
 

 



Note that the peek method works equally well when a deque is used as a queue or a stack; in either case, elements are drawn from the beginning of the deque. 
This interface provides two methods to remove interior elements, removeFirstOccurrence and removeLastOccurrence. 
Unlike the List interface, this interface does not provide support for indexed access to elements. 
While Deque implementations are not strictly required to prohibit the insertion of null elements, they are strongly encouraged to do so. Users of any Deque implementations that do allow null elements are strongly encouraged not to take advantage of the ability to insert nulls. This is so because null is used as a special return value by various methods to indicate that the deque is empty. 
Deque implementations generally do not define element-based versions of the equals and hashCode methods, but instead inherit the identity-based versions from class Object. 
This interface is a member of the

Java Collections Framework.

Author:Doug Lea, Josh Bloch
Type parameters:
  • <E> – the type of elements held in this deque
Since: 1.6
/**
 * A linear collection that supports element insertion and removal at
 * both ends.  The name <i>deque</i> is short for "double ended queue"
 * and is usually pronounced "deck".  Most {@code Deque}
 * implementations place no fixed limits on the number of elements
 * they may contain, but this interface supports capacity-restricted
 * deques as well as those with no fixed size limit.
 *
 * <p>This interface defines methods to access the elements at both
 * ends of the deque.  Methods are provided to insert, remove, and
 * examine the element.  Each of these methods exists in two forms:
 * one throws an exception if the operation fails, the other returns a
 * special value (either {@code null} or {@code false}, depending on
 * the operation).  The latter form of the insert operation is
 * designed specifically for use with capacity-restricted
 * {@code Deque} implementations; in most implementations, insert
 * operations cannot fail.
 *
 * <p>The twelve methods described above are summarized in the
 * following table:
 *
 * <table class="striped">
 * <caption>Summary of Deque methods</caption>
 *  <thead>
 *  <tr>
 *    <td rowspan="2"></td>
 *    <th scope="col" colspan="2"> First Element (Head)</th>
 *    <th scope="col" colspan="2"> Last Element (Tail)</th>
 *  </tr>
 *  <tr>
 *    <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
 *    <th scope="col" style="font-weight:normal; font-style:italic">Special value</th>
 *    <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
 *    <th scope="col" style="font-weight:normal; font-style:italic">Special value</th>
 *  </tr>
 *  </thead>
 *  <tbody>
 *  <tr>
 *    <th scope="row">Insert</th>
 *    <td>{@link #addFirst(Object) addFirst(e)}</td>
 *    <td>{@link #offerFirst(Object) offerFirst(e)}</td>
 *    <td>{@link #addLast(Object) addLast(e)}</td>
 *    <td>{@link #offerLast(Object) offerLast(e)}</td>
 *  </tr>
 *  <tr>
 *    <th scope="row">Remove</th>
 *    <td>{@link #removeFirst() removeFirst()}</td>
 *    <td>{@link #pollFirst() pollFirst()}</td>
 *    <td>{@link #removeLast() removeLast()}</td>
 *    <td>{@link #pollLast() pollLast()}</td>
 *  </tr>
 *  <tr>
 *    <th scope="row">Examine</th>
 *    <td>{@link #getFirst() getFirst()}</td>
 *    <td>{@link #peekFirst() peekFirst()}</td>
 *    <td>{@link #getLast() getLast()}</td>
 *    <td>{@link #peekLast() peekLast()}</td>
 *  </tr>
 *  </tbody>
 * </table>
 *
 * <p>This interface extends the {@link Queue} interface.  When a deque is
 * used as a queue, FIFO (First-In-First-Out) behavior results.  Elements are
 * added at the end of the deque and removed from the beginning.  The methods
 * inherited from the {@code Queue} interface are precisely equivalent to
 * {@code Deque} methods as indicated in the following table:
 *
 * <table class="striped">
 * <caption>Comparison of Queue and Deque methods</caption>
 *  <thead>
 *  <tr>
 *    <th scope="col"> {@code Queue} Method</th>
 *    <th scope="col"> Equivalent {@code Deque} Method</th>
 *  </tr>
 *  </thead>
 *  <tbody>
 *  <tr>
 *    <th scope="row">{@link #add(Object) add(e)}</th>
 *    <td>{@link #addLast(Object) addLast(e)}</td>
 *  </tr>
 *  <tr>
 *    <th scope="row">{@link #offer(Object) offer(e)}</th>
 *    <td>{@link #offerLast(Object) offerLast(e)}</td>
 *  </tr>
 *  <tr>
 *    <th scope="row">{@link #remove() remove()}</th>
 *    <td>{@link #removeFirst() removeFirst()}</td>
 *  </tr>
 *  <tr>
 *    <th scope="row">{@link #poll() poll()}</th>
 *    <td>{@link #pollFirst() pollFirst()}</td>
 *  </tr>
 *  <tr>
 *    <th scope="row">{@link #element() element()}</th>
 *    <td>{@link #getFirst() getFirst()}</td>
 *  </tr>
 *  <tr>
 *    <th scope="row">{@link #peek() peek()}</th>
 *    <td>{@link #peekFirst() peekFirst()}</td>
 *  </tr>
 *  </tbody>
 * </table>
 *
 * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks.  This
 * interface should be used in preference to the legacy {@link Stack} class.
 * When a deque is used as a stack, elements are pushed and popped from the
 * beginning of the deque.  Stack methods are equivalent to {@code Deque}
 * methods as indicated in the table below:
 *
 * <table class="striped">
 * <caption>Comparison of Stack and Deque methods</caption>
 *  <thead>
 *  <tr>
 *    <th scope="col"> Stack Method</th>
 *    <th scope="col"> Equivalent {@code Deque} Method</th>
 *  </tr>
 *  </thead>
 *  <tbody>
 *  <tr>
 *    <th scope="row">{@link #push(Object) push(e)}</th>
 *    <td>{@link #addFirst(Object) addFirst(e)}</td>
 *  </tr>
 *  <tr>
 *    <th scope="row">{@link #pop() pop()}</th>
 *    <td>{@link #removeFirst() removeFirst()}</td>
 *  </tr>
 *  <tr>
 *    <th scope="row">{@link #peek() peek()}</th>
 *    <td>{@link #getFirst() getFirst()}</td>
 *  </tr>
 *  </tbody>
 * </table>
 *
 * <p>Note that the {@link #peek peek} method works equally well when
 * a deque is used as a queue or a stack; in either case, elements are
 * drawn from the beginning of the deque.
 *
 * <p>This interface provides two methods to remove interior
 * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and
 * {@link #removeLastOccurrence removeLastOccurrence}.
 *
 * <p>Unlike the {@link List} interface, this interface does not
 * provide support for indexed access to elements.
 *
 * <p>While {@code Deque} implementations are not strictly required
 * to prohibit the insertion of null elements, they are strongly
 * encouraged to do so.  Users of any {@code Deque} implementations
 * that do allow null elements are strongly encouraged <i>not</i> to
 * take advantage of the ability to insert nulls.  This is so because
 * {@code null} is used as a special return value by various methods
 * to indicate that the deque is empty.
 *
 * <p>{@code Deque} implementations generally do not define
 * element-based versions of the {@code equals} and {@code hashCode}
 * methods, but instead inherit the identity-based versions from class
 * {@code Object}.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @author Doug Lea
 * @author Josh Bloch
 * @since  1.6
 * @param <E> the type of elements held in this deque
 */
public interface Deque<E> extends Queue<E> {
    
