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Revision 1.8 - (download) (annotate)
Mon Jul 28 04:11:54 2003 UTC (14 years, 4 months ago) by dholmes
Branch: MAIN
Changes since 1.7: +42 -30 lines
Significant doc updates:
 - inherit comments where appropriate
 - ensure runtime exception comments inherited (overriding as needed)
 - consistent descriptions
 - introduce head and tail terminology
/*
 * 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.
 * Besides basic {@link Collection} operations, queues provide
 * additional insertion, extraction, and inspection operations.
 *
 * <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()}.
 * Every Queue implementation must specify its ordering guarantees.
 *
 * <p>The {@link #offer(E)} method adds an element if possible, otherwise
 * returning <tt>false</tt>.  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 &quot;bounded&quot;) 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 <tt>remove()</tt> and
 * <tt>poll()</tt> methods differ only in their behavior when the
 * queue is empty: the <tt>remove()</tt> method throws an exception,
 * while the <tt>poll()</tt> method returns <tt>null</tt>.
 *
 * <p>The {@link #element()} and {@link #peek()} methods return, but do
 * not delete, the head of the queue.
 *
 * <p>The <tt>Queue</tt> 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><tt>Queue</tt> implementations generally do not allow insertion
 * of <tt>null</tt> elements, although some implementations, such as
 * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
 * Even in the implementations that permit it, <tt>null</tt> should
 * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
 * used as a special return value by the <tt>poll</tt> method to
 * indicate that the queue contains no elements.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @see Collection
 * @see LinkedList
 * @see PriorityQueue
 * @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<E> extends Collection<E> {

    /**
     * Add the specified element to this queue, if possible.
     *
     * @param element the element to add.
     * @return <tt>true</tt> if it was possible to add the element to 
     * this queue.
     */
    boolean offer(E element);

    /**
     * Retrieve and remove the head of this queue, if it is available.
     *
     * @return the head of this queue, or <tt>null</tt> if this
     *         queue is empty. 
     */
    E poll();

    /**
     * Retrieve and remove the head of this queue.  
     * This method differs
     * from the <tt>poll</tt> method in that it throws an exception if the
     * queue is empty. 
     *
     * @return the head of this queue.
     * @throws NoSuchElementException if this queue is empty.
     */
    E remove() throws NoSuchElementException;

    /**
     * Retrieve, but do not remove, the head of this queue, or <tt>null</tt>
     * if this queue is empty.  This method differs from the <tt>poll</tt> 
     * method only in that this method does not remove the element from 
     * this queue.
     *
     * @return the head of this queue, or <tt>null</tt> if this queue is empty.
     */
    E peek();

    /**
     * Retrieve, but do not remove, the head of this queue.  This method
     * differs from the <tt>peek</tt> method 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() throws NoSuchElementException;
}











Doug Lea
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