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Summary: Ctors | Methods | Inherited Methods

# DelayQueue

`public class DelayQueue`
`` `` `extends AbstractQueue<E extends Delayed>` `implements BlockingQueue<E extends Delayed>`

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

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.

## 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(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.
`E` | `poll()` Retrieves and removes the head of this queue, or returns `null` if this queue has no elements with an expired delay.
`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 the number of elements in this collection.
`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.

### Inherited methods
From class `java.util.AbstractQueue`
`boolean` | `add(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
`boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this queue.
`void` | `clear()` Removes all of the elements from this queue.
`E` | `element()` Retrieves, but does not remove, the head of this queue.
`E` | `remove()` Retrieves and removes the head of this queue.
From class `java.util.AbstractCollection`
`boolean` | `add(E e)` Ensures that this collection contains the specified element (optional operation). This implementation always throws an `UnsupportedOperationException`.
`boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
`void` | `clear()` Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the `Iterator.remove` operation.
`boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
`boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
`boolean` | `isEmpty()` Returns `true` if this collection contains no elements. This implementation returns `size() == 0`.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements contained in this collection.
`boolean` | `remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
`boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`abstract int` | `size()` Returns the number of elements in this collection.
`<T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index `0`.
`Object[]` | `toArray()` Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index `0`.
`String` | `toString()` Returns a string representation of this collection.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
From interface `java.util.Queue`
`abstract boolean` | `add(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
`abstract E` | `element()` Retrieves, but does not remove, the head of this queue.
`abstract boolean` | `offer(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
`abstract E` | `peek()` Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
`abstract E` | `poll()` Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
`abstract E` | `remove()` Retrieves and removes the head of this queue.
From interface `java.util.Collection`
`abstract boolean` | `add(E e)` Ensures that this collection contains the specified element (optional operation).
`abstract boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`abstract void` | `clear()` Removes all of the elements from this collection (optional operation).
`abstract boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element.
`abstract boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`abstract boolean` | `equals(Object o)` Compares the specified object with this collection for equality.
`abstract int` | `hashCode()` Returns the hash code value for this collection.
`abstract boolean` | `isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements in this collection.
`default Stream<E>` | `parallelStream()` Returns a possibly parallel `Stream` with this collection as its source.
`abstract boolean` | `remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`abstract boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`default boolean` | `removeIf(Predicate<? super E> filter)` Removes all of the elements of this collection that satisfy the given predicate.
`abstract boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`abstract int` | `size()` Returns the number of elements in this collection.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements in this collection.
`default Stream<E>` | `stream()` Returns a sequential `Stream` with this collection as its source.
`abstract <T> T[]` | `toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`abstract Object[]` | `toArray()` Returns an array containing all of the elements in this collection.
From interface `java.util.concurrent.BlockingQueue`
`abstract boolean` | `add(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
`abstract boolean` | `contains(Object o)` Returns `true` if this queue contains the specified element.
`abstract int` | `drainTo(Collection<? super E> c)` Removes all available elements from this queue and adds them to the given collection.
`abstract 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.
`abstract boolean` | `offer(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and `false` if no space is currently available.
`abstract boolean` | `offer(E e, long timeout, TimeUnit unit)` Inserts the specified element into this queue, waiting up to the specified wait time if necessary for space to become available.
`abstract E` | `poll(long timeout, TimeUnit unit)` Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
`abstract void` | `put(E e)` Inserts the specified element into this queue, waiting if necessary for space to become available.
`abstract int` | `remainingCapacity()` Returns the number of additional elements that this queue can ideally (in the absence of memory or resource constraints) accept without blocking, or `Integer.MAX_VALUE` if there is no intrinsic limit.
`abstract boolean` | `remove(Object o)` Removes a single instance of the specified element from this queue, if it is present.
`abstract E` | `take()` Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
From interface `java.lang.Iterable`
`default void` | `forEach(Consumer<? super T> action)` Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
`abstract Iterator<E extends Delayed>` | `iterator()` Returns an iterator over elements of type `T`.
`default Spliterator<E extends Delayed>` | `spliterator()` Creates a `Spliterator` over the elements described by this `Iterable`.

## Public 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` | `Collection`: the collection of elements to initially contain

Throws
`NullPointerException` | if the specified collection or any of its elements are null

## Public methods

### add

    public boolean add (E e)

Inserts the specified element into this delay queue.

Parameters
`e` | `E`: the element to add

Returns
`boolean` | `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<? super E> c,
                    int maxElements)

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` | `Collection`: the collection to transfer elements into
`maxElements` | `int`: the maximum number of elements to transfer

Returns
`int` | the number of elements transferred

Throws
`UnsupportedOperationException` |
`ClassCastException` |
`NullPointerException` |
`IllegalArgumentException` |

### drainTo

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

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` | `Collection`: the collection to transfer elements into

Returns
`int` | the number of elements transferred

Throws
`UnsupportedOperationException` |
`ClassCastException` |
`NullPointerException` |
`IllegalArgumentException` |

### 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.

Returns
`Iterator<E>` | an iterator over the elements in this queue

### offer

    public boolean offer (E e)

Inserts the specified element into this delay queue.

Parameters
`e` | `E`: the element to add

Returns
`boolean` | `true`

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.

Parameters
`e` | `E`: the element to add
`timeout` | `long`: This parameter is ignored as the method never blocks
`unit` | `TimeUnit`: This parameter is ignored as the method never blocks

Returns
`boolean` | `true`

Throws
`NullPointerException` |

### 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.

Returns
`E` | the head of this queue, or `null` if this queue is empty

### poll

    public 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.

Parameters
`timeout` | `long`: how long to wait before giving up, in units of `unit`
`unit` | `TimeUnit`: a `TimeUnit` determining how to interpret the `timeout` parameter

Returns
`E` | the head of this queue, or `null` if the specified waiting time elapses before an element with an expired delay becomes available

Throws
`InterruptedException` |

### 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.

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

### put

    public void put (E e)

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

Parameters
`e` | `E`: the element to add

Throws
`NullPointerException` |

### remainingCapacity

    public int remainingCapacity ()

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

Returns
`int` | `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.

Parameters
`o` | `Object`: element to be removed from this collection, if present

Returns
`boolean` | `true` if an element was removed as a result of this call

### size

    public int size ()

Returns the number of elements in this collection. If this collection contains more than `Integer.MAX_VALUE` elements, returns `Integer.MAX_VALUE`.

Returns
`int` | the number of elements in this collection

### take

    public 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.

Returns
`E` | the head of this queue

Throws
`InterruptedException` |

### toArray

    public 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()`.

Parameters
`a` | `T`: 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
`T[]` | 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

### 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
`Object[]` | an array containing all of the elements in this queue

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