/*

  * 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 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 <tt>null</tt> or <tt>false</tt>, depending on the

  * operation).  The latter form of the insert operation is designed

  * specifically for use with capacity-restricted <tt>Queue</tt>

  * implementations; in most implementations, insert operations cannot

  * fail.

  *

  * <p>

  * <table BORDER CELLPADDING=3 CELLSPACING=1>

  *  <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 #add add(e)}</td>

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

  *  </tr>

  *  <tr>

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

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

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

  *  </tr>

  *  <tr>

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

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

  *    <td>{@link #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 <tt>Queue</tt> implementation

  * must specify its ordering properties.

  *

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

  * otherwise returning <tt>false</tt>.  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

  * <tt>offer</tt> 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 <tt>remove()</tt> and

  * <tt>poll()</tt> methods differ only in their behavior when the

  * queue is empty: the <tt>remove()</tt> method throws an exception,

  * while the <tt>poll()</tt> method returns <tt>null</tt>.

  *

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

  * not remove, the head of the queue.

  *

  * <p>The <tt>Queue</tt> 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><tt>Queue</tt> implementations generally do not allow insertion

  * of <tt>null</tt> elements, although some implementations, such as

  * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.

  * Even in the implementations that permit it, <tt>null</tt> should

  * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also

  * used as a special return value by the <tt>poll</tt> method to

  * indicate that the queue contains no elements.

  *

  * <p><tt>Queue</tt> implementations generally do not define

  * element-based versions of methods <tt>equals</tt> and

  * <tt>hashCode</tt> but instead inherit the identity based versions

  * from class <tt>Object</tt>, 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

      * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>

      * if no space is currently available.

      *

      * @param e the element to add

      * @return <tt>true</tt> (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 <tt>true</tt> if the element was added to this queue, else

      *         <tt>false</tt>

      * @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 <tt>null</tt> if this queue is empty.

      *

      * @return the head of this queue, or <tt>null</tt> 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 <tt>null</tt> if this queue is empty.

      *

      * @return the head of this queue, or <tt>null</tt> if this queue is empty

      */

     E peek();

 }