| /* | |
| * 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 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 class="striped"> | |
| * <caption>Summary of Queue methods</caption> | |
| * <thead> | |
| * <tr> | |
| * <td></td> | |
| * <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th> | |
| * <th scope="col" style="font-weight:normal; font-style:italic">Returns special value</th> | |
| * </tr> | |
| * </thead> | |
| * <tbody> | |
| * <tr> | |
| * <th scope="row">Insert</th> | |
| * <td>{@link #add(Object) add(e)}</td> | |
| * <td>{@link #offer(Object) offer(e)}</td> | |
| * </tr> | |
| * <tr> | |
| * <th scope="row">Remove</th> | |
| * <td>{@link #remove() remove()}</td> | |
| * <td>{@link #poll() poll()}</td> | |
| * </tr> | |
| * <tr> | |
| * <th scope="row">Examine</th> | |
| * <td>{@link #element() element()}</td> | |
| * <td>{@link #peek() peek()}</td> | |
| * </tr> | |
| * </tbody> | |
| * </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}/java.base/java/util/package-summary.html#CollectionsFramework"> | |
| * Java Collections Framework</a>. | |
| * | |
| * @since 1.5 | |
| * @author Doug Lea | |
| * @param <E> the type of elements held in this queue | |
| */ | |
| 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(); | |
| } |