--- jsr166/src/main/java/util/Queue.java 2003/07/31 07:18:02 1.10 +++ jsr166/src/main/java/util/Queue.java 2011/03/15 19:47:03 1.36 @@ -1,31 +1,66 @@ /* * 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. + * 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. + * Besides basic {@link java.util.Collection 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 null or false, depending on the + * operation). The latter form of the insert operation is designed + * specifically for use with capacity-restricted Queue + * implementations; in most implementations, insert operations cannot + * fail. + * + *

+ * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + *
Throws exceptionReturns special value
Insert{@link #add add(e)}{@link #offer offer(e)}
Remove{@link #remove remove()}{@link #poll poll()}
Examine{@link #element element()}{@link #peek peek()}
* *

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()}. - * 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. + * 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. @@ -53,78 +88,102 @@ 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 * @see java.util.concurrent.PriorityBlockingQueue * @since 1.5 * @author Doug Lea + * @param the type of elements held in this collection */ public interface Queue extends Collection { - /** - * Add the specified element to this queue, if possible. + * Inserts the specified element into this queue if it is possible to do so + * immediately without violating capacity restrictions, returning + * true upon success and throwing an IllegalStateException + * if no space is currently available. * - * @param o the element to add. - * @return true if it was possible to add the element to - * this queue, else false + * @param e the element to add + * @return 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 offer(E o); + boolean add(E e); /** - * Retrieve and remove the head of this queue, if it is available. + * 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. * - * @return the head of this queue, or null if this - * queue is empty. + * @param e the element to add + * @return true if the element was added to this queue, else + * 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 */ - E poll(); + boolean offer(E e); /** - * Retrieve and remove the head of this queue. - * This method differs - * from the poll method in that it throws an exception if this + * 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. + * @return the head of this queue + * @throws NoSuchElementException if this queue is empty */ E remove(); /** - * Retrieve, but do not remove, the head of this queue. - * This method differs from the poll - * method only in that this method does not remove the head element from - * this queue. + * Retrieves and removes the head of this queue, + * or returns null if this queue is empty. * - * @return the head of this queue, or null if this queue is empty. + * @return the head of this queue, or null if this queue is empty */ - E peek(); + E poll(); /** - * 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. + * 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. + * @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 null if this queue is empty. + * + * @return the head of this queue, or null if this queue is empty + */ + E peek(); +}