AbstractQueue简介

本文详细介绍了Java中Queue接口的抽象实现类AbstractQueue,包括其核心方法如add、remove、element等的功能与实现原理,以及如何通过子类实现具体队列。

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Queue接口定义:[url]http://donald-draper.iteye.com/blog/2363491[/url]
/*
* 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;

/**
* This class provides skeletal implementations of some {@link Queue}
* operations. The implementations in this class are appropriate when
* the base implementation does [i]not[/i] allow <tt>null</tt>
* elements. Methods {@link #add add}, {@link #remove remove}, and
* {@link #element element} are based on {@link #offer offer}, {@link
* #poll poll}, and {@link #peek peek}, respectively, but throw
* exceptions instead of indicating failure via <tt>false</tt> or
* <tt>null</tt> returns.
AbstractQueue提供了Queue的操作实现的基础。AbstractQueue可以作为不允许
元素为null时,Queue的简单实现。add,remove,element方法分别基于offer,
poll,peek的实现,但是当队列为null时,抛出异常,而不是返回false或null。

* <p>A <tt>Queue</tt> implementation that extends this class must
* minimally define a method {@link Queue#offer} which does not permit
* insertion of <tt>null</tt> elements, along with methods {@link
* Queue#peek}, {@link Queue#poll}, {@link Collection#size}, and
* {@link Collection#iterator}. Typically, additional methods will be
* overridden as well. If these requirements cannot be met, consider
* instead subclassing {@link AbstractCollection}.
*
Queue的实现必须实现offer方法,并且允许插入null;peek,poll,size方法最好
重写,如果这些要求不能满足,可以用AbstractCollection代替。
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @since 1.5
* @author Doug Lea
* @param <E> the type of elements held in this collection
*/
public abstract class AbstractQueue<E>
extends AbstractCollection<E>
implements Queue<E> {

/**
* Constructor for use by subclasses.
*/
protected AbstractQueue() {
}

/**
* 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.
*
* <p>This implementation returns <tt>true</tt> if <tt>offer</tt> succeeds,
* else throws an <tt>IllegalStateException</tt>.
*
* @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
*/
添加一个元素,成功返回ture,否则抛出异常
public boolean add(E e) {
if (offer(e))
return true;
else
throw new IllegalStateException("Queue full");
}

/**
* 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.
*
* <p>This implementation returns the result of <tt>poll</tt>
* unless the queue is empty.
*
* @return the head of this queue
* @throws NoSuchElementException if this queue is empty
*/
消费队列头元素,有则返回元素,为null,则抛出异常
public E remove() {
E x = poll();
if (x != null)
return x;
else
throw new NoSuchElementException();
}

/**
* 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.
*
* <p>This implementation returns the result of <tt>peek</tt>
* unless the queue is empty.
*
* @return the head of this queue
* @throws NoSuchElementException if this queue is empty
*/
检查是否有元素,有则返回元素,无则抛出异常
public E element() {
E x = peek();
if (x != null)
return x;
else
throw new NoSuchElementException();
}

/**
* Removes all of the elements from this queue.
* The queue will be empty after this call returns.
*
* <p>This implementation repeatedly invokes {@link #poll poll} until it
* returns <tt>null</tt>.
*/
清空,循环poll,直到为空
public void clear() {
while (poll() != null)
;
}

/**
* Adds all of the elements in the specified collection to this
* queue. Attempts to addAll of a queue to itself result in
* <tt>IllegalArgumentException</tt>. Further, the behavior of
* this operation is undefined if the specified collection is
* modified while the operation is in progress.
*
* <p>This implementation iterates over the specified collection,
* and adds each element returned by the iterator to this
* queue, in turn. A runtime exception encountered while
* trying to add an element (including, in particular, a
* <tt>null</tt> element) may result in only some of the elements
* having been successfully added when the associated exception is
* thrown.
*
* @param c collection containing elements to be added to this queue
* @return <tt>true</tt> if this queue changed as a result of the call
* @throws ClassCastException if the class of an element of the specified
* collection prevents it from being added to this queue
* @throws NullPointerException if the specified collection contains a
* null element and this queue does not permit null elements,
* or if the specified collection is null
* @throws IllegalArgumentException if some property of an element of the
* specified collection prevents it from being added to this
* queue, or if the specified collection is this queue
* @throws IllegalStateException if not all the elements can be added at
* this time due to insertion restrictions
* @see #add(Object)
*/
//循环遍历集合元素,add到队列
public boolean addAll(Collection<? extends E> c) {
if (c == null)
throw new NullPointerException();
if (c == this)
throw new IllegalArgumentException();
boolean modified = false;
for (E e : c)
if (add(e))
modified = true;
return modified;
}
}

总结:
[color=green]AbstractQueue的add,remove,element方法分别基于offer,poll,peek的实现,但是当队列为null时,抛出异常,而不是返回false或null。offer,poll,peek,并没有实现待子类扩展。清空,循环poll,直到为空。addAll为循环遍历集合元素,add到队列;[/color]
附:AbstractCollection

[img]http://dl2.iteye.com/upload/attachment/0123/7520/76ff9288-0d1c-3e46-ae29-ce59e4a48ea2.png[/img]
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