AbstractQueue.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
 * 2 along with this work; if not, write to the Free Software Foundation,
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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;

/**
 * This class provides skeletal implementations of some {@link Queue}
 * operations. The implementations in this class are appropriate when
 * the base implementation does <em>not</em> allow {@code null}
 * 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 {@code false} or
 * {@code null} returns.
 *
 * <p>A {@code Queue} implementation that extends this class must
 * minimally define a method {@link Queue#offer} which does not permit
 * insertion of {@code null} 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}.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements held in this queue
 *
 * @author Doug Lea
 * @since 1.5
 */
// 队列容器的一个抽象实现
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
     * {@code true} upon success and throwing an {@code IllegalStateException}
     * if no space is currently available.
     *
     * <p>This implementation returns {@code true} if {@code offer} succeeds,
     * else throws an {@code IllegalStateException}.
     *
     * @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
     */
    // 入队，无法入队时扩容或抛异常，不阻塞
    public boolean add(E e) {
        if(offer(e)) {
            return true;
        } else {
            throw new IllegalStateException("Queue full");
        }
    }
    
    /**
     * Adds all of the elements in the specified collection to this
     * queue.  Attempts to addAll of a queue to itself result in
     * {@code IllegalArgumentException}. 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
     * {@code null} 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 {@code true} 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)
     */
    // 批量添加。将指定容器中的所有元素入队，不阻塞，但可能会抛出异常
    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;
    }
    
    /**
     * 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 {@code poll}
     * unless the queue is empty.
     *
     * @return the head of this queue
     *
     * @throws NoSuchElementException if this queue is empty
     */
    // 出队，无法出队时抛异常，不阻塞
    public E remove() {
        E x = poll();
        
        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 {@code null}.
     */
    // 清空。将所有元素移除，不阻塞，不抛异常
    public void clear() {
        while(poll() != null)
            ;
    }
    
    /**
     * 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 {@code peek}
     * unless the queue is empty.
     *
     * @return the head of this queue
     *
     * @throws NoSuchElementException if this queue is empty
     */
    // 查看队头元素，如果队列为空，抛出异常
    public E element() {
        E x = peek();
        if(x != null) {
            return x;
        } else {
            throw new NoSuchElementException();
        }
    }
    
}