Added in API level 21

Summary: Ctors | Methods | Inherited Methods

LinkedTransferQueue

public class LinkedTransferQueue
extends AbstractQueue<E> implements TransferQueue<E>, Serializable

java.lang.Object
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractQueue<E>
		↳	java.util.concurrent.LinkedTransferQueue<E>

An unbounded TransferQueue based on linked nodes. This queue orders elements FIFO (first-in-first-out) with respect to any given producer. The head of the queue is that element that has been on the queue the longest time for some producer. The tail of the queue is that element that has been on the queue the shortest time for some producer.

Beware that, unlike in most collections, the size method is NOT a constant-time operation. Because of the asynchronous nature of these queues, determining the current number of elements requires a traversal of the elements, and so may report inaccurate results if this collection is modified during traversal.

Bulk operations that add, remove, or examine multiple elements, such as AbstractQueue.addAll(Collection), removeIf(Predicate) or forEach(Consumer), are not guaranteed to be performed atomically. For example, a forEach traversal concurrent with an addAll operation might observe only some of the added elements.

This class and its iterator implement all of the optional methods of the Collection and Iterator interfaces.

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a LinkedTransferQueue happen-before actions subsequent to the access or removal of that element from the LinkedTransferQueue in another thread.

This class is a member of the Java Collections Framework.

Summary

Public constructors
`LinkedTransferQueue()` Creates an initially empty `LinkedTransferQueue`.
`LinkedTransferQueue(Collection<? extends E> c)` Creates a `LinkedTransferQueue` initially containing the elements of the given collection, added in traversal order of the collection's iterator.

Public methods
`boolean`	`add(E e)` Inserts the specified element at the tail of this queue.
`void`	`clear()` Removes all of the elements from this queue.
`boolean`	`contains(Object o)` Returns `true` if this queue contains the specified element.
`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.
`void`	`forEach(Consumer<? super E> action)`
`int`	`getWaitingConsumerCount()` Returns an estimate of the number of consumers waiting to receive elements via `BlockingQueue.take()` or timed `poll`.
`boolean`	`hasWaitingConsumer()` Returns `true` if there is at least one consumer waiting to receive an element via `BlockingQueue.take()` or timed `poll`.
`boolean`	`isEmpty()` Returns `true` if this queue contains no elements.
`Iterator<E>`	`iterator()` Returns an iterator over the elements in this queue in proper sequence.
`boolean`	`offer(E e, long timeout, TimeUnit unit)` Inserts the specified element at the tail of this queue.
`boolean`	`offer(E e)` Inserts the specified element at the tail of this 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 up to the specified wait time if necessary for an element to become available.
`E`	`poll()` Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
`void`	`put(E e)` Inserts the specified element at the tail of this queue.
`int`	`remainingCapacity()` Always returns `Integer.MAX_VALUE` because a `LinkedTransferQueue` is not capacity constrained.
`boolean`	`remove(Object o)` Removes a single instance of the specified element from this queue, if it is present.
`boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`boolean`	`removeIf(Predicate<? super E> filter)` Removes all of the elements of this collection that satisfy the given predicate.
`boolean`	`retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`int`	`size()` Returns the number of elements in this queue.
`Spliterator<E>`	`spliterator()` Returns a `Spliterator` over the elements in this queue.
`E`	`take()` Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
`Object[]`	`toArray()` Returns an array containing all of the elements in this queue, in proper sequence.
`<T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this queue, in proper sequence; the runtime type of the returned array is that of the specified array.
`String`	`toString()` Returns a string representation of this collection.
`void`	`transfer(E e)` Transfers the element to a consumer, waiting if necessary to do so.
`boolean`	`tryTransfer(E e)` Transfers the element to a waiting consumer immediately, if possible.
`boolean`	`tryTransfer(E e, long timeout, TimeUnit unit)` Transfers the element to a consumer if it is possible to do so before the timeout elapses.

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).
`boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`void`	`clear()` Removes all of the elements from this collection (optional operation).
`boolean`	`contains(Object o)` Returns `true` if this collection contains the specified element.
`boolean`	`containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`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).
`boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`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.
`<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.
`Object[]`	`toArray()` Returns an array containing all of the elements in this collection.
`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 timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

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.
`default <T> T[]`	`toArray(IntFunction<T[]> generator)` Returns an array containing all of the elements in this collection, using the provided `generator` function to allocate the returned array.

