jaroslav@597: /*
jaroslav@597:  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
jaroslav@597:  *
jaroslav@597:  * This code is free software; you can redistribute it and/or modify it
jaroslav@597:  * under the terms of the GNU General Public License version 2 only, as
jaroslav@597:  * published by the Free Software Foundation.  Oracle designates this
jaroslav@597:  * particular file as subject to the "Classpath" exception as provided
jaroslav@597:  * by Oracle in the LICENSE file that accompanied this code.
jaroslav@597:  *
jaroslav@597:  * This code is distributed in the hope that it will be useful, but WITHOUT
jaroslav@597:  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
jaroslav@597:  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
jaroslav@597:  * version 2 for more details (a copy is included in the LICENSE file that
jaroslav@597:  * accompanied this code).
jaroslav@597:  *
jaroslav@597:  * You should have received a copy of the GNU General Public License version
jaroslav@597:  * 2 along with this work; if not, write to the Free Software Foundation,
jaroslav@597:  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
jaroslav@597:  *
jaroslav@597:  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
jaroslav@597:  * or visit www.oracle.com if you need additional information or have any
jaroslav@597:  * questions.
jaroslav@597:  */
jaroslav@597: 
jaroslav@597: /*
jaroslav@597:  * This file is available under and governed by the GNU General Public
jaroslav@597:  * License version 2 only, as published by the Free Software Foundation.
jaroslav@597:  * However, the following notice accompanied the original version of this
jaroslav@597:  * file:
jaroslav@597:  *
jaroslav@597:  * Written by Doug Lea with assistance from members of JCP JSR-166
jaroslav@597:  * Expert Group and released to the public domain, as explained at
jaroslav@597:  * http://creativecommons.org/publicdomain/zero/1.0/
jaroslav@597:  */
jaroslav@597: 
jaroslav@597: package java.util;
jaroslav@597: 
jaroslav@597: /**
jaroslav@597:  * A collection designed for holding elements prior to processing.
jaroslav@597:  * Besides basic {@link java.util.Collection Collection} operations,
jaroslav@597:  * queues provide additional insertion, extraction, and inspection
jaroslav@597:  * operations.  Each of these methods exists in two forms: one throws
jaroslav@597:  * an exception if the operation fails, the other returns a special
jaroslav@597:  * value (either <tt>null</tt> or <tt>false</tt>, depending on the
jaroslav@597:  * operation).  The latter form of the insert operation is designed
jaroslav@597:  * specifically for use with capacity-restricted <tt>Queue</tt>
jaroslav@597:  * implementations; in most implementations, insert operations cannot
jaroslav@597:  * fail.
jaroslav@597:  *
jaroslav@597:  * <p>
jaroslav@597:  * <table BORDER CELLPADDING=3 CELLSPACING=1>
jaroslav@597:  *  <tr>
jaroslav@597:  *    <td></td>
jaroslav@597:  *    <td ALIGN=CENTER><em>Throws exception</em></td>
jaroslav@597:  *    <td ALIGN=CENTER><em>Returns special value</em></td>
jaroslav@597:  *  </tr>
jaroslav@597:  *  <tr>
jaroslav@597:  *    <td><b>Insert</b></td>
jaroslav@597:  *    <td>{@link #add add(e)}</td>
jaroslav@597:  *    <td>{@link #offer offer(e)}</td>
jaroslav@597:  *  </tr>
jaroslav@597:  *  <tr>
jaroslav@597:  *    <td><b>Remove</b></td>
jaroslav@597:  *    <td>{@link #remove remove()}</td>
jaroslav@597:  *    <td>{@link #poll poll()}</td>
jaroslav@597:  *  </tr>
jaroslav@597:  *  <tr>
jaroslav@597:  *    <td><b>Examine</b></td>
jaroslav@597:  *    <td>{@link #element element()}</td>
jaroslav@597:  *    <td>{@link #peek peek()}</td>
jaroslav@597:  *  </tr>
jaroslav@597:  * </table>
jaroslav@597:  *
jaroslav@597:  * <p>Queues typically, but do not necessarily, order elements in a
jaroslav@597:  * FIFO (first-in-first-out) manner.  Among the exceptions are
jaroslav@597:  * priority queues, which order elements according to a supplied
jaroslav@597:  * comparator, or the elements' natural ordering, and LIFO queues (or
jaroslav@597:  * stacks) which order the elements LIFO (last-in-first-out).
