ConcurrentLinkedDeque

public class ConcurrentLinkedDeque extends AbstractCollection<E>
implements Deque<E> Serializable

An unbounded concurrent {@linkplain Deque deque} based on linked nodes. Concurrent insertion, removal, and access operations execute safely across multiple threads. A ConcurrentLinkedDeque is an appropriate choice when many threads will share access to a common collection. Like most other concurrent collection implementations, this class does not permit the use of null elements.

Iterators and spliterators are weakly consistent.

Beware that, unlike in most collections, the size method is NOT a constant-time operation. Because of the asynchronous nature of these deques, 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. Additionally, the bulk operations addAll, removeAll, retainAll, containsAll, equals, and toArray are not guaranteed to be performed atomically. For example, an iterator operating concurrently with an addAll operation might view only some of the added elements.

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

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

Public Constructor Summary

	ConcurrentLinkedDeque() Constructs an empty deque.
	ConcurrentLinkedDeque(Collection<? extends E> c) Constructs a deque initially containing the elements of the given collection, added in traversal order of the collection's iterator.

Public Method Summary

boolean	add(E e) Inserts the specified element at the tail of this deque.
boolean	addAll(Collection<? extends E> c) Appends all of the elements in the specified collection to the end of this deque, in the order that they are returned by the specified collection's iterator.
void	addFirst(E e) Inserts the specified element at the front of this deque.
void	addLast(E e) Inserts the specified element at the end of this deque.
void	clear() Removes all of the elements from this deque.
boolean	contains(Object o) Returns `true` if this deque contains the specified element.
Iterator<E>	descendingIterator() Returns an iterator over the elements in this deque in reverse sequential order.
E	element() Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque).
E	getFirst() Retrieves, but does not remove, the first element of this deque.
E	getLast() Retrieves, but does not remove, the last element of this deque.
boolean	isEmpty() Returns `true` if this collection contains no elements.
Iterator<E>	iterator() Returns an iterator over the elements in this deque in proper sequence.
boolean	offer(E e) Inserts the specified element at the tail of this deque.
boolean	offerFirst(E e) Inserts the specified element at the front of this deque.
boolean	offerLast(E e) Inserts the specified element at the end of this deque.
E	peek() Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returns `null` if this deque is empty.
E	peekFirst() Retrieves, but does not remove, the first element of this deque, or returns `null` if this deque is empty.
E	peekLast() Retrieves, but does not remove, the last element of this deque, or returns `null` if this deque is empty.
E	poll() Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returns `null` if this deque is empty.
E	pollFirst() Retrieves and removes the first element of this deque, or returns `null` if this deque is empty.
E	pollLast() Retrieves and removes the last element of this deque, or returns `null` if this deque is empty.
E	pop() Pops an element from the stack represented by this deque.
void	push(E e) Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing an `IllegalStateException` if no space is currently available.
E	remove() Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).
boolean	remove(Object o) Removes the first occurrence of the specified element from this deque.
E	removeFirst() Retrieves and removes the first element of this deque.
boolean	removeFirstOccurrence(Object o) Removes the first occurrence of the specified element from this deque.
E	removeLast() Retrieves and removes the last element of this deque.
boolean	removeLastOccurrence(Object o) Removes the last occurrence of the specified element from this deque.
int	size() Returns the number of elements in this deque.
Spliterator<E>	spliterator() Returns a `Spliterator` over the elements in this deque.
Object[]	toArray() Returns an array containing all of the elements in this deque, in proper sequence (from first to last element).
<T> T[]	toArray(T[] a) Returns an array containing all of the elements in this deque, in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.
String	toString() Returns a string representation of this collection.

Protected Method Summary

void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.

Inherited Method Summary

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[] contents) 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) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of `Class` that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object.

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.
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).
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.
Spliterator<E>	spliterator() Creates a `Spliterator` over the elements in this collection.
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.Deque