From interface


        
          java.util.concurrent.TransferQueue

`abstract int`	`getWaitingConsumerCount()` Returns an estimate of the number of consumers waiting to receive elements via `BlockingQueue.take()` or timed `poll`.
`abstract boolean`	`hasWaitingConsumer()` Returns `true` if there is at least one consumer waiting to receive an element via `BlockingQueue.take()` or timed `poll`.
`abstract void`	`transfer(E e)` Transfers the element to a consumer, waiting if necessary to do so.
`abstract boolean`	`tryTransfer(E e)` Transfers the element to a waiting consumer immediately, if possible.
`abstract boolean`	`tryTransfer(E e, long timeout, TimeUnit unit)` Transfers the element to a consumer if it is possible to do so before the timeout elapses.

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>`	`iterator()` Returns an iterator over elements of type `T`.
`default Spliterator<E>`	`spliterator()` Creates a `Spliterator` over the elements described by this `Iterable`.

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.

Public constructors

LinkedTransferQueue

Added in API level 21

public LinkedTransferQueue ()

Creates an initially empty LinkedTransferQueue.

LinkedTransferQueue

Added in API level 21

public LinkedTransferQueue (Collection<? extends E> c)

Creates a LinkedTransferQueue initially containing the elements of the given collection, added in traversal order of the collection's iterator.

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

Added in API level 21

public boolean add (E e)

Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never throw IllegalStateException or return false.

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

Returns
`boolean`	`true` (as specified by `Collection#add`)

Throws
`NullPointerException`	if the specified element is null

clear

Added in API level 21

public void clear ()

Removes all of the elements from this queue. The queue will be empty after this call returns.

This implementation repeatedly invokes poll until it returns null.

contains

Added in API level 21

public boolean contains (Object o)

Returns true if this queue contains the specified element. More formally, returns true if and only if this queue contains at least one element e such that o.equals(e).

Parameters
`o`	`Object`: object to be checked for containment in this queue

Returns
`boolean`	`true` if this queue contains the specified element

drainTo

Added in API level 21

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
`NullPointerException`
`IllegalArgumentException`

drainTo

Added in API level 21

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
`NullPointerException`
`IllegalArgumentException`

forEach

Added in API level 24

public void forEach (Consumer<? super E> action)

Parameters
`action`	`Consumer`

Throws
`NullPointerException`

getWaitingConsumerCount

Added in API level 21

public int getWaitingConsumerCount ()

Returns an estimate of the number of consumers waiting to receive elements via BlockingQueue.take() or timed poll. The return value is an approximation of a momentary state of affairs, that may be inaccurate if consumers have completed or given up waiting. The value may be useful for monitoring and heuristics, but not for synchronization control. Implementations of this method are likely to be noticeably slower than those for hasWaitingConsumer().

Returns
`int`	the number of consumers waiting to receive elements

hasWaitingConsumer

Added in API level 21

public boolean hasWaitingConsumer ()

Returns true if there is at least one consumer waiting to receive an element via BlockingQueue.take() or timed poll. The return value represents a momentary state of affairs.

Returns
`boolean`	`true` if there is at least one waiting consumer

isEmpty

Added in API level 21

public boolean isEmpty ()

Returns true if this queue contains no elements.

Returns
`boolean`	`true` if this queue contains no elements

iterator

Added in API level 21

public Iterator<E> iterator ()

Returns an iterator over the elements in this queue in proper sequence. The elements will be returned in order from first (head) to last (tail).

The returned iterator is weakly consistent.

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

offer

Added in API level 21

public boolean offer (E e, 
                long timeout, 
                TimeUnit unit)

Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never block or return false.

Parameters
`e`	`E`: the element to add
`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
`boolean`	`true` (as specified by `BlockingQueue.offer`)

Throws
`NullPointerException`	if the specified element is null

offer

Added in API level 21

public boolean offer (E e)

Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never return false.

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

Returns
`boolean`	`true` (as specified by `Queue#offer`)

Throws
`NullPointerException`	if the specified element is null

peek

Added in API level 21

public E peek ()

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

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

poll

Added in API level 21

public 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.

