/*

 * 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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/*

 * 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 <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 BORDER CELLPADDING=3 CELLSPACING=1>

 * <caption>Summary of Deque methods</caption>

 *  <tr>

 *    <td></td>

 *    <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td>

 *    <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td>

 *  </tr>

 *  <tr>

 *    <td></td>

 *    <td ALIGN=CENTER><em>Throws exception</em></td>

 *    <td ALIGN=CENTER><em>Special value</em></td>

 *    <td ALIGN=CENTER><em>Throws exception</em></td>

 *    <td ALIGN=CENTER><em>Special value</em></td>

 *  </tr>

 *  <tr>

 *    <td><b>Insert</b></td>

 *    <td>{@link Deque#addFirst addFirst(e)}</td>

 *    <td>{@link Deque#offerFirst offerFirst(e)}</td>

 *    <td>{@link Deque#addLast addLast(e)}</td>

 *    <td>{@link Deque#offerLast offerLast(e)}</td>

 *  </tr>

 *  <tr>

 *    <td><b>Remove</b></td>

 *    <td>{@link Deque#removeFirst removeFirst()}</td>

 *    <td>{@link Deque#pollFirst pollFirst()}</td>

 *    <td>{@link Deque#removeLast removeLast()}</td>

 *    <td>{@link Deque#pollLast pollLast()}</td>

 *  </tr>

 *  <tr>

 *    <td><b>Examine</b></td>

 *    <td>{@link Deque#getFirst getFirst()}</td>

 *    <td>{@link Deque#peekFirst peekFirst()}</td>

 *    <td>{@link Deque#getLast getLast()}</td>

 *    <td>{@link Deque#peekLast peekLast()}</td>

 *  </tr>

 * </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 BORDER CELLPADDING=3 CELLSPACING=1>

 * <caption>Comparison of Queue and Deque methods</caption>

 *  <tr>

 *    <td ALIGN=CENTER> <b>{@code Queue} Method</b></td>

 *    <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#add add(e)}</td>

 *    <td>{@link #addLast addLast(e)}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#offer offer(e)}</td>

 *    <td>{@link #offerLast offerLast(e)}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#remove remove()}</td>

 *    <td>{@link #removeFirst removeFirst()}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#poll poll()}</td>

 *    <td>{@link #pollFirst pollFirst()}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#element element()}</td>

 *    <td>{@link #getFirst getFirst()}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link java.util.Queue#peek peek()}</td>

 *    <td>{@link #peek peekFirst()}</td>

 *  </tr>

 * </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 precisely equivalent to

 * {@code Deque} methods as indicated in the table below:

 *

 * <table BORDER CELLPADDING=3 CELLSPACING=1>

 * <caption>Comparison of Stack and Deque methods</caption>

 *  <tr>

 *    <td ALIGN=CENTER> <b>Stack Method</b></td>

 *    <td ALIGN=CENTER> <b>Equivalent {@code Deque} Method</b></td>

 *  </tr>

 *  <tr>

 *    <td>{@link #push push(e)}</td>

 *    <td>{@link #addFirst addFirst(e)}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link #pop pop()}</td>

 *    <td>{@link #removeFirst removeFirst()}</td>

 *  </tr>

 *  <tr>

 *    <td>{@link #peek peek()}</td>

 *    <td>{@link #peekFirst peekFirst()}</td>

 *  </tr>

 * </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 indicated 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}/../technotes/guides/collections/index.html"> Java Collections

 * Framework</a>.

 *

 * @author Doug Lea

 * @author Josh Bloch

 * @since  1.6

 * @param <E> the type of elements held in this collection

 */

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 {@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 {@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 {@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 {@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 {@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 {@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 {@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 {@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 {@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 {@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 {@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 {@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 {@code e} such that

     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>

     * (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="Collection.html#optional-restrictions">optional</a>)

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements

     * (<a href="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 {@code e} such that

     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>

     * (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="Collection.html#optional-restrictions">optional</a>)

     * @throws NullPointerException if the specified element is null and this

     *         deque does not permit null elements

     * (<a href="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

     * {@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

     * {@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 {@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

     * {@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 {@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 {@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();

    // *** 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

     * {@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.

     *

     * <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();