java/util/AbstractQueue.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

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 <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 a
 * {@link Collection#iterator} supporting {@link
 * Iterator#remove}. Typically, additional methods will be overriden
 * as well. If these requirements cannot be met, consider instead
 * subclassing {@link AbstractCollection}.
 *
 * @since 1.5
 * @author Doug Lea
 */
public abstract class AbstractQueue<E>
    extends AbstractCollection<E>
    implements Queue<E> {

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


    /**
     * Adds the specified element to this queue. This implementation
     * returns <tt>true</tt> if <tt>offer</tt> succeeds, else
     * throws an IllegalStateException. 
     * th
     * @param o the element
     * @return <tt>true</tt> (as per the general contract of
     *         <tt>Collection.add</tt>).
     *
     * @throws NullPointerException if the specified element is <tt>null</tt>
     * @throws IllegalStateException if element cannot be added
     */
    public boolean add(E o) {
        if (offer(o))
            return true;
        else
            throw new IllegalStateException("Queue full");
    }

    /**
     * Retrieves and removes the head of this queue.
     * 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();
    }


    /**
     * Retrieves, but does not remove, the head of this queue.  
     * 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();
    }

    /**
     * Removes all of the elements from this collection.
     * The collection 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)
            ;
    }
}
Revision:	1.16
Committed:	Sat Sep 13 18:51:06 2003 UTC (20 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.15:	+5 -96 lines
Log Message:	Proofreading pass -- many minor adjustments
#	User	Rev	Content
1	dl	1.2	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain. Use, modify, and
4			* redistribute this code in any way without acknowledgement.
5			*/
6
7	tim	1.1	package java.util;
8
9			/**
10	dl	1.14	* This class provides skeletal implementations of some {@link Queue}
11			* operations. The implementations in this class are appropriate when
12			* the base implementation does <em>not</em> allow <tt>null</tt>
13			* elements. Methods {@link #add add}, {@link #remove remove}, and
14			* {@link #element element} are based on {@link #offer offer}, {@link
15			* #poll poll}, and {@link #peek peek}, respectively but throw
16			* exceptions instead of indicating failure via <tt>false</tt> or
17	dholmes	1.9	* <tt>null</tt> returns.
18	dl	1.14	*
19			* <p> A <tt>Queue</tt> implementation that extends this class must
20			* minimally define a method {@link Queue#offer} which does not permit
21			* insertion of <tt>null</tt> elements, along with methods {@link
22	dl	1.16	* Queue#peek}, {@link Queue#poll}, {@link Collection#size}, and a
23			* {@link Collection#iterator} supporting {@link
24			* Iterator#remove}. Typically, additional methods will be overriden
25			* as well. If these requirements cannot be met, consider instead
26			* subclassing {@link AbstractCollection}.
27	dl	1.14	*
28	dl	1.4	* @since 1.5
29			* @author Doug Lea
30	tim	1.1	*/
31	tim	1.10	public abstract class AbstractQueue<E>
32			extends AbstractCollection<E>
33	dholmes	1.9	implements Queue<E> {
34	tim	1.5
35	dl	1.14	/**
36			* Constructor for use by subclasses.
37			*/
38			protected AbstractQueue() {
39			}
40	tim	1.1
41	dholmes	1.12
42			/**
43	dl	1.14	* Adds the specified element to this queue. This implementation
44			* returns <tt>true</tt> if <tt>offer</tt> succeeds, else
45			* throws an IllegalStateException.
46			* th
47			* @param o the element
48	tim	1.10	* @return <tt>true</tt> (as per the general contract of
49	dl	1.14	* <tt>Collection.add</tt>).
50	dholmes	1.9	*
51	dholmes	1.11	* @throws NullPointerException if the specified element is <tt>null</tt>
52	dl	1.14	* @throws IllegalStateException if element cannot be added
53	dholmes	1.9	*/
54			public boolean add(E o) {
55			if (offer(o))
56	tim	1.1	return true;
57			else
58			throw new IllegalStateException("Queue full");
59			}
60
61	dholmes	1.9	/**
62	dl	1.14	* Retrieves and removes the head of this queue.
63			* This implementation returns the result of <tt>poll</tt>
64			* unless the queue is empty.
65	dholmes	1.9	*
66	dl	1.14	* @return the head of this queue.
67			* @throws NoSuchElementException if this queue is empty.
68	dholmes	1.9	*/
69	tim	1.8	public E remove() {
70	tim	1.1	E x = poll();
71			if (x != null)
72			return x;
73			else
74			throw new NoSuchElementException();
75			}
76
77	dl	1.14
78			/**
79			* Retrieves, but does not remove, the head of this queue.
80			* This implementation returns the result of <tt>peek</tt>
81			* unless the queue is empty.
82			*
83			* @return the head of this queue.
84			* @throws NoSuchElementException if this queue is empty.
85			*/
86	tim	1.8	public E element() {
87	tim	1.1	E x = peek();
88			if (x != null)
89			return x;
90			else
91			throw new NoSuchElementException();
92			}
93
94	dholmes	1.9	/**
95			* Removes all of the elements from this collection.
96			* The collection will be empty after this call returns.
97			* <p>This implementation repeatedly invokes {@link #poll poll} until it
98			* returns <tt>null</tt>.
99			*/
100	tim	1.1	public void clear() {
101			while (poll() != null)
102			;
103			}
104	dl	1.14	}