java.util.concurrent

## Class ConcurrentLinkedDeque<E>

- 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
  Author:
    Doug Lea, Martin Buchholz
  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

    All Methods Instance Methods Concrete 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, parallelStream, removeAll, removeIf, retainAll, spliterator, stream`

    - ### Methods inherited from interface java.lang.Iterable

      `forEach`

- - ### 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>`
      Type Parameters:
        `T` - the runtime type of the array to contain the collection
      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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