java.util

Class AbstractQueue<E>

Implemented Interfaces:
Collection<E>, Iterable<T>, Queue<E>
Known Direct Subclasses:
ArrayBlockingQueue<E>, ConcurrentLinkedQueue<E>, DelayQueue<E,extends,Delayed>, LinkedBlockingQueue<E>, PriorityBlockingQueue<E>, PriorityQueue<E>, SynchronousQueue<E>

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.

This class is a member of the ../../../guide/collections/index.html"> Java Collections Framework.

Since:
1.5

Constructor Summary

AbstractQueue()
Constructor for use by subclasses.

Method Summary

boolean
add(E o)
Adds the specified element to this queue.
boolean
addAll(E> c)
Adds all of the elements in the specified collection 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<E>

T[] toArray, add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

AbstractQueue

protected AbstractQueue()
Constructor for use by subclasses.

Method Details

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<E>
Overrides:
add in interface AbstractCollection<E>
Parameters:
o - the element
Returns:
true (as per the general contract of Collection.add).
Throws:
NullPointerException - if the specified element is null
IllegalStateException - if element cannot be added

addAll

public boolean addAll(E> c)
Adds all of the elements in the specified collection to this queue. Attempts to addAll of a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

This implementation iterates over the specified collection, and adds each element returned by the iterator to this collection, in turn. A runtime exception encountered while trying to add an element (including, in particular, a null element) may result in only some of the elements having been successfully added when the associated exception is thrown.

Specified by:
addAll in interface Collection<E>
Overrides:
addAll in interface AbstractCollection<E>
Parameters:
c - collection whose elements are to be added to this collection.
Returns:
true if this collection changed as a result of the call.
Throws:
NullPointerException - if the specified collection or any of its elements are null.
IllegalArgumentException - if c is this queue.
See Also:
add(Object)

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<E>
Overrides:
clear in interface AbstractCollection<E>

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<E>
Returns:
the head of this queue.
Throws:
NoSuchElementException - if this queue is empty.

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<E>
Returns:
the head of this queue.
Throws:
NoSuchElementException - if this queue is empty.