abstract boolean	add(E e) Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque) 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 void	addFirst(E e) Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions, throwing an `IllegalStateException` if no space is currently available.
abstract void	addLast(E e) Inserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions, throwing an `IllegalStateException` if no space is currently available.
abstract boolean	contains(Object o) Returns `true` if this deque contains the specified element.
abstract Iterator<E>	descendingIterator() Returns an iterator over the elements in this deque in reverse sequential order.
abstract E	element() Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque).
abstract E	getFirst() Retrieves, but does not remove, the first element of this deque.
abstract E	getLast() Retrieves, but does not remove, the last element of this deque.
abstract Iterator<E>	iterator() Returns an iterator over the elements in this deque in proper sequence.
abstract boolean	offer(E e) Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque) 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	offerFirst(E e) Inserts the specified element at the front of this deque unless it would violate capacity restrictions.
abstract boolean	offerLast(E e) Inserts the specified element at the end of this deque unless it would violate capacity restrictions.
abstract E	peek() Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returns `null` if this deque is empty.
abstract E	peekFirst() Retrieves, but does not remove, the first element of this deque, or returns `null` if this deque is empty.
abstract E	peekLast() Retrieves, but does not remove, the last element of this deque, or returns `null` if this deque is empty.
abstract E	poll() Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returns `null` if this deque is empty.
abstract E	pollFirst() Retrieves and removes the first element of this deque, or returns `null` if this deque is empty.
abstract E	pollLast() Retrieves and removes the last element of this deque, or returns `null` if this deque is empty.
abstract E	pop() Pops an element from the stack represented by this deque.
abstract void	push(E e) Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing an `IllegalStateException` if no space is currently available.
abstract E	remove() Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).
abstract boolean	remove(Object o) Removes the first occurrence of the specified element from this deque.
abstract E	removeFirst() Retrieves and removes the first element of this deque.
abstract boolean	removeFirstOccurrence(Object o) Removes the first occurrence of the specified element from this deque.
abstract E	removeLast() Retrieves and removes the last element of this deque.
abstract boolean	removeLastOccurrence(Object o) Removes the last occurrence of the specified element from this deque.
abstract int	size() Returns the number of elements in this deque.

From interface java.lang.Iterable

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`.
Spliterator<E>	spliterator() Creates a `Spliterator` over the elements described by this `Iterable`.

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.

Public Constructors

public ConcurrentLinkedDeque ()

Constructs an empty deque.

public ConcurrentLinkedDeque (Collection<? extends E> c)

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

Parameters

c	the collection of elements to initially contain

Throws

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

Public Methods

public boolean add (E e)

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

Parameters

e	element whose presence in this collection is to be ensured

Returns

true (as specified by Collection.add(E))

Throws

NullPointerException	if the specified element is null

public boolean addAll (Collection<? extends E> c)

Appends all of the elements in the specified collection to the end of this deque, in the order that they are returned by the specified collection's iterator. Attempts to addAll of a deque to itself result in IllegalArgumentException.

Parameters

c	the elements to be inserted into this deque

Returns

true if this deque changed as a result of the call

Throws

NullPointerException	if the specified collection or any of its elements are null
IllegalArgumentException	if the collection is this deque

public void addFirst (E e)

Inserts the specified element at the front of this deque. As the deque is unbounded, this method will never throw IllegalStateException.

Parameters

e	the element to add

Throws

NullPointerException	if the specified element is null

public void addLast (E e)

Inserts the specified element at the end of this deque. As the deque is unbounded, this method will never throw IllegalStateException.

This method is equivalent to add(E).

Parameters

e	the element to add

Throws

NullPointerException	if the specified element is null

public void clear ()

Removes all of the elements from this deque.

public boolean contains (Object o)

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

Parameters

o	element whose presence in this deque is to be tested

Returns

true if this deque contains the specified element

public Iterator<E> descendingIterator ()

Returns an iterator over the elements in this deque in reverse sequential order. The elements will be returned in order from last (tail) to first (head).

The returned iterator is weakly consistent.

Returns

an iterator over the elements in this deque in reverse order

public E element ()

Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque). This method differs from peek only in that it throws an exception if this deque is empty.

This method is equivalent to getFirst().

Returns

the head of the queue represented by this deque

Throws

NoSuchElementException

public E getFirst ()

Retrieves, but does not remove, the first element of this deque. This method differs from peekFirst only in that it throws an exception if this deque is empty.

Returns

the head of this deque

Throws

NoSuchElementException

public E getLast ()

Retrieves, but does not remove, the last element of this deque. This method differs from peekLast only in that it throws an exception if this deque is empty.

Returns

the tail of this deque

Throws

NoSuchElementException

public boolean isEmpty ()

Returns true if this collection contains no elements.

Returns

true if this collection contains no elements

public Iterator<E> iterator ()

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

The returned iterator is weakly consistent.

Returns

an iterator over the elements in this deque in proper sequence

public boolean offer (E e)

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

Parameters

e	the element to add

Returns

true (as specified by Queue.offer(E))

Throws

NullPointerException	if the specified element is null

public boolean offerFirst (E e)

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

Parameters

e	the element to add

Returns

true (as specified by Deque.offerFirst(E))

Throws

NullPointerException	if the specified element is null

public boolean offerLast (E e)

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

This method is equivalent to add(E).

Parameters

e	the element to add

Returns

true (as specified by Deque.offerLast(E))

Throws

NullPointerException	if the specified element is null

public E peek ()

Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returns null if this deque is empty.

This method is equivalent to peekFirst().

Returns

the head of the queue represented by this deque, or null if this deque is empty

public E peekFirst ()

Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.

