DelayQueue (Java SE 11 & JDK 11 )
Keywords: java.util.concurrent.DelayQueue class

Module java.base
Package java.util.concurrent

## Class DelayQueue<E extends Delayed>

- java.lang.Object
- - java.util.AbstractCollection<E>
  - - java.util.AbstractQueue<E>
    - - java.util.concurrent.DelayQueue<E>

- 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

- - ### Constructor Summary

    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.

  - ### Method Summary

    All Methods Instance Methods Concrete 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`

- - ### Constructor Detail

    - #### 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

  - ### Method Detail

    - #### 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