ConcurrentLinkedDeque

java.lang.Object
- java.util.AbstractCollection<E>
- - java.util.concurrent.ConcurrentLinkedDeque<E>

Type Parameters:
E - the type of elements held in this collection

All Implemented Interfaces:

Serializable, Iterable<E>, Collection<E>, Deque<E>, Queue<E>
```
public class ConcurrentLinkedDeque<E>
extends AbstractCollection<E>
implements Deque<E>, Serializable
```
An unbounded concurrent 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.
This class is a member of the Java Collections Framework.

Since:

1.7

See Also:
Serialized Form

Constructor Summary

Constructors
Constructor and Description
`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.

Method Summary

Methods
Modifier and Type	Method and Description
`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, returning `true` upon success and 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.

Methods inherited from class java.util.AbstractCollection
containsAll, removeAll, retainAll, toString

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Collection
containsAll, equals, hashCode, removeAll, retainAll

- Constructor Detail
  - ConcurrentLinkedDeque
```
public ConcurrentLinkedDeque()
```
    Constructs an empty deque.
  - ConcurrentLinkedDeque
```
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
- Method Detail
  - addFirst
```
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.
    
    Specified by:
    
    addFirst in interface Deque<E>
    
    Parameters:
    e - the element to add
    
    Throws:
    
    NullPointerException - if the specified element is null
  - addLast
```
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).
    
    Specified by:
    
    addLast in interface Deque<E>
    
    Parameters:
    e - the element to add
    
    Throws:
    
    NullPointerException - if the specified element is null
  - offerFirst
```
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.
    
    Specified by:
    
    offerFirst in interface Deque<E>
    
    Parameters:
    e - the element to add
    
    Returns:
    true (as specified by Deque.offerFirst(E))
    
    Throws:
    
    NullPointerException - if the specified element is null
  - offerLast
```
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).
    
    Specified by:
    
    offerLast in interface Deque<E>
    
    Parameters:
    e - the element to add
    
    Returns:
    true (as specified by Deque.offerLast(E))
    
    Throws:
    
    NullPointerException - if the specified element is null
  - peekFirst
```
public E peekFirst()
```
    Description copied from interface: Deque
    
    Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.
    
    Specified by:
    
    peekFirst in interface Deque<E>
    
    Returns:
    the head of this deque, or null if this deque is empty
  - peekLast
```
public E peekLast()
```
    Description copied from interface: Deque
    
    Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.
    
    Specified by:
    
    peekLast in interface Deque<E>
    
    Returns:
    the tail of this deque, or null if this deque is empty
  - getFirst
```
public E getFirst()
```
    Description copied from interface: Deque
    
    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.
    
    Specified by:
    
    getFirst in interface Deque<E>
    
    Returns:
    the head of this deque
    
    Throws:
    
    NoSuchElementException - if this deque is empty
  - getLast
```
public E getLast()
```
    Description copied from interface: Deque
    
    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.
    
    Specified by:
    
    getLast in interface Deque<E>
    
    Returns:
    the tail of this deque
    
    Throws:
    
    NoSuchElementException - if this deque is empty
  - pollFirst
```
public E pollFirst()
```
    Description copied from interface: Deque
    
    Retrieves and removes the first element of this deque, or returns null if this deque is empty.
    
    Specified by:
    
    pollFirst in interface Deque<E>
    
    Returns:
    the head of this deque, or null if this deque is empty
  - pollLast
```
public E pollLast()
```
    Description copied from interface: Deque
    
    Retrieves and removes the last element of this deque, or returns null if this deque is empty.
    
    Specified by:
    
    pollLast in interface Deque<E>
    
    Returns:
    the tail of this deque, or null if this deque is empty
  - removeFirst
```
public E removeFirst()
```
    Description copied from interface: Deque
    
    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.
    
    Specified by:
    
    removeFirst in interface Deque<E>
    
    Returns:
    the head of this deque
    
    Throws:
    
    NoSuchElementException - if this deque is empty
  - removeLast
```
public E removeLast()
```
    Description copied from interface: Deque
    
    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.
    
    Specified by:
    
    removeLast in interface Deque<E>
    
    Returns:
    the tail of this deque
    
    Throws:
    
    NoSuchElementException - if this deque is empty
  - offer
```
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.
    
    Specified by:
    
    offer in interface Deque<E>
    
    Specified by:
    
    offer in interface Queue<E>
    
    Parameters:
    e - the element to add
    
    Returns:
    true (as specified by Queue.offer(E))
    
    Throws:
    
