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 * published by the Free Software Foundation.  Oracle designates this

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 * 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

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 * accompanied this code).

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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 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 collection designed for holding elements prior to processing.

 * Besides basic {@link java.util.Collection Collection} operations,

 * queues provide additional insertion, extraction, and inspection

 * operations.  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 Queue}

 * implementations; in most implementations, insert operations cannot

 * fail.

 *

 * <table BORDER CELLPADDING=3 CELLSPACING=1>

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

 *  <tr>

 *    <td></td>

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

 *    <td ALIGN=CENTER><em>Returns special value</em></td>

 *  </tr>

 *  <tr>

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

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

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

 *  </tr>

 *  <tr>

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

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

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

 *  </tr>

 *  <tr>

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

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

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

 *  </tr>

 * </table>

 *

 * <p>Queues typically, but do not necessarily, order elements in a

 * FIFO (first-in-first-out) manner.  Among the exceptions are

 * priority queues, which order elements according to a supplied

 * comparator, or the elements' natural ordering, and LIFO queues (or

 * stacks) which order the elements LIFO (last-in-first-out).

 * Whatever the ordering used, the <em>head</em> of the queue is that

 * element which would be removed by a call to {@link #remove() } or

 * {@link #poll()}.  In a FIFO queue, all new elements are inserted at

 * the <em>tail</em> of the queue. Other kinds of queues may use

 * different placement rules.  Every {@code Queue} implementation

 * must specify its ordering properties.

 *

 * <p>The {@link #offer offer} method inserts an element if possible,

 * otherwise returning {@code false}.  This differs from the {@link

 * java.util.Collection#add Collection.add} method, which can fail to

 * add an element only by throwing an unchecked exception.  The

 * {@code offer} method is designed for use when failure is a normal,

 * rather than exceptional occurrence, for example, in fixed-capacity

 * (or "bounded") queues.

 *

 * <p>The {@link #remove()} and {@link #poll()} methods remove and

 * return the head of the queue.

 * Exactly which element is removed from the queue is a

 * function of the queue's ordering policy, which differs from

 * implementation to implementation. The {@code remove()} and

 * {@code poll()} methods differ only in their behavior when the

 * queue is empty: the {@code remove()} method throws an exception,

 * while the {@code poll()} method returns {@code null}.

 *

 * <p>The {@link #element()} and {@link #peek()} methods return, but do

 * not remove, the head of the queue.

 *

 * <p>The {@code Queue} interface does not define the <i>blocking queue

 * methods</i>, which are common in concurrent programming.  These methods,

 * which wait for elements to appear or for space to become available, are

 * defined in the {@link java.util.concurrent.BlockingQueue} interface, which

 * extends this interface.

 *

 * <p>{@code Queue} implementations generally do not allow insertion

 * of {@code null} elements, although some implementations, such as

 * {@link LinkedList}, do not prohibit insertion of {@code null}.

 * Even in the implementations that permit it, {@code null} should

 * not be inserted into a {@code Queue}, as {@code null} is also

 * used as a special return value by the {@code poll} method to

 * indicate that the queue contains no elements.

 *

 * <p>{@code Queue} implementations generally do not define

 * element-based versions of methods {@code equals} and

 * {@code hashCode} but instead inherit the identity based versions

 * from class {@code Object}, because element-based equality is not

 * always well-defined for queues with the same elements but different

 * ordering properties.

 *

 *

 * <p>This interface is a member of the

 * <a href="{@docRoot}/../technotes/guides/collections/index.html">

 * Java Collections Framework</a>.

 *

 * @see java.util.Collection

 * @see LinkedList

 * @see PriorityQueue

 * @see java.util.concurrent.LinkedBlockingQueue

 * @see java.util.concurrent.BlockingQueue

 * @see java.util.concurrent.ArrayBlockingQueue

 * @see java.util.concurrent.LinkedBlockingQueue

 * @see java.util.concurrent.PriorityBlockingQueue

 * @since 1.5

 * @author Doug Lea

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

 */

public interface Queue<E> extends Collection<E> {

    /**

     * Inserts the specified element into this queue 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.

     *

     * @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 queue

     * @throws NullPointerException if the specified element is null and

     *         this queue does not permit null elements

     * @throws IllegalArgumentException if some property of this element

     *         prevents it from being added to this queue

     */

    boolean add(E e);

    /**

     * Inserts the specified element into this queue if it is possible to do

     * so immediately without violating capacity restrictions.

     * When using a capacity-restricted queue, this method is generally

     * preferable to {@link #add}, 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 queue, else

     *         {@code false}

     * @throws ClassCastException if the class of the specified element

     *         prevents it from being added to this queue

     * @throws NullPointerException if the specified element is null and

     *         this queue does not permit null elements

     * @throws IllegalArgumentException if some property of this element

     *         prevents it from being added to this queue

     */

    boolean offer(E e);

    /**

     * Retrieves and removes the head of this queue.  This method differs

     * from {@link #poll poll} only in that it throws an exception if this

     * queue is empty.

     *

     * @return the head of this queue

     * @throws NoSuchElementException if this queue is empty

     */

    E remove();

    /**

     * Retrieves and removes the head of this queue,

     * or returns {@code null} if this queue is empty.

     *

     * @return the head of this queue, or {@code null} if this queue is empty

     */

    E poll();

    /**

     * Retrieves, but does not remove, the head of this queue.  This method

     * differs from {@link #peek peek} only in that it throws an exception

     * if this queue is empty.

     *

     * @return the head of this queue

     * @throws NoSuchElementException if this queue is empty

     */

    E element();

    /**

     * Retrieves, but does not remove, the head of this queue,

     * or returns {@code null} if this queue is empty.

     *

     * @return the head of this queue, or {@code null} if this queue is empty

     */

    E peek();

}