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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/licenses/publicdomain

 */

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 <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();

}