Module java.base
Package java.util.concurrent

Class ConcurrentLinkedDeque<E>

  • Type Parameters:
    E - the type of elements held in this deque
    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 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.

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

    This class and its iterator implement all of the optional methods of the 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 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 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 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).
      void forEach​(Consumer<? super E> action)
      Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
      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.
      boolean removeAll​(Collection<?> c)
      Removes all of this collection's elements that are also contained in the specified collection (optional operation).
      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.
      boolean removeIf​(Predicate<? super E> filter)
      Removes all of the elements of this collection that satisfy the given predicate.
      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.
      boolean retainAll​(Collection<?> c)
      Retains only the elements in this collection that are contained in the specified collection (optional operation).
      int size​()
      Returns the number of elements in this 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.

    • 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

      • 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

      • 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

      • 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
        Throws:
        NoSuchElementException - if this deque is empty

      • 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)
        Throws:
        NoSuchElementException - 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
        Throws:
        NoSuchElementException - if this deque is empty

      • 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, 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
        Throws:
        NullPointerException - if the specified element is null and this deque does not permit null elements

      • removeFirstOccurrence

        public boolean removeFirstOccurrence​(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).
        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 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).
        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 the specified element. More formally, returns true if and only 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 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).

        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>
        Specified by:
        addAll in interface Deque<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)

      • toString

        public String toString​()
        Description copied from class: AbstractCollection
        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).
        Overrides:
        toString in class AbstractCollection<E>
        Returns:
        a string representation of this collection

      • 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

      • 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 weakly consistent.

        Specified by:
        descendingIterator in interface Deque<E>
        Returns:
        an iterator over the elements in this deque in reverse order

      • removeIf

        public boolean removeIf​(Predicate<? super E> filter)
        Description copied from interface: Collection
        Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.
        Specified by:
        removeIf in interface Collection<E>
        Parameters:
        filter - a predicate which returns true for elements to be removed
        Returns:
        true if any elements were removed
        Throws:
        NullPointerException - if the specified filter is null

      • removeAll

        public boolean removeAll​(Collection<?> c)
        Description copied from class: AbstractCollection
        Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.
        Specified by:
        removeAll in interface Collection<E>
        Overrides:
        removeAll in class AbstractCollection<E>
        Parameters:
        c - collection containing elements to be removed from this collection
        Returns:
        true if this collection changed as a result of the call
        Throws:
        NullPointerException - if this collection contains one or more null elements and the specified collection does not support null elements (optional), or if the specified collection is null
        See Also:
        AbstractCollection.remove(Object), AbstractCollection.contains(Object)

      • retainAll

        public boolean retainAll​(Collection<?> c)
        Description copied from class: AbstractCollection
        Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.
        Specified by:
        retainAll in interface Collection<E>
        Overrides:
        retainAll in class AbstractCollection<E>
        Parameters:
        c - collection containing elements to be retained in this collection
        Returns:
        true if this collection changed as a result of the call
        Throws:
        NullPointerException - if this collection contains one or more null elements and the specified collection does not permit null elements (optional), or if the specified collection is null
        See Also:
        AbstractCollection.remove(Object), AbstractCollection.contains(Object)

      • forEach

        public void forEach​(Consumer<? super E> action)
        Description copied from interface: Iterable
        Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.

        The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy.

        Specified by:
        forEach in interface Iterable<E>
        Parameters:
        action - The action to be performed for each element
        Throws:
        NullPointerException - if the specified action is null