Java tutorial
/* * 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(); }