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 method offerFirst. 
Params:
  • e – the element to add
Throws:
  • IllegalStateException – if the element cannot be added at this
        time due to capacity restrictions
  • ClassCastException – if the class of the specified element
        prevents it from being added to this deque
  • NullPointerException – if the specified element is null and this
        deque does not permit null elements
  • IllegalArgumentException – if some property of the specified
        element prevents it from being added to this deque
/**
     * 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 method {@link #offerFirst}.
     *
     * @param e the element to add
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to capacity restrictions
     * @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 and this
     *         deque does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    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 method offerLast. 
This method is equivalent to add. 
Params:
  • e – the element to add
Throws:
  • IllegalStateException – if the element cannot be added at this
        time due to capacity restrictions
  • ClassCastException – if the class of the specified element
        prevents it from being added to this deque
  • NullPointerException – if the specified element is null and this
        deque does not permit null elements
  • IllegalArgumentException – if some property of the specified
        element prevents it from being added to this deque
/**
     * 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 method {@link #offerLast}.
     *
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to capacity restrictions
     * @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 and this
     *         deque does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    void addLast(E e);

    
Inserts the specified element at the front of this deque unless it would violate capacity restrictions. 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:
  • ClassCastException – if the class of the specified element
        prevents it from being added to this deque
  • NullPointerException – if the specified element is null and this
        deque does not permit null elements
  • IllegalArgumentException – if some property of the specified
        element prevents it from being added to this deque
Returns:true if the element was added to this deque, else false
/**
     * Inserts the specified element at the front of this deque unless it would
     * violate capacity restrictions.  When using a capacity-restricted deque,
     * this method is generally preferable to the {@link #addFirst} method,
     * which can fail to insert an element only by throwing an exception.
     *
     * @param e the element to add
     * @return {@code true} if the element was added to this deque, else
     *         {@code false}
     * @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 and this
     *         deque does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    boolean offerFirst(E e);

