--- jsr166/src/main/java/util/Queue.java 2003/06/24 14:34:30 1.7 +++ jsr166/src/main/java/util/Queue.java 2003/09/15 12:02:23 1.19 @@ -7,38 +7,41 @@ package java.util; /** - * A Collection designed for holding elements prior to processing. - * Besides basic {@link Collection} operations, queues provide + * 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. -0 * + * *

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. Every Queue - * implementation must specify its ordering guarantees. + * 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 #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. - - * *

The {@link #remove()} and {@link #poll()} methods remove and - * return an element in accord with the implementation's ordering - * policy. Exactly which element is removed from the queue is a + * 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. Possible orderings include (but - * are not limited to) first-in-first-out (FIFO), last-in-first-out - * (LIFO), element priority, and arbitrary. The remove() and + * 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 element that would be obtained by a call to - * the remove and poll methods respectively. + *

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, @@ -54,14 +57,22 @@ package java.util; * 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 Collection + * @see java.util.Collection * @see LinkedList * @see PriorityQueue - * @see java.util.concurrent.LinkedQueue + * @see java.util.concurrent.LinkedBlockingQueue * @see java.util.concurrent.BlockingQueue * @see java.util.concurrent.ArrayBlockingQueue * @see java.util.concurrent.LinkedBlockingQueue @@ -70,49 +81,55 @@ package java.util; * @author Doug Lea */ public interface Queue extends Collection { + /** - * Add the specified element to this queue, if possible. + * Inserts the specified element into this queue, if possible. When + * using queues that may impose insertion restrictions (for + * example capacity bounds), method offer is generally + * preferable to method {@link Collection#add}, which can fail to + * insert an element only by throwing an exception. * - * @param element the element to add. - * @return true if it was possible to add the element to the queue. + * @param o the element to insert. + * @return true if it was possible to add the element to + * this queue, else false */ - boolean offer(E element); + boolean offer(E o); /** - * Remove and return an element from the queue if one is available. + * Retrieves and removes the head of this queue, or null + * if this queue is empty. * - * @return an element previously on the queue, or null if the - * queue is empty. + * @return the head of this queue, or null if this + * queue is empty. */ E poll(); /** - * Remove and return an element from the queue. This method differs - * from the poll method in that it throws an exception if the - * queue is empty. + * Retrieves and removes the head of this queue. This method + * differs from the poll method in that it throws an + * exception if this 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. */ - 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. + * Retrieves, but does not remove, the head of this queue, + * returning null if this queue is empty. * - * @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. */ 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. + * Retrieves, but does 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. */ - E element() throws NoSuchElementException; + E element(); }