ConcurrentLinkedDeque - JRE Emulation | JRE Emulation
public class

ConcurrentLinkedDeque

extends AbstractCollection<E>
implements Deque<E> Serializable
java.lang.Object
   ↳ java.util.AbstractCollection<E>
     ↳ java.util.concurrent.ConcurrentLinkedDeque<E>

Class Overview

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 are weakly consistent, returning elements reflecting the state of the deque at some point at or since the creation of the iterator. They do not throw ConcurrentModificationException, and may proceed concurrently with other operations.

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.

Summary

Public Constructors
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 Methods
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 at least one element e such that o.equals(e).
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 element e such that o.equals(e), if such an element exists in this deque.
E removeFirst()
Retrieves and removes the first element of this deque.
boolean removeFirstOccurrence(Object o)
Removes the first element e such that o.equals(e), if such an element exists in this deque.
E removeLast()
Retrieves and removes the last element of this deque.
boolean removeLastOccurrence(Object o)
Removes the last element e such that o.equals(e), if such an element exists in this deque.
int size()
Returns the number of 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.
Protected Methods
void finalize()
Invoked when the garbage collector has detected that this instance is no longer reachable.
Inherited Methods
[Expand]
From class java.util.AbstractCollection
From class java.lang.Object
From interface java.util.Collection
From interface java.util.Deque
From interface java.lang.Iterable
From interface java.util.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
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 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 a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

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

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

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

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 a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

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
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
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
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)

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

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

public boolean remove (Object o)

Removes the first element e such that o.equals(e), if such an element exists in this deque. If the deque does not contain the element, it is unchanged.

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

public boolean removeFirstOccurrence (Object o)

Removes the first element e such that o.equals(e), if such an element exists in this deque. If the deque does not contain the element, it is unchanged.

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

public boolean removeLastOccurrence (Object o)

Removes the last element e such that o.equals(e), if such an element exists in this deque. If the deque does not contain the element, it is unchanged.

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 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

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