/* * @(#)Queue.java 1.5 03/12/19 * * Copyright 2004 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ 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. * *
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 head 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 tail of the queue. Other kinds of queues may use * different placement rules. Every Queue implementation * must specify its ordering properties. * *
The {@link #offer offer} method inserts an element if possible, * otherwise returning 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 * offer method is designed for use when failure is a normal, * rather than exceptional occurrence, for example, in fixed-capacity * (or "bounded") queues. * *
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 remove() and * poll() methods differ only in their behavior when the * queue is empty: the remove() method throws an exception, * while the poll() method returns null. * *
The {@link #element()} and {@link #peek()} methods return, but do * not remove, the head of the queue. * *
The Queue interface does not define the blocking queue * methods, 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. * *
Queue implementations generally do not allow insertion * of null elements, although some implementations, such as * {@link LinkedList}, do not prohibit insertion of null. * Even in the implementations that permit it, null should * not be inserted into a Queue, as null is also * used as a special return value by the poll method to * indicate that the queue contains no elements. * *
Queue implementations generally do not define * element-based versions of methods equals and * hashCode but instead inherit the identity based versions * from class Object, because element-based equality is not * always well-defined for queues with the same elements but different * ordering properties. * * *
This interface is a member of the
*
* Java Collections Framework.
*
* @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