java.util.concurrent

Class DelayQueue<E,extends,Delayed>

Implemented Interfaces:
BlockingQueue<E>, Collection<E>, Iterable<E>, Queue<E>

public class DelayQueue<E,extends,Delayed>
extends AbstractQueue<E>
implements BlockingQueue<E>

An unbounded blocking queue of Delayed elements, in which an element can only be taken when its delay has expired. The head of the queue is that Delayed element whose delay expired furthest in the past. If no delay has expired there is no head and poll will return null. Expiration occurs when an element's getDelay(TimeUnit.NANOSECONDS) method returns a value less than or equal to zero. Even though unexpired elements cannot be removed using take or poll, they are otherwise treated as normal elements. For example, the size method returns the count of both expired and unexpired elements. This queue does not permit null elements.

This class and its iterator implement all of the optional methods of the Collection and Iterator interfaces.

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

Since:
1.5

Constructor Summary

DelayQueue()
Creates a new DelayQueue that is initially empty.
DelayQueue(E> c)
Creates a DelayQueue initially containing the elements of the given collection of Delayed instances.

Method Summary

T[] toArray(T[] a)
Returns an array containing all of the elements in this queue; the runtime type of the returned array is that of the specified array.
boolean
add(E e)
Inserts the specified element into this delay queue.
void
clear()
Atomically removes all of the elements from this delay queue.
int
drainTo(E> c)
int
drainTo(E> c, int maxElements)
Iterator
iterator()
Returns an iterator over all the elements (both expired and unexpired) in this queue.
boolean
offer(E e)
Inserts the specified element into this delay queue.
boolean
offer(E e, long timeout, TimeUnit unit)
Inserts the specified element into this delay queue.
E
peek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
E
poll()
Retrieves and removes the head of this queue, or returns null if this queue has no elements with an expired delay.
E
poll(long timeout, TimeUnit unit)
Retrieves and removes the head of this queue, waiting if necessary until an element with an expired delay is available on this queue, or the specified wait time expires.
void
put(E e)
Inserts the specified element into this delay queue.
int
remainingCapacity()
Always returns Integer.MAX_VALUE because a DelayQueue is not capacity constrained.
boolean
remove(Object o)
Removes a single instance of the specified element from this queue, if it is present, whether or not it has expired.
int
size()
E
take()
Retrieves and removes the head of this queue, waiting if necessary until an element with an expired delay is available on this queue.
Object[]
toArray()
Returns an array containing all of the elements in this queue.

Methods inherited from class java.util.AbstractQueue<E>

add, addAll, clear, element, remove

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

DelayQueue

public DelayQueue()
Creates a new DelayQueue that is initially empty.

DelayQueue

public DelayQueue(E> c)
Creates a DelayQueue initially containing the elements of the given collection of Delayed instances.
Parameters:
c - the collection of elements to initially contain
Throws:
NullPointerException - if the specified collection or any of its elements are null

Method Details

T[] toArray

public  T[] toArray(T[] a)
Returns an array containing all of the elements in this queue; the runtime type of the returned array is that of the specified array. The returned array elements are in no particular order. If the queue fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this queue.

If this queue fits in the specified array with room to spare (i.e., the array has more elements than this queue), the element in the array immediately following the end of the queue is set to null.

Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

The following code can be used to dump a delay queue into a newly allocated array of Delayed:

