edu.emory.mathcs.backport.java.util.concurrent

Class DelayQueue

public class DelayQueue extends AbstractQueue implements BlockingQueue

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 Java Collections Framework.

Since: 1.5

Author: Doug Lea

Constructor Summary
DelayQueue()
Creates a new DelayQueue that is initially empty.
DelayQueue(Collection c)
Creates a DelayQueue initially containing the elements of the given collection of Delayed instances.
Method Summary
booleanadd(Object e)
Inserts the specified element into this delay queue.
voidclear()
Atomically removes all of the elements from this delay queue.
intdrainTo(Collection c)
intdrainTo(Collection c, int maxElements)
Iteratoriterator()
Returns an iterator over all the elements (both expired and unexpired) in this queue.
booleanoffer(Object e)
Inserts the specified element into this delay queue.
booleanoffer(Object e, long timeout, TimeUnit unit)
Inserts the specified element into this delay queue.
Objectpeek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
Objectpoll()
Retrieves and removes the head of this queue, or returns null if this queue has no elements with an expired delay.
Objectpoll(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.
voidput(Object e)
Inserts the specified element into this delay queue.
intremainingCapacity()
Always returns Integer.MAX_VALUE because a DelayQueue is not capacity constrained.
booleanremove(Object o)
Removes a single instance of the specified element from this queue, if it is present, whether or not it has expired.
intsize()
Objecttake()
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.
Object[]toArray(Object[] 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.

Constructor Detail

DelayQueue

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

DelayQueue

public DelayQueue(Collection 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 Detail

add

public boolean add(Object e)
Inserts the specified element into this delay queue.

Parameters: e the element to add

Returns: true (as specified by Collection#add)

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.

drainTo

public int drainTo(Collection c)

Throws: UnsupportedOperationException {@inheritDoc } ClassCastException {@inheritDoc } NullPointerException {@inheritDoc } IllegalArgumentException {@inheritDoc }

drainTo

public int drainTo(Collection c, int maxElements)

Throws: UnsupportedOperationException {@inheritDoc } ClassCastException {@inheritDoc } NullPointerException {@inheritDoc } IllegalArgumentException {@inheritDoc }

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 java.util.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

offer

public boolean offer(Object e)
Inserts the specified element into this delay queue.

Parameters: e the element to add

Returns: true

Throws: NullPointerException if the specified element is null

offer

public boolean offer(Object e, long timeout, TimeUnit unit)
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 {@inheritDoc }

peek

public Object 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.

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

poll

public Object poll()
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

poll

public Object 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.

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 {@inheritDoc }

put

public void put(Object e)
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 {@inheritDoc }

remainingCapacity

public int remainingCapacity()
Always returns Integer.MAX_VALUE because a DelayQueue is not capacity constrained.

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.

size

public int size()

take

public Object take()
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 {@inheritDoc }

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.

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

toArray

public Object[] toArray(Object[] 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().

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