jaroslav@597:  * Whatever the ordering used, the <em>head</em> of the queue is that
jaroslav@597:  * element which would be removed by a call to {@link #remove() } or
jaroslav@597:  * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
jaroslav@597:  * the <em> tail</em> of the queue. Other kinds of queues may use
jaroslav@597:  * different placement rules.  Every <tt>Queue</tt> implementation
jaroslav@597:  * must specify its ordering properties.
jaroslav@597:  *
jaroslav@597:  * <p>The {@link #offer offer} method inserts an element if possible,
jaroslav@597:  * otherwise returning <tt>false</tt>.  This differs from the {@link
jaroslav@597:  * java.util.Collection#add Collection.add} method, which can fail to
jaroslav@597:  * add an element only by throwing an unchecked exception.  The
jaroslav@597:  * <tt>offer</tt> method is designed for use when failure is a normal,
jaroslav@597:  * rather than exceptional occurrence, for example, in fixed-capacity
jaroslav@597:  * (or &quot;bounded&quot;) queues.
jaroslav@597:  *
jaroslav@597:  * <p>The {@link #remove()} and {@link #poll()} methods remove and
jaroslav@597:  * return the head of the queue.
jaroslav@597:  * Exactly which element is removed from the queue is a
jaroslav@597:  * function of the queue's ordering policy, which differs from
jaroslav@597:  * implementation to implementation. The <tt>remove()</tt> and
jaroslav@597:  * <tt>poll()</tt> methods differ only in their behavior when the
jaroslav@597:  * queue is empty: the <tt>remove()</tt> method throws an exception,
jaroslav@597:  * while the <tt>poll()</tt> method returns <tt>null</tt>.
jaroslav@597:  *
jaroslav@597:  * <p>The {@link #element()} and {@link #peek()} methods return, but do
jaroslav@597:  * not remove, the head of the queue.
jaroslav@597:  *
jaroslav@597:  * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
jaroslav@597:  * methods</i>, which are common in concurrent programming.  These methods,
jaroslav@597:  * which wait for elements to appear or for space to become available, are
jaroslav@597:  * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
jaroslav@597:  * extends this interface.
jaroslav@597:  *
jaroslav@597:  * <p><tt>Queue</tt> implementations generally do not allow insertion
jaroslav@597:  * of <tt>null</tt> elements, although some implementations, such as
jaroslav@597:  * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
jaroslav@597:  * Even in the implementations that permit it, <tt>null</tt> should
jaroslav@597:  * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
jaroslav@597:  * used as a special return value by the <tt>poll</tt> method to
jaroslav@597:  * indicate that the queue contains no elements.
jaroslav@597:  *
jaroslav@597:  * <p><tt>Queue</tt> implementations generally do not define
jaroslav@597:  * element-based versions of methods <tt>equals</tt> and
jaroslav@597:  * <tt>hashCode</tt> but instead inherit the identity based versions
jaroslav@597:  * from class <tt>Object</tt>, because element-based equality is not
jaroslav@597:  * always well-defined for queues with the same elements but different
jaroslav@597:  * ordering properties.
jaroslav@597:  *
jaroslav@597:  *
jaroslav@597:  * <p>This interface is a member of the
jaroslav@597:  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
jaroslav@597:  * Java Collections Framework</a>.