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 is available

Throws
`InterruptedException`

poll

Added in API level 21

public E poll ()

Retrieves and removes the head of this queue, or returns null if this queue is empty.

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

put

Added in API level 21

public void put (E e)

Inserts the specified element at the tail of this queue. As the queue is unbounded, this method will never block.

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

Throws
`NullPointerException`	if the specified element is null

remainingCapacity

Added in API level 21

public int remainingCapacity ()

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

Returns
`int`	`Integer.MAX_VALUE` (as specified by `BlockingQueue#remainingCapacity()`)

remove

Added in API level 21

public boolean remove (Object o)

Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element e such that o.equals(e), if this queue contains one or more such elements. Returns true if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).

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

Returns
`boolean`	`true` if this queue changed as a result of the call

removeAll

Added in API level 21

public boolean removeAll (Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.

Parameters
`c`	`Collection`: collection containing elements to be removed from this collection

Returns
`boolean`	`true` if this collection changed as a result of the call

Throws
`NullPointerException`

removeIf

Added in API level 24

public boolean removeIf (Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.

Parameters
`filter`	`Predicate`: a predicate which returns `true` for elements to be removed

Returns
`boolean`	`true` if any elements were removed

Throws
`NullPointerException`

retainAll

Added in API level 21

public boolean retainAll (Collection<?> c)

Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.

Parameters
`c`	`Collection`: collection containing elements to be retained in this collection

Returns
`boolean`	`true` if this collection changed as a result of the call

Throws
`NullPointerException`

size

Added in API level 21

public int size ()

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

Beware that, unlike in most collections, this method is NOT a constant-time operation. Because of the asynchronous nature of these queues, determining the current number of elements requires an O(n) traversal.

Returns
`int`	the number of elements in this queue

spliterator

Added in API level 24

public Spliterator<E> spliterator ()

Returns a Spliterator over the elements in this queue.

The returned spliterator is weakly consistent.

The Spliterator reports Spliterator#CONCURRENT, Spliterator#ORDERED, and Spliterator#NONNULL.

Implementation Note:

The Spliterator implements trySplit to permit limited parallelism.

Returns
`Spliterator<E>`	a `Spliterator` over the elements in this queue

take

Added in API level 21

public E take ()

Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.

Returns
`E`	the head of this queue

Throws
`InterruptedException`

toArray

Added in API level 21

public Object[] toArray ()

Returns an array containing all of the elements in this queue, in proper sequence.

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

toArray

Added in API level 21

public T[] toArray (T[] a)

Returns an array containing all of the elements in this queue, in proper sequence; the runtime type of the returned array is that of the specified array. 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.

Suppose x is a queue known to contain only strings. The following code can be used to dump the queue into a newly allocated array of String:

 String[] y = x.toArray(new String[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

toString

Added in API level 21

public String toString ()

Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String#valueOf(Object).

Returns
`String`	a string representation of this collection

transfer

Added in API level 21

public void transfer (E e)

Transfers the element to a consumer, waiting if necessary to do so.

More precisely, transfers the specified element immediately if there exists a consumer already waiting to receive it (in take() or timed poll), else inserts the specified element at the tail of this queue and waits until the element is received by a consumer.

Parameters
`e`	`E`: the element to transfer

Throws
`NullPointerException`	if the specified element is null
`InterruptedException`

tryTransfer

Added in API level 21

public boolean tryTransfer (E e)

Transfers the element to a waiting consumer immediately, if possible.

More precisely, transfers the specified element immediately if there exists a consumer already waiting to receive it (in take() or timed poll), otherwise returning false without enqueuing the element.

Parameters
`e`	`E`: the element to transfer

Returns
`boolean`	`true` if the element was transferred, else `false`

Throws
`NullPointerException`	if the specified element is null

tryTransfer

Added in API level 21

public boolean tryTransfer (E e, 
                long timeout, 
                TimeUnit unit)

Transfers the element to a consumer if it is possible to do so before the timeout elapses.

Parameters
`e`	`E`: the element to transfer
`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
`boolean`	`true` if successful, or `false` if the specified waiting time elapses before completion, in which case the element is not left enqueued

Throws
`NullPointerException`	if the specified element is null
`InterruptedException`