Quelle Queue.java

Sprache: JAVA

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*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.  Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
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*
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* 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 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 class="striped">
* <caption>Summary of Queue methods</caption>
*  <thead>
*  <tr>
*    <td></td>
*    <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
*    <th scope="col" style="font-weight:normal; font-style:italic">Returns special value</th>
*  </tr>
*  </thead>
*  <tbody>
*  <tr>
*    <th scope="row">Insert</th>
*    <td>{@link #add(Object) add(e)}</td>
*    <td>{@link #offer(Object) offer(e)}</td>
*  </tr>
*  <tr>
*    <th scope="row">Remove</th>
*    <td>{@link #remove() remove()}</td>
*    <td>{@link #poll() poll()}</td>
*  </tr>
*  <tr>
*    <th scope="row">Examine</th>
*    <td>{@link #element() element()}</td>
*    <td>{@link #peek() peek()}</td>
*  </tr>
*  </tbody>
* </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 "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 {@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}/java.base/java/util/package-summary.html#CollectionsFramework">
* Java Collections Framework</a>.
*
* @since 1.5
* @author Doug Lea
* @param <E> the type of elements held in this queue
*/
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();
}

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