    
Inserts the specified element at the end of this deque unless it would violate capacity restrictions. 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:
  • ClassCastException – if the class of the specified element
        prevents it from being added to this deque
  • NullPointerException – if the specified element is null and this
        deque does not permit null elements
  • IllegalArgumentException – if some property of the specified
        element prevents it from being added to this deque
Returns:true if the element was added to this deque, else false
/**
     * Inserts the specified element at the end of this deque unless it would
     * violate capacity restrictions.  When using a capacity-restricted deque,
     * this method is generally preferable to the {@link #addLast} method,
     * which can fail to insert an element only by throwing an exception.
     *
     * @param e the element to add
     * @return {@code true} if the element was added to this deque, else
     *         {@code false}
     * @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 and this
     *         deque does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    boolean offerLast(E e);

    
Retrieves and removes the first element of this deque. This method differs from pollFirst only in that it throws an exception if this deque is empty. 
Throws:
Returns:the head of this deque
/**
     * Retrieves and removes the first element of this deque.  This method
     * differs from {@link #pollFirst pollFirst} only in that it throws an
     * exception if this deque is empty.
     *
     * @return the head of this deque
     * @throws NoSuchElementException if this deque is empty
     */
    E removeFirst();

    
Retrieves and removes the last element of this deque. This method differs from pollLast only in that it throws an exception if this deque is empty. 
Throws:
Returns:the tail of this deque
/**
     * Retrieves and removes the last element of this deque.  This method
     * differs from {@link #pollLast pollLast} only in that it throws an
     * exception if this deque is empty.
     *
     * @return the tail of this deque
     * @throws NoSuchElementException if this deque is empty
     */
    E removeLast();

    
Retrieves and removes the first element of this deque, or returns null if this deque is empty. 
Returns:the head of this deque, or null if this deque is empty
/**
     * Retrieves and removes the first element of this deque,
     * or returns {@code null} if this deque is empty.
     *
     * @return the head of this deque, or {@code null} if this deque is empty
     */
    E pollFirst();

    
Retrieves and removes the last element of this deque, or returns null if this deque is empty. 
Returns:the tail of this deque, or null if this deque is empty
/**
     * Retrieves and removes the last element of this deque,
     * or returns {@code null} if this deque is empty.
     *
     * @return the tail of this deque, or {@code null} if this deque is empty
     */
    E pollLast();

    
Retrieves, but does not remove, the first element of this deque. This method differs from peekFirst only in that it throws an exception if this deque is empty. 
Throws:
Returns:the head of this deque
/**
     * Retrieves, but does not remove, the first element of this deque.
     *
     * This method differs from {@link #peekFirst peekFirst} only in that it
     * throws an exception if this deque is empty.
     *
     * @return the head of this deque
     * @throws NoSuchElementException if this deque is empty
     */
    E getFirst();

    
Retrieves, but does not remove, the last element of this deque. This method differs from peekLast only in that it throws an exception if this deque is empty. 
Throws:
Returns:the tail of this deque
/**
     * Retrieves, but does not remove, the last element of this deque.
     * This method differs from {@link #peekLast peekLast} only in that it
     * throws an exception if this deque is empty.
     *
     * @return the tail of this deque
     * @throws NoSuchElementException if this deque is empty
     */
    E getLast();

    
Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty. 
Returns:the head of this deque, or null if this deque is empty
/**
     * Retrieves, but does not remove, the first element of this deque,
     * or returns {@code null} if this deque is empty.
     *
     * @return the head of this deque, or {@code null} if this deque is empty
     */
    E peekFirst();

    
Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty. 
Returns:the tail of this deque, or null if this deque is empty
/**
     * Retrieves, but does not remove, the last element of this deque,
     * or returns {@code null} if this deque is empty.
     *
     * @return the tail of this deque, or {@code null} if this deque is empty
     */
    E peekLast();

    
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 Objects.equals(o, 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 Objects.equals(o, 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 and this
     *         deque does not permit null elements
     * (<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 Objects.equals(o, 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 Objects.equals(o, 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 and this
     *         deque does not permit null elements
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    boolean removeLastOccurrence(Object o);

