Class DelayQueue<E extends Delayed>

Type Parameters:
E - the type of elements held in this queue
All Implemented Interfaces:
Iterable<E>, Collection<E>, BlockingQueue<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. The Iterator provided in method iterator() is not guaranteed to traverse the elements of the DelayQueue in any particular order.

This class is a member of the Java Collections Framework.

Since:
1.5

Constructors

Constructor Description
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.

Methods

Modifier and Type Method Description
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)

Removes all available elements from this queue and adds them to the given collection.
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.
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.
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.
<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.

Methods declared in class java.util.AbstractQueue

addAll, element, remove

Methods declared in class java.util.AbstractCollection

contains, containsAll, isEmpty, removeAll, retainAll, toString

Methods declared in class java.lang.Object

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

Methods declared in interface java.util.concurrent.BlockingQueue

contains

Methods declared in interface java.util.Collection

addAll, containsAll, equals, hashCode, isEmpty, parallelStream, removeAll, removeIf, retainAll, size, spliterator, stream, toArray

Methods declared in interface java.lang.Iterable

forEach

Methods declared in interface java.util.Queue

element, remove

Constructors

DelayQueue

public DelayQueue()

Creates a new DelayQueue that is initially empty.

DelayQueue

public DelayQueue(Collection<? extends 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

Methods

add

public boolean add(E e)

Inserts the specified element into this delay queue.

Specified by:
add in interface BlockingQueue<E extends Delayed>
Specified by:
add in interface Collection<E extends Delayed>
Specified by:
add in interface Queue<E extends Delayed>
Overrides:
add in class AbstractQueue<E extends Delayed>
Parameters:
e - the element to add
Returns:
true (as specified by Collection.add(E))
Throws:
NullPointerException - if the specified element is null

offer

public boolean offer(E e)

Inserts the specified element into this delay queue.

Specified by:
offer in interface BlockingQueue<E extends Delayed>
Specified by:
offer in interface Queue<E extends Delayed>
Parameters:
e - the element to add
Returns:
true
Throws:
NullPointerException - if the specified element is null

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 extends Delayed>
Parameters:
e - the element to add
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 extends Delayed>
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 - if the specified element is null

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 extends Delayed>
Returns:
the head of this queue, or null if this queue has no elements with an expired delay

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 extends Delayed>
Returns:
the head of this queue
Throws:
InterruptedException - if interrupted while waiting

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 extends Delayed>
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 - if interrupted while waiting

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 extends Delayed>
Returns:
the head of this queue, or null if this queue is empty

drainTo

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

Description copied from interface: BlockingQueue

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.

Specified by:
drainTo in interface BlockingQueue<E extends Delayed>
Parameters:
c - the collection to transfer elements into
Returns:
the number of elements transferred
Throws:
UnsupportedOperationException - if addition of elements is not supported by the specified collection
ClassCastException - if the class of an element of this queue prevents it from being added to the specified collection
NullPointerException - if the specified collection is null
IllegalArgumentException - if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection

drainTo

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

Description copied from interface: BlockingQueue

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.

Specified by:
drainTo in interface BlockingQueue<E extends Delayed>
Parameters:
c - the collection to transfer elements into
maxElements - the maximum number of elements to transfer
Returns:
the number of elements transferred
Throws:
UnsupportedOperationException - if addition of elements is not supported by the specified collection
ClassCastException - if the class of an element of this queue prevents it from being added to the specified collection
NullPointerException - if the specified collection is null
IllegalArgumentException - if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection

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 extends Delayed>
Overrides:
clear in class AbstractQueue<E extends Delayed>

remainingCapacity

public int remainingCapacity()

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

Specified by:
remainingCapacity in interface BlockingQueue<E extends Delayed>
Returns:
Integer.MAX_VALUE

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 extends Delayed>
Overrides:
toArray in class AbstractCollection<E extends Delayed>
Returns:
an array containing all of the elements in this queue

toArray

public <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. 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:
toArray in interface Collection<E extends Delayed>
Overrides:
toArray in class AbstractCollection<E extends Delayed>
Type Parameters:
T - the component type of the array to contain the collection
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

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 extends Delayed>
Specified by:
remove in interface Collection<E extends Delayed>
Overrides:
remove in class AbstractCollection<E extends Delayed>
Parameters:
o - element to be removed from this collection, if present
Returns:
true if an element was removed as a result of this call

iterator

public Iterator<E> 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 weakly consistent.

Specified by:
iterator in interface Collection<E extends Delayed>
Specified by:
iterator in interface Iterable<E extends Delayed>
Specified by:
iterator in class AbstractCollection<E extends Delayed>
Returns:
an iterator over the elements in this queue

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/DelayQueue.html