Queue.java

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
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 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package com.ta.common;

import java.util.Collection;

/**
 * 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. Each of these methods
 * exists in two forms: one throws an exception if the operation fails, the
 * other returns a special value (either <tt>null</tt> or <tt>false</tt>,
 * depending on the operation). The latter form of the insert operation is
 * designed specifically for use with capacity-restricted <tt>Queue</tt>
 * implementations; in most implementations, insert operations cannot fail.
 * 
 * <p>
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <tr>
 * <td></td>
 * <td ALIGN=CENTER><em>Throws exception</em></td>
 * <td ALIGN=CENTER><em>Returns special value</em></td>
 * </tr>
 * <tr>
 * <td><b>Insert</b></td>
 * <td>{@link #add add(e)}</td>
 * <td>{@link #offer offer(e)}</td>
 * </tr>
 * <tr>
 * <td><b>Remove</b></td>
 * <td>{@link #remove remove()}</td>
 * <td>{@link #poll poll()}</td>
 * </tr>
 * <tr>
 * <td><b>Examine</b></td>
 * <td>{@link #element element()}</td>
 * <td>{@link #peek peek()}</td>
 * </tr>
 * </table>
 * 
 * <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()}. In a FIFO queue, all new elements are inserted at the
 * <em> tail</em> of the queue. Other kinds of queues may use different
 * placement rules. Every <tt>Queue</tt> implementation must specify its
 * ordering properties.
 * 
 * <p>
 * The {@link #offer offer} method inserts an element if possible, otherwise
 * returning <tt>false</tt>. 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 <tt>offer</tt> method
 * is designed for use when failure is a normal, rather than exceptional
 * occurrence, for example, in fixed-capacity (or "bounded") 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 remove,
 * 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>
 * <tt>Queue</tt> implementations generally do not define element-based versions
 * of methods <tt>equals</tt> and <tt>hashCode</tt> but instead inherit the
 * identity based versions from class <tt>Object</tt>, because element-based
 * equality is not always well-defined for queues with the same elements but
 * different ordering properties.
 * 
 * 
 * <p>
 * This interface is a member of the <a href="{@docRoot}
 * /../technotes/guides/collections/index.html"> Java Collections Framework</a>.
 * 
 * @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 <E>
 *            the type of elements held in this collection
 */
public interface Queue<E> extends Collection<E>
{
	/**
	 * Inserts the specified element into this queue if it is possible to do so
	 * immediately without violating capacity restrictions, returning
	 * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
	 * if no space is currently available.
	 * 
	 * @param e
	 *            the element to add
	 * @return <tt>true</tt> (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 add(E e);

	/**
	 * 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.
	 * 
	 * @param e
	 *            the element to add
	 * @return <tt>true</tt> if the element was added to this queue, else
	 *         <tt>false</tt>
	 * @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 e);

	/**
	 * 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
	 */
	E remove();

	/**
	 * Retrieves and removes the head of this queue, or returns <tt>null</tt> if
	 * this queue is empty.
	 * 
	 * @return the head of this queue, or <tt>null</tt> if this queue is empty
	 */
	E poll();

	/**
	 * 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
	 */
	E element();

	/**
	 * Retrieves, but does not remove, the head of this queue, or returns
	 * <tt>null</tt> if this queue is empty.
	 * 
	 * @return the head of this queue, or <tt>null</tt> if this queue is empty
	 */
	E peek();
}