Returns

the head of this deque, or null if this deque is empty

public E peekLast ()

Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.

Returns

the tail of this deque, or null if this deque is empty

public E poll ()

Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returns null if this deque is empty.

This method is equivalent to pollFirst().

Returns

the first element of this deque, or null if this deque is empty

public E pollFirst ()

Retrieves and removes the first element of this deque, or returns null if this deque is empty.

Returns

the head of this deque, or null if this deque is empty

public E pollLast ()

Retrieves and removes the last element of this deque, or returns null if this deque is empty.

Returns

the tail of this deque, or null if this deque is empty

public E pop ()

Pops an element from the stack represented by this deque. In other words, removes and returns the first element of this deque.

This method is equivalent to removeFirst().

Returns

the element at the front of this deque (which is the top of the stack represented by this deque)

Throws

NoSuchElementException

public void push (E e)

Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing an IllegalStateException if no space is currently available.

This method is equivalent to addFirst(E).

Parameters

e	the element to push

Throws

NullPointerException

public E remove ()

Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque). This method differs from poll only in that it throws an exception if this deque is empty.

This method is equivalent to removeFirst().

Returns

the head of the queue represented by this deque

Throws

NoSuchElementException

public boolean remove (Object o)

Removes the first occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the first element e such that o.equals(e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).

This method is equivalent to removeFirstOccurrence(Object).

Parameters

o	element to be removed from this deque, if present

Returns

true if the deque contained the specified element

Throws

NullPointerException	if the specified element is null

public E removeFirst ()

Retrieves and removes the first element of this deque. This method differs from pollFirst only in that it throws an exception if this deque is empty.

Returns

the head of this deque

Throws

NoSuchElementException

public boolean removeFirstOccurrence (Object o)

Parameters

o	element to be removed from this deque, if present

Returns

true if the deque contained the specified element

Throws

NullPointerException	if the specified element is null

public E removeLast ()

Retrieves and removes the last element of this deque. This method differs from pollLast only in that it throws an exception if this deque is empty.

Returns

the tail of this deque

Throws

NoSuchElementException

public boolean removeLastOccurrence (Object o)

Removes the last occurrence of the specified element from this deque. If the deque does not contain the element, it is unchanged. More formally, removes the last element e such that o.equals(e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).

Parameters

o	element to be removed from this deque, if present

Returns

true if the deque contained the specified element

Throws

NullPointerException	if the specified element is null

public int size ()

Returns the number of elements in this deque. If this deque contains more than Integer.MAX_VALUE elements, it 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 deques, determining the current number of elements requires traversing them all to count them. Additionally, it is possible for the size to change during execution of this method, in which case the returned result will be inaccurate. Thus, this method is typically not very useful in concurrent applications.

Returns

the number of elements in this deque

public Spliterator<E> spliterator ()

Returns a Spliterator over the elements in this deque.

The returned spliterator is weakly consistent.

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

Returns

a Spliterator over the elements in this deque

public Object[] toArray ()

Returns an array containing all of the elements in this deque, in proper sequence (from first to last element).

The returned array will be "safe" in that no references to it are maintained by this deque. (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

an array containing all of the elements in this deque

public T[] toArray (T[] a)

Returns an array containing all of the elements in this deque, in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the deque 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 deque.

If this deque fits in the specified array with room to spare (i.e., the array has more elements than this deque), the element in the array immediately following the end of the deque 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 deque known to contain only strings. The following code can be used to dump the deque 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	the array into which the elements of the deque 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 deque

Throws

ArrayStoreException	if the runtime type of the specified array is not a supertype of the runtime type of every element in this deque
NullPointerException	if the specified array is null

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

a string representation of this collection

Protected Methods

protected void finalize ()

Invoked when the garbage collector has detected that this instance is no longer reachable. The default implementation does nothing, but this method can be overridden to free resources.

Note that objects that override finalize are significantly more expensive than objects that don't. Finalizers may be run a long time after the object is no longer reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup. Note also that finalizers are run on a single VM-wide finalizer thread, so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary for a class that has a native peer and needs to call a native method to destroy that peer. Even then, it's better to provide an explicit close method (and implement Closeable), and insist that callers manually dispose of instances. This works well for something like files, but less well for something like a BigInteger where typical calling code would have to deal with lots of temporaries. Unfortunately, code that creates lots of temporaries is the worst kind of code from the point of view of the single finalizer thread.

If you must use finalizers, consider at least providing your own ReferenceQueue and having your own thread process that queue.

Unlike constructors, finalizers are not automatically chained. You are responsible for calling super.finalize() yourself.

Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer thread. See Effective Java Item 7, "Avoid finalizers" for more.

Throws

Throwable