    // *** Queue 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:
  • IllegalStateException – if the element cannot be added at this
        time due to capacity restrictions
  • ClassCastException – if the class of the specified element
        prevents it from being added to this deque
  • NullPointerException – if the specified element is null and this
        deque does not permit null elements
  • IllegalArgumentException – if some property of the specified
        element prevents it from being added to this deque
Returns:true (as specified by Collection.add)
/**
     * 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}.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to capacity restrictions
     * @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 and this
     *         deque does not permit null elements
     * @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:
  • ClassCastException – if the class of the specified element
        prevents it from being added to this deque
  • NullPointerException – if the specified element is null and this
        deque does not permit null elements
  • IllegalArgumentException – if some property of the specified
        element prevents it from being added to this deque
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) 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}.
     *
     * @param e the element to add
     * @return {@code true} if the element was added to this deque, else
     *         {@code false}
     * @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 and this
     *         deque does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    boolean offer(E e);

    
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()}.
     *
     * @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 pollFirst(). 
Returns:the first element 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 first element of this deque, or {@code null} if
     *         this deque is empty
     */
    E poll();

    
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 the queue represented by 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()}.
     *
     * @return the head of the queue represented by 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 the queue represented by 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()}.
     *
     * @return the head of the queue represented by this deque, or
     *         {@code null} if this deque is empty
     */
    E peek();

    
Adds all of the elements in the specified collection at the end of this deque, as if by calling addLast on each one, in the order that they are returned by the collection's iterator. 
When using a capacity-restricted deque, it is generally preferable to call offer separately on each element. 
An exception encountered while trying to add an element may result
in only some of the elements having been successfully added when
the associated exception is thrown.

Params:
  • c – the elements to be inserted into this deque
Throws:
  • IllegalStateException – if not all the elements can be added at
        this time due to insertion restrictions
  • ClassCastException – if the class of an element of the specified
        collection prevents it from being added to this deque
  • NullPointerException – if the specified collection contains a
        null element and this deque does not permit null elements,
        or if the specified collection is null
  • IllegalArgumentException – if some property of an element of the
        specified collection prevents it from being added to this deque
Returns:true if this deque changed as a result of the call
/**
     * Adds all of the elements in the specified collection at the end
     * of this deque, as if by calling {@link #addLast} on each one,
     * in the order that they are returned by the collection's iterator.
     *
     * <p>When using a capacity-restricted deque, it is generally preferable
     * to call {@link #offer(Object) offer} separately on each element.
     *
     * <p>An exception encountered while trying to add an element may result
     * in only some of the elements having been successfully added when
     * the associated exception is thrown.
     *
     * @param c the elements to be inserted into this deque
     * @return {@code true} if this deque changed as a result of the call
     * @throws IllegalStateException if not all the elements can be added at
     *         this time due to insertion restrictions
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this deque
     * @throws NullPointerException if the specified collection contains a
     *         null element and this deque does not permit null elements,
     *         or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this deque
     */
    boolean addAll(Collection<? extends E> c);

    // *** 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. 
Params:
  • e – the element to push
Throws:
  • IllegalStateException – if the element cannot be added at this
        time due to capacity restrictions
  • ClassCastException – if the class of the specified element
        prevents it from being added to this deque
  • NullPointerException – if the specified element is null and this
        deque does not permit null elements
  • IllegalArgumentException – if some property of the specified
        element prevents it from being added to this deque
/**
     * 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}.
     *
     * @param e the element to push
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to capacity restrictions
     * @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 and this
     *         deque does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this deque
     */
    void push(E e);

    
Pops an element from the stack represented by this deque.  In other
words, removes and returns the first element of this deque.

This method is equivalent to removeFirst(). 
Throws:
Returns:the element at the front of this deque (which is the top
        of the stack represented by this deque)
/**
     * Pops an element from the stack represented by this deque.  In other
     * words, removes and returns the first element of this deque.
     *
     * <p>This method is equivalent to {@link #removeFirst()}.
     *
     * @return the element at the front of this deque (which is the top
     *         of the stack represented by this deque)
     * @throws NoSuchElementException if this deque is empty
     */
    E pop();


    // *** 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 Objects.equals(o, 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(Object). 
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 Objects.equals(o, e)} (if such an element exists).
     * Returns {@code true} if this deque contained the specified element
     * (or equivalently, if this deque changed as a result of the call).
     *
     * <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
     *
     * @param o element to be removed from this deque, if present
     * @return {@code true} if 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 and this
     *         deque does not permit null elements
     * (<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 Objects.equals(o, e). 
Params:
  • o – element whose presence in this deque is to be tested
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 Objects.equals(o, e)}.
     *
     * @param o element whose presence in this deque is to be tested
     * @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 and this
     *         deque does not permit null elements
     * (<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();

    
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).

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

}