| /* |
| * 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 |
| * or visit www.oracle.com if you need additional information or have any |
| * 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 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(); |
| } |
