--- jsr166/src/main/java/util/Queue.java 2003/05/18 18:10:02 1.2 +++ jsr166/src/main/java/util/Queue.java 2003/07/28 19:53:49 1.9 @@ -1,33 +1,43 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain. Use, modify, and + * redistribute this code in any way without acknowledgement. + */ + package java.util; /** - * A Collection designed for holding elements prior to processing. + * A collection designed for holding elements prior to processing. * Besides basic {@link 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 comparators, or - * the elements natural ordering. Every Queue implementation must specify - * its ordering guarantees. + * 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()}. + * Every Queue implementation must specify its ordering guarantees. * *

The {@link #offer(E)} method adds an element if possible, otherwise * returning false. This differs from the {@link * Collections#add(Object)} method, which throws an unchecked exception upon * failure. It is designed for use in collections in which failure to * add is a normal, rather than exceptional occurrence, for example, - * in fixed-capacity (or “bounded”) queues. + * in fixed-capacity (or "bounded") queues. * - *

The {@link #remove()} and {@link #poll()} methods remove and return an - * element in accord with the implementation's ordering policy. For example, - * in FIFO queues, they remove and return the oldest element in the queue. - * 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 delete the element that would be obtained by a call to - * the remove and poll methods respectively. + *

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 delete, the head of the queue. * *

The Queue interface does not define the blocking queue * methods, which are common in concurrent programming. These methods, @@ -35,11 +45,13 @@ package java.util; * defined in the {@link java.util.concurrent.BlockingQueue} interface, which * extends this interface. * - *

Queue implementations generally do not allow insertion of - * null elements. Even in the few implementations that permit it, - * it is a bad idea, 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 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. * *

This interface is a member of the * @@ -48,60 +60,71 @@ package java.util; * @see Collection * @see LinkedList * @see PriorityQueue - * @see LinkedQueue + * @see java.util.concurrent.LinkedQueue * @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 */ public interface Queue extends Collection { + /** * Add the specified element to this queue, if possible. * * @param element the element to add. - * @return true if it was possible to add the element to the queue. + * @return true if it was possible to add the element to + * this queue. */ - public boolean offer(E element); + boolean offer(E element); /** - * Remove and return an element from the queue if one is available. - * Exactly which element is removed from the queue is a function - * of the queue's ordering policy, which differs from implementation - * to implementation. Possible orderings include (but are not limited - * to) first-in-first-out (FIFO), element priority, and arbitrary. + * Retrieve and remove the head of this queue, if it is available. * - * @return an element previously on the queue, or null if the + * @return the head of this queue, or null if this * queue is empty. */ - public E poll(); + E poll(); /** - * Remove and return an element from the queue. This method differs + * Retrieve and remove the head of this queue. + * This method differs * from the poll method in that it throws an exception if the * queue is empty. * - * @return an element previously on the queue. - * @throws NoSuchElementException if the queue is empty. + * @return the head of this queue. + * @throws NoSuchElementException if this queue is empty. */ - public E remove() throws NoSuchElementException; + E remove(); /** - * Return, but do not remove, an element from the queue, or null - * if the queue is empty. This method returns the same object reference - * that would be returned by by the poll method. The two methods - * differ in that this method does not remove the element from the queue. + * Retrieve, but do not remove, the head of this queue, or null + * if this queue is empty. This method differs from the poll + * method only in that this method does not remove the element from + * this queue. * - * @return an element on the queue, or null if the queue is empty. + * @return the head of this queue, or null if this queue is empty. */ - public E peek(); + E peek(); /** - * Return, but do not remove, an element from the queue. This method - * differs from the peek method in that it throws an exception if - * the queue is empty. + * Retrieve, but do not remove, the head of this queue. This method + * differs from the peek method only in that it throws an + * exception if this queue is empty. * - * @return an element on the queue. - * @throws NoSuchElementException if the queue is empty. + * @return the head of this queue. + * @throws NoSuchElementException if this queue is empty. */ - public E element() throws NoSuchElementException; + E element(); } + + + + + + + + + +