     Delayed[] a = q.toArray(new Delayed[0]);
Note that toArray(new Object[0]) is identical in function to toArray().
Specified by:
T[] toArray in interface Collection<E>
Overrides:
T[] toArray in interface AbstractCollection<E>
Parameters:
a - the array into which the elements of the queue are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose
Returns:
an array containing all of the elements in this queue
Throws:
ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this queue
NullPointerException - if the specified array is null

add

public boolean add(E e)
Inserts the specified element into this delay queue.
Specified by:
add in interface BlockingQueue<E>
add in interface Queue<E>
add in interface Collection<E>
Overrides:
add in interface AbstractQueue<E>
Parameters:
e - the element to add
Returns:
true (as specified by Collection.add(E))
Throws:
NullPointerException - if the specified element is null

clear

public void clear()
Atomically removes all of the elements from this delay queue. The queue will be empty after this call returns. Elements with an unexpired delay are not waited for; they are simply discarded from the queue.
Specified by:
clear in interface Collection<E>
Overrides:
clear in interface AbstractQueue<E>

drainTo

public int drainTo(E> c)
Specified by:
drainTo in interface BlockingQueue<E>

drainTo

public int drainTo(E> c,
                   int maxElements)
Specified by:
drainTo in interface BlockingQueue<E>

iterator

public Iterator iterator()
Returns an iterator over all the elements (both expired and unexpired) in this queue. The iterator does not return the elements in any particular order. The returned Iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.
Specified by:
iterator in interface Collection<E>
iterator in interface Iterable<E>
Overrides:
iterator in interface AbstractCollection<E>
Returns:
an iterator over the elements in this queue

offer

public boolean offer(E e)
Inserts the specified element into this delay queue.
Specified by:
offer in interface BlockingQueue<E>
offer in interface Queue<E>
Parameters:
e - the element to add
Returns:
true
Throws:
NullPointerException - if the specified element is null

offer

public boolean offer(E e,
                     long timeout,
                     TimeUnit unit)
Inserts the specified element into this delay queue. As the queue is unbounded this method will never block.
Specified by:
offer in interface BlockingQueue<E>
Parameters:
e - the element to add
timeout - This parameter is ignored as the method never blocks
unit - This parameter is ignored as the method never blocks
Returns:
true

peek

public E peek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty. Unlike poll, if no expired elements are available in the queue, this method returns the element that will expire next, if one exists.
Specified by:
peek in interface Queue<E>
Returns:
the head of this queue, or null if this queue is empty.

poll

public E poll()
Retrieves and removes the head of this queue, or returns null if this queue has no elements with an expired delay.
Specified by:
poll in interface Queue<E>
Returns:
the head of this queue, or null if this queue has no elements with an expired delay

poll

public E poll(long timeout,
              TimeUnit unit)
            throws InterruptedException
Retrieves and removes the head of this queue, waiting if necessary until an element with an expired delay is available on this queue, or the specified wait time expires.
Specified by:
poll in interface BlockingQueue<E>
Returns:
the head of this queue, or null if the specified waiting time elapses before an element with an expired delay becomes available

put

public void put(E e)
Inserts the specified element into this delay queue. As the queue is unbounded this method will never block.
Specified by:
put in interface BlockingQueue<E>
Parameters:
e - the element to add

remainingCapacity

public int remainingCapacity()
Always returns Integer.MAX_VALUE because a DelayQueue is not capacity constrained.
Specified by:
remainingCapacity in interface BlockingQueue<E>
Returns:
Integer.MAX_VALUE

remove

public boolean remove(Object o)
Removes a single instance of the specified element from this queue, if it is present, whether or not it has expired.
Specified by:
remove in interface BlockingQueue<E>
remove in interface Collection<E>
Overrides:
remove in interface AbstractCollection<E>

size

public int size()
Specified by:
size in interface Collection<E>
Overrides:
size in interface AbstractCollection<E>

take

public E take()
            throws InterruptedException
Retrieves and removes the head of this queue, waiting if necessary until an element with an expired delay is available on this queue.
Specified by:
take in interface BlockingQueue<E>
Returns:
the head of this queue

toArray

public Object[] toArray()
Returns an array containing all of the elements in this queue. The returned array elements are in no particular order.

The returned array will be "safe" in that no references to it are maintained by this queue. (In other words, this method must allocate a new array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs.

Specified by:
toArray in interface Collection<E>
Overrides:
toArray in interface AbstractCollection<E>
Returns:
an array containing all of the elements in this queue