/*
 * 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 java.util;

/**
 * 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 {@code null} or {@code false}, depending on the
 * operation). The latter form of the insert operation is designed
 * specifically for use with capacity-restricted {@code Queue}
 * implementations; in most implementations, insert operations cannot
 * fail.
 *
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Summary of Queue methods</caption>
 * <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 Queue#add add(e)}</td>
 * <td>{@link Queue#offer offer(e)}</td>
 * </tr>
 * <tr>
 * <td><b>Remove</b></td>
 * <td>{@link Queue#remove remove()}</td>
 * <td>{@link Queue#poll poll()}</td>
 * </tr>
 * <tr>
 * <td><b>Examine</b></td>
 * <td>{@link Queue#element element()}</td>
 * <td>{@link Queue#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 {@code Queue} implementation
 * must specify its ordering properties.
 *
 * <p>The {@link #offer offer} method inserts an element if possible,
 * otherwise returning {@code 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
 * {@code offer} method is designed for use when failure 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 {@code remove()} and
 * {@code poll()} methods differ only in their behavior when the
 * queue is empty: the {@code remove()} method throws an exception,
 * while the {@code poll()} method returns {@code null}.
 *
 * <p>The {@link #element()} and {@link #peek()} methods return, but do
 * not remove, the head of the queue.
 *
 * <p>The {@code Queue} 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>{@code Queue} implementations generally do not allow insertion
 * of {@code null} elements, although some implementations, such as
 * {@link LinkedList}, do not prohibit insertion of {@code null}.
 * Even in the implementations that permit it, {@code null} should
 * not be inserted into a {@code Queue}, as {@code null} is also
 * used as a special return value by the {@code poll} method to
 * indicate that the queue contains no elements.
 *
 * <p>{@code Queue} implementations generally do not define
 * element-based versions of methods {@code equals} and
 * {@code hashCode} but instead inherit the identity based versions
 * from class {@code Object}, 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
 * {@code true} upon success and throwing an {@code IllegalStateException}
 * if no space is currently available.
 *
 * @param e the element to add
 * @return {@code 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 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 {@code true} if the element was added to this queue, else
 * {@code 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
 */
 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 {@code null} if this queue is empty.
 *
 * @return the head of this queue, or {@code null} 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 {@code null} if this queue is empty.
 *
 * @return the head of this queue, or {@code null} if this queue is empty
 */
 E peek();
}