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 generally becomes eligible for removal when its delay has expired.

An element is considered expired when its getDelay(TimeUnit.NANOSECONDS) method would return a value less than or equal to zero.

An element is considered the head of the queue if it is the element with the earliest expiration time, whether in the past or the future, if there is such an element.

An element is considered the expired head of the queue if it is the expired element with the earliest expiration time in the past, if there is such an element. The expired head, when present, is also the head.

While this class implements the BlockingQueue interface, it intentionally violates the general contract of BlockingQueue, in that the following methods disregard the presence of unexpired elements and only ever remove the expired head:

All other methods operate on both expired and unexpired elements. For example, the size() method returns the count of all elements. Method peek() may return the (non-null) head even when take() would block waiting for that element to expire.

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 Details

    • 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 Details

    • 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 expired head of this queue, or returns null if this queue has no expired elements.
      Specified by:
      poll in interface Queue<E extends Delayed>
      Returns:
      the expired 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 expired head of this queue, waiting if necessary until an expired element is available on this queue.
      Specified by:
      take in interface BlockingQueue<E extends Delayed>
      Returns:
      the expired head of this queue
      Throws:
      InterruptedException - if interrupted while waiting

    • poll

      public E poll(long timeout, TimeUnit unit) throws InterruptedException
      Retrieves and removes the expired head of this queue, waiting if necessary until an expired element 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 expired 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

    • remove

      public E remove()
      Retrieves and removes the expired head of this queue, or throws an exception if this queue has no expired elements.
      Specified by:
      remove in interface Queue<E extends Delayed>
      Overrides:
      remove in class AbstractQueue<E extends Delayed>
      Returns:
      the expired head of this queue
      Throws:
      NoSuchElementException - if this queue has no elements with an expired delay

    • 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

    • size

      public int size()
      Description copied from interface: Collection
      Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
      Specified by:
      size in interface Collection<E extends Delayed>
      Returns:
      the number of elements in this collection

    • 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