Abstract Class - java.util.AbstractQueue

Created by : Mr Dk.

2019 / 11 / 11 23:08

Nanjing, Jiangsu, China

Definition

public abstract class AbstractQueue<E>
    extends AbstractCollection<E>
    implements Queue<E> {
}

提供了 Queue 接口的基本实现：

add() 基于 offer()
remove() 基于 poll()
element() 基于 peek()

但对于失败的操作会抛出异常，而不是返回 null 或 false。

/**
 * 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 <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.
 *
 * <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}.
 *
 * <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
 */

Constructor

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

Add

在 offer() 返回失败（即意味着没有空间）时抛出异常。

/**
 * 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
 */
public boolean add(E e) {
    if (offer(e))
        return true;
    else
        throw new IllegalStateException("Queue full");
}

Remove

在队列已经为空时，抛出异常。

/**
 * 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
 */
public E remove() {
    E x = poll();
    if (x != null)
        return x;
    else
        throw new NoSuchElementException();
}

Element

看一下队头元素，如果为空，则抛出异常。

/**
 * 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
 */
public E element() {
    E x = peek();
    if (x != null)
        return x;
    else
        throw new NoSuchElementException();
}

Clear

把所有的元素全部都移除。

/**
 * 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>.
 */
public void clear() {
    while (poll() != null)
        ;
}

Add All

/**
 * 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)
 */
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;
}