public class

DelayQueue

extends AbstractQueue<E>
implements BlockingQueue<E>
java.lang.Object
   ↳ java.util.AbstractCollection<E>
     ↳ java.util.AbstractQueue<E>
       ↳ java.util.concurrent.DelayQueue<E extends java.util.concurrent.Delayed>

Class Overview

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.

Summary

Public Constructors
DelayQueue()
Creates a new DelayQueue that is initially empty.
DelayQueue(Collection<? extends E> c)
Creates a DelayQueue initially containing the elements of the given collection of Delayed instances.
Public Methods
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(Collection<? super E> c, int maxElements)
Removes at most the given number of available elements from this queue and adds them to the given collection.
int drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them to the given collection.
Iterator<E> 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()
Returns a count of how many objects this Collection contains.
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.
<T> 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.
Object[] toArray()
Returns an array containing all of the elements in this queue.
[Expand]
Inherited Methods
From class java.util.AbstractQueue
From class java.util.AbstractCollection
From class java.lang.Object
From interface java.lang.Iterable
From interface java.util.Collection
From interface java.util.Queue
From interface java.util.concurrent.BlockingQueue

Public Constructors

public DelayQueue ()

Since: API Level 1

Creates a new DelayQueue that is initially empty.

public DelayQueue (Collection<? extends E> c)

Since: API Level 1

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

Public Methods

public boolean add (E e)

Since: API Level 1

Inserts the specified element into this delay queue.

Parameters
e the element to add
Returns
Throws
NullPointerException if the specified element is null

public void clear ()

Since: API Level 1

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.

public int drainTo (Collection<? super E> c, int maxElements)

Since: API Level 1

Removes at most the given number of available elements from this queue and adds them to the given collection. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain 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.

Parameters
c the collection to transfer elements into
maxElements the maximum number of elements to transfer
Returns
  • the number of elements transferred
Throws
UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

public int drainTo (Collection<? super E> c)

Since: API Level 1

Removes all available elements from this queue and adds them to the given collection. This operation may be more efficient than repeatedly polling this queue. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain 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.

Parameters
c the collection to transfer elements into
Returns
  • the number of elements transferred
Throws
UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

public Iterator<E> iterator ()

Since: API Level 1

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.

Returns
  • an iterator over the elements in this queue

public boolean offer (E e)

Since: API Level 1

Inserts the specified element into this delay queue.

Parameters
e the element to add
Returns
  • true
Throws
NullPointerException if the specified element is null

public boolean offer (E e, long timeout, TimeUnit unit)

Since: API Level 1

Inserts the specified element into this delay queue. As the queue is unbounded this method will never block.

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
Throws
NullPointerException

public E peek ()

Since: API Level 1

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.

Returns
  • the head of this queue, or null if this queue is empty.

public E poll ()

Since: API Level 1

Retrieves and removes the head of this queue, or returns null if this queue has no elements with an expired delay.

Returns
  • the head of this queue, or null if this queue has no elements with an expired delay

public E poll (long timeout, TimeUnit unit)

Since: API Level 1

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.

Parameters
timeout how long to wait before giving up, in units of unit
unit a TimeUnit determining how to interpret the timeout parameter
Returns
  • the head of this queue, or null if the specified waiting time elapses before an element with an expired delay becomes available
Throws
InterruptedException

public void put (E e)

Since: API Level 1

Inserts the specified element into this delay queue. As the queue is unbounded this method will never block.

Parameters
e the element to add
Throws
NullPointerException

public int remainingCapacity ()

Since: API Level 1

Always returns Integer.MAX_VALUE because a DelayQueue is not capacity constrained.

Returns
  • Integer.MAX_VALUE

public boolean remove (Object o)

Since: API Level 1

Removes a single instance of the specified element from this queue, if it is present, whether or not it has expired.

Parameters
o the object to remove.
Returns
  • true if this Collection is modified, false otherwise.

public int size ()

Since: API Level 1

Returns a count of how many objects this Collection contains.

In this class this method is declared abstract and has to be implemented by concrete Collection implementations.

Returns
  • how many objects this Collection contains, or Integer.MAX_VALUE if there are more than Integer.MAX_VALUE elements in this Collection.

public E take ()

Since: API Level 1

Retrieves and removes the head of this queue, waiting if necessary until an element with an expired delay is available on this queue.

Returns
  • the head of this queue
Throws
InterruptedException

public T[] toArray (T[] a)

Since: API Level 1

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().

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

public Object[] toArray ()

Since: API Level 1

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.

Returns
  • an array containing all of the elements in this queue