Queue接口定义

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 * 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 <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.
 *
Queue优先处理元素，除了Collection的基本操作之外，Queue添加了另外插入，拉取，
检查３个操作。每一种方法都有两种格式，一种操作失败，则抛出异常，另一种返回一个
特殊的值，null或false，依赖于操作。后一种形式的插入操作是用capacity-restricted
Queue来实现的，在大多数的实现中，插入操作不会失败。
 * <p>
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 *  <tr>
 *    <td></td>
 *    <td ALIGN=CENTER>[i]Throws exception[/i]</td>
 *    <td ALIGN=CENTER>[i]Returns special value[/i]</td>
 *  </tr>
 *  <tr> 插入，提供
 *    <td><b>Insert</b></td>
 *    <td>{@link #add add(e)}</td>
 *    <td>{@link #offer offer(e)}</td>
 *  </tr>
 *  <tr>移除，拉取
 *    <td><b>Remove</b></td>
 *    <td>{@link #remove remove()}</td>
 *    <td>{@link #poll poll()}</td>
 *  </tr>
 *  <tr>检查
 *    <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 [i]head[/i] 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 [i] tail[/i] of the queue. Other kinds of queues may use
 * different placement rules.  Every <tt>Queue</tt> implementation
 * must specify its ordering properties.
 *
Queues除了非必要之外，元素的模式为FIFO。其中的一个例外是优先级队列，元素根据
比较器或者本身的顺序添加队列（ LIFO queues）中。无论什么顺序，对头的元素将会
被#remove或#poll消费/移除。在先进先出队列中，所有的新元素插入到队尾，其他队列
可能会用其他的策略添加新元素。每一种队列的实现要明确排序策略。

 * <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.
 *
 如果可能，offer操作，插入元素时，会返回false。这个与集合类的add方法不同的时，
 集合类add方法，当失败时，则抛出一个unchecked异常。offer操作设计为在一个正常模式下的失败，
 而不异常发生，比如队列容量有限。
 * <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.
 *
remove和poll元素将从队列头部消费一个元素。不同的队列实现消费队列元素顺序时，可能有不同的
策略。remove和poll操作不同的时，当队列为空时，remove抛出异常 ，poll则返回null。

 * <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.
 *
Queue没有定义阻塞队列的方法，这些方法会在并发编程中用到。这些方法，比如，在队列为空时，
消费者阻塞等待队列有元素可利用，这些方法在BlockingQueue接口中定义。
 * <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.
 队列的实现通常不允许插入null，尽管LinkedList不禁止插入null值。即使允许插入null，
 我们也利用利用poll操作，来暗示队列中没有元素。
 *
 * <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.
 *
 Queue的实现通常不会定义equals和hashCode方法，而是继承自Object，因为hash值和值相等
 ，但是顺序属性可能不同。
 *
 * <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.
     *
     在有界队列未满的情况下，添加元素，返回true，没空间可利用，则抛出异常IllegalStateException
     * @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.
     *
      在有界队列未满的情况下，添加元素，立即成功；当队列为有界是，add更好，因为越界抛出异常
     * @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.
     *
     从对头消费一个元素，当队列为空，抛出异常
     * @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.
     *
     从对头消费一个元素，当队列为空，返回null
     * @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.
     *
     检查队列中是否有元素，当队列为空，抛出异常
     * @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.
     *
      检查队列中是否有元素，当队列为空，返回null
     * @return the head of this queue, or <tt>null</tt> if this queue is empty
     */
    E peek();
}

附：Collection