java.util
Class AbstractQueue<E>

java.lang.Object
  java.util.AbstractCollection
      java.util.AbstractQueue

All Implemented Interfaces:

Collection, java.lang.Iterable, Queue

Direct Known Subclasses:

ArrayBlockingQueue, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingQueue, PriorityBlockingQueue, PriorityQueue, SynchronousQueue

public abstract class AbstractQueue<E>

extends AbstractCollection<E>

implements Queue<E>

This class provides skeletal implementations of some Queue operations. The implementations in this class are appropriate when the base implementation does not allow null elements. Methods add, remove, and element are based on offer, poll, and peek, respectively but throw exceptions instead of indicating failure via false or null returns.

A Queue implementation that extends this class must minimally define a method Queue.offer(E) which does not permit insertion of null elements, along with methods Queue.peek(), Queue.poll(), Collection.size(), and a Collection.iterator() supporting Iterator.remove(). Typically, additional methods will be overridden as well. If these requirements cannot be met, consider instead subclassing AbstractCollection.

Since:

1.5

Author:

Doug Lea

Constructor Summary

protected

AbstractQueue() Constructor for use by subclasses.

Method Summary

boolean	add(E o) Adds the specified element to this queue.
void	clear() Removes all of the elements from this collection.
E	element() Retrieves, but does not remove, the head of this queue.
E	remove() Retrieves and removes the head of this queue.

Methods inherited from class java.util.AbstractCollection

addAll, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Queue

offer, peek, poll

Methods inherited from interface java.util.Collection

addAll, contains, containsAll, equals, hashCode, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray

Constructor Detail

AbstractQueue

protected AbstractQueue()

Constructor for use by subclasses.

Method Detail

add

public boolean add(E o)

Adds the specified element to this queue. This implementation returns true if offer succeeds, else throws an IllegalStateException.

Specified by:

add in interface Collection

Overrides:

add in class AbstractCollection

Parameters:

o - the element

Returns:

true (as per the general contract of Collection.add).

Throws:

java.lang.NullPointerException - if the specified element is null

java.lang.IllegalStateException - if element cannot be added

remove

public E remove()

Retrieves and removes the head of this queue. This implementation returns the result of poll unless the queue is empty.

Specified by:

remove in interface Queue

Returns:

the head of this queue.

Throws:

NoSuchElementException - if this queue is empty.

element

public E element()

Retrieves, but does not remove, the head of this queue. This implementation returns the result of peek unless the queue is empty.

Specified by:

element in interface Queue

Returns:

the head of this queue.

Throws:

NoSuchElementException - if this queue is empty.

clear

public void clear()

Removes all of the elements from this collection. The collection will be empty after this call returns.

This implementation repeatedly invokes poll until it returns null.

Specified by:

clear in interface Collection

Overrides:

clear in class AbstractCollection