    NullPointerException - if the specified element is null
  - add
```
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.
    
    Specified by:
    
    add in interface Collection<E>
    
    Specified by:
    
    add in interface Deque<E>
    
    Specified by:
    
    add in interface Queue<E>
    
    Overrides:
    
    add in class AbstractCollection<E>
    
    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
  - poll
```
public E poll()
```
    Description copied from interface: Deque
    
    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 Deque.pollFirst().
    
    Specified by:
    
    poll in interface Deque<E>
    
    Specified by:
    
    poll in interface Queue<E>
    
    Returns:
    the first element of this deque, or null if this deque is empty
  - remove
```
public E remove()
```
    Description copied from interface: Deque
    
    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 Deque.removeFirst().
    
    Specified by:
    
    remove in interface Deque<E>
    
    Specified by:
    
    remove in interface Queue<E>
    
    Returns:
    the head of the queue represented by this deque
  - peek
```
public E peek()
```
    Description copied from interface: Deque
    
    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 Deque.peekFirst().
    
    Specified by:
    
    peek in interface Deque<E>
    
    Specified by:
    
    peek in interface Queue<E>
    
    Returns:
    the head of the queue represented by this deque, or null if this deque is empty
  - element
```
public E element()
```
    Description copied from interface: Deque
    
    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 Deque.getFirst().
    
    Specified by:
    
    element in interface Deque<E>
    
    Specified by:
    
    element in interface Queue<E>
    
    Returns:
    the head of the queue represented by this deque
  - push
```
public void push(E e)
```
    Description copied from interface: Deque
    
    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, returning true upon success and throwing an IllegalStateException if no space is currently available.
    This method is equivalent to Deque.addFirst(E).
    
    Specified by:
    
    push in interface Deque<E>
    
    Parameters:
    e - the element to push
  - pop
```
public E pop()
```
    Description copied from interface: Deque
    
    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 Deque.removeFirst().
    
    Specified by:
    
    pop in interface Deque<E>
    
    Returns:
    the element at the front of this deque (which is the top of the stack represented by this deque)
  - removeFirstOccurrence
```
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.
    
    Specified by:
    
    removeFirstOccurrence in interface Deque<E>
    
    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
  - removeLastOccurrence
```
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.
    
    Specified by:
    
    removeLastOccurrence in interface Deque<E>
    
    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
  - contains
```
public boolean contains(Object o)
```
    Returns true if this deque contains at least one element e such that o.equals(e).
    
    Specified by:
    
    contains in interface Collection<E>
    
    Specified by:
    
    contains in interface Deque<E>
    
    Overrides:
    
    contains in class AbstractCollection<E>
    
    Parameters:
    o - element whose presence in this deque is to be tested
    
    Returns:
    true if this deque contains the specified element
  - isEmpty
```
public boolean isEmpty()
```
    Returns true if this collection contains no elements.
    
    Specified by:
    
    isEmpty in interface Collection<E>
    
    Overrides:
    
    isEmpty in class AbstractCollection<E>
    
    Returns:
    true if this collection contains no elements
  - size
```
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.
    
    Specified by:
    
    size in interface Collection<E>
    
    Specified by:
    
    size in interface Deque<E>
    
    Specified by:
    
    size in class AbstractCollection<E>
    
    Returns:
    the number of elements in this deque
  - remove
```
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.
    
    Specified by:
    
    remove in interface Collection<E>
    
    Specified by:
    
    remove in interface Deque<E>
    
    Overrides:
    
    remove in class AbstractCollection<E>
    
    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
  - addAll
```
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.
    
    Specified by:
    
    addAll in interface Collection<E>
    
    Overrides:
    
    addAll in class AbstractCollection<E>
    
    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
    See Also:
    AbstractCollection.add(Object)
  - clear
```
public void clear()
```
    Removes all of the elements from this deque.
    
    Specified by:
    
    clear in interface Collection<E>
    
    Overrides:
    
    clear in class AbstractCollection<E>
  - toArray
```
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.
    
    Specified by:
    
    toArray in interface Collection<E>
    
    Overrides:
    
    toArray in class AbstractCollection<E>
    
    Returns:
    an array containing all of the elements in this deque
  - toArray
```
public <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. 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().
    Specified by:
    
    toArray in interface Collection<E>
    
    Overrides:
    
    toArray in class AbstractCollection<E>
    
    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
  - iterator
```
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.
    
    Specified by:
    
    iterator in interface Iterable<E>
    
    Specified by:
    
    iterator in interface Collection<E>
    
    Specified by:
    
    iterator in interface Deque<E>
    
    Specified by:
    
    iterator in class AbstractCollection<E>
    
    Returns:
    an iterator over the elements in this deque in proper sequence
  - descendingIterator
```
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.
    
    Specified by:
    
    descendingIterator in interface Deque<E>
    
    Returns:
    an iterator over the elements in this deque in reverse order

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Class ConcurrentLinkedDeque<E>

Constructor Summary

Method Summary

Methods inherited from class java.util.AbstractCollection

Methods inherited from class java.lang.Object

Methods inherited from interface java.util.Collection

Constructor Detail

ConcurrentLinkedDeque

ConcurrentLinkedDeque

Method Detail

addFirst

addLast

offerFirst

offerLast

peekFirst

peekLast

getFirst

getLast

pollFirst

pollLast

removeFirst

removeLast

offer

add

poll

remove

peek

element

push

pop

removeFirstOccurrence

removeLastOccurrence

contains

isEmpty

size

remove

addAll

clear

toArray

toArray

iterator

descendingIterator