jaroslav@597:  *
jaroslav@597:  * @see java.util.Collection
jaroslav@597:  * @see LinkedList
jaroslav@597:  * @see PriorityQueue
jaroslav@597:  * @see java.util.concurrent.LinkedBlockingQueue
jaroslav@597:  * @see java.util.concurrent.BlockingQueue
jaroslav@597:  * @see java.util.concurrent.ArrayBlockingQueue
jaroslav@597:  * @see java.util.concurrent.LinkedBlockingQueue
jaroslav@597:  * @see java.util.concurrent.PriorityBlockingQueue
jaroslav@597:  * @since 1.5
jaroslav@597:  * @author Doug Lea
jaroslav@597:  * @param <E> the type of elements held in this collection
jaroslav@597:  */
jaroslav@597: public interface Queue<E> extends Collection<E> {
jaroslav@597:     /**
jaroslav@597:      * Inserts the specified element into this queue if it is possible to do so
jaroslav@597:      * immediately without violating capacity restrictions, returning
jaroslav@597:      * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
jaroslav@597:      * if no space is currently available.
jaroslav@597:      *
jaroslav@597:      * @param e the element to add
jaroslav@597:      * @return <tt>true</tt> (as specified by {@link Collection#add})
jaroslav@597:      * @throws IllegalStateException if the element cannot be added at this
jaroslav@597:      *         time due to capacity restrictions
jaroslav@597:      * @throws ClassCastException if the class of the specified element
jaroslav@597:      *         prevents it from being added to this queue
jaroslav@597:      * @throws NullPointerException if the specified element is null and
jaroslav@597:      *         this queue does not permit null elements
jaroslav@597:      * @throws IllegalArgumentException if some property of this element
jaroslav@597:      *         prevents it from being added to this queue
jaroslav@597:      */
jaroslav@597:     boolean add(E e);
jaroslav@597: 
jaroslav@597:     /**
jaroslav@597:      * Inserts the specified element into this queue if it is possible to do
jaroslav@597:      * so immediately without violating capacity restrictions.
jaroslav@597:      * When using a capacity-restricted queue, this method is generally
jaroslav@597:      * preferable to {@link #add}, which can fail to insert an element only
jaroslav@597:      * by throwing an exception.
jaroslav@597:      *
jaroslav@597:      * @param e the element to add
jaroslav@597:      * @return <tt>true</tt> if the element was added to this queue, else
jaroslav@597:      *         <tt>false</tt>
jaroslav@597:      * @throws ClassCastException if the class of the specified element
jaroslav@597:      *         prevents it from being added to this queue
jaroslav@597:      * @throws NullPointerException if the specified element is null and
jaroslav@597:      *         this queue does not permit null elements
jaroslav@597:      * @throws IllegalArgumentException if some property of this element
jaroslav@597:      *         prevents it from being added to this queue
jaroslav@597:      */
jaroslav@597:     boolean offer(E e);
jaroslav@597: 
jaroslav@597:     /**
jaroslav@597:      * Retrieves and removes the head of this queue.  This method differs
jaroslav@597:      * from {@link #poll poll} only in that it throws an exception if this
jaroslav@597:      * queue is empty.
jaroslav@597:      *
jaroslav@597:      * @return the head of this queue
jaroslav@597:      * @throws NoSuchElementException if this queue is empty
jaroslav@597:      */
jaroslav@597:     E remove();
jaroslav@597: 
jaroslav@597:     /**
jaroslav@597:      * Retrieves and removes the head of this queue,
jaroslav@597:      * or returns <tt>null</tt> if this queue is empty.
jaroslav@597:      *
jaroslav@597:      * @return the head of this queue, or <tt>null</tt> if this queue is empty
jaroslav@597:      */
jaroslav@597:     E poll();
jaroslav@597: 
jaroslav@597:     /**
jaroslav@597:      * Retrieves, but does not remove, the head of this queue.  This method
jaroslav@597:      * differs from {@link #peek peek} only in that it throws an exception
jaroslav@597:      * if this queue is empty.
jaroslav@597:      *
jaroslav@597:      * @return the head of this queue
jaroslav@597:      * @throws NoSuchElementException if this queue is empty
jaroslav@597:      */
jaroslav@597:     E element();
jaroslav@597: 
jaroslav@597:     /**
jaroslav@597:      * Retrieves, but does not remove, the head of this queue,
jaroslav@597:      * or returns <tt>null</tt> if this queue is empty.
jaroslav@597:      *
jaroslav@597:      * @return the head of this queue, or <tt>null</tt> if this queue is empty
jaroslav@597:      */
jaroslav@597:     E peek();
jaroslav@597: }