Module java.base
Package java.util

# Class ArrayDeque<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.ArrayDeque<E>

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

All Implemented Interfaces:
  `Serializable`, `Cloneable`, `Iterable<E>`, `Collection<E>`, `Deque<E>`, `Queue<E>`, `SequencedCollection<E>`

public class ArrayDeque<E> extends AbstractCollection<E> implements Deque<E>, Cloneable, Serializable
Resizable-array implementation of the `Deque` interface. Array deques have no capacity restrictions; they grow as necessary to support usage. They are not thread-safe; in the absence of external synchronization, they do not support concurrent access by multiple threads. Null elements are prohibited. This class is likely to be faster than `Stack` when used as a stack, and faster than `LinkedList` when used as a queue.

Most `ArrayDeque` operations run in amortized constant time. Exceptions include `remove`, `removeFirstOccurrence`, `removeLastOccurrence`, `contains`, `iterator.remove()`, and the bulk operations, all of which run in linear time.

The iterators returned by this class's `iterator` method are fail-fast: If the deque is modified at any time after the iterator is created, in any way except through the iterator's own `remove` method, the iterator will generally throw a `ConcurrentModificationException`. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw `ConcurrentModificationException` on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class and its iterator implement all of the optional methods of the `Collection`, `SequencedCollection`, and `Iterator` interfaces.

This class is a member of the Java Collections Framework.

Since:
  1.6
See Also:
  - Serialized Form

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `ArrayDeque()`
  Constructs an empty array deque with an initial capacity sufficient to hold 16 elements.
  `ArrayDeque(int numElements)`
  Constructs an empty array deque with an initial capacity sufficient to hold the specified number of elements.
  `ArrayDeque(Collection<? extends E> c)`
  Constructs a deque containing the elements of the specified collection, in the order they are returned by the collection's iterator.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `add(E e)`
  Inserts the specified element at the end of this deque.
  `boolean`
  `addAll(Collection<? extends E> c)`
  Adds all of the elements in the specified collection at the end of this deque, as if by calling `addLast(E)` on each one, in the order that they are returned by the 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.
  `ArrayDeque<E>`
  `clone()`
  Returns a copy of 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.
  `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 deque contains no elements.
  `Iterator<E>`
  `iterator()`
  Returns an iterator over the elements in this deque.
  `boolean`
  `offer(E e)`
  Inserts the specified element at the end 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, 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.
  `E`
  `remove()`
  Retrieves and removes the head of the queue represented by this deque.
  `boolean`
  `remove(Object o)`
  Removes a single instance 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 in this deque (when traversing the deque from head to tail).
  `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 in this deque (when traversing the deque from head to tail).
  `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()`
  Creates a late-binding and fail-fast `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.

  ### Methods declared in class java.util.AbstractCollection

  `containsAll, toString`

  ### Methods declared in class java.lang.Object

  `equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

  ### Methods declared in interface java.util.Collection

  `containsAll, equals, hashCode, parallelStream, stream, toArray`

  ### Methods declared in interface java.util.Deque

  `reversed`

- ## Constructor Details

  - ### ArrayDeque

    public ArrayDeque()
    Constructs an empty array deque with an initial capacity sufficient to hold 16 elements.

  - ### ArrayDeque

    public ArrayDeque(int numElements)
    Constructs an empty array deque with an initial capacity sufficient to hold the specified number of elements.
    Parameters:
      `numElements` - lower bound on initial capacity of the deque

  - ### ArrayDeque

    public ArrayDeque(Collection<? extends E> c)
    Constructs a deque containing the elements of the specified collection, in the order they are returned by the collection's iterator. (The first element returned by the collection's iterator becomes the first element, or front of the deque.)
    Parameters:
      `c` - the collection whose elements are to be placed into the deque
    Throws:
      `NullPointerException` - if the specified collection is null

- ## Method Details

  - ### addFirst

    public void addFirst(E e)
    Inserts the specified element at the front of this deque.
    Specified by:
      `addFirst` in interface `Deque<E>`
    Specified by:
      `addFirst` in interface `SequencedCollection<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.

    This method is equivalent to `add(E)`.

    Specified by:
      `addLast` in interface `Deque<E>`
    Specified by:
      `addLast` in interface `SequencedCollection<E>`
    Parameters:
      `e` - the element to add
    Throws:
      `NullPointerException` - if the specified element is null

  - ### addAll

    public boolean addAll(Collection<? extends E> c)
    Adds all of the elements in the specified collection at the end of this deque, as if by calling `addLast(E)` on each one, in the order that they are returned by the collection's iterator.
    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
    See Also:
      - `AbstractCollection.add(Object)`

  - ### offerFirst

    public boolean offerFirst(E e)
    Inserts the specified element at the front of this deque.
    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.
    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

  - ### 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>`
    Specified by:
      `removeFirst` in interface `SequencedCollection<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>`
    Specified by:
      `removeLast` in interface `SequencedCollection<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

  - ### 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>`
    Specified by:
      `getFirst` in interface `SequencedCollection<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>`
    Specified by:
      `getLast` in interface `SequencedCollection<E>`
    Returns:
      the tail of this deque
    Throws:
      `NoSuchElementException` - if this deque is empty

  - ### 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

  - ### removeFirstOccurrence

    public boolean removeFirstOccurrence(Object o)
    Removes the first occurrence of the specified element in this deque (when traversing the deque from head to tail). 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

  - ### removeLastOccurrence

    public boolean removeLastOccurrence(Object o)
    Removes the last occurrence of the specified element in this deque (when traversing the deque from head to tail). 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

  - ### add

    public boolean add(E e)
    Inserts the specified element at the end of this deque.

    This method is equivalent to `addLast(E)`.

    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` - the element to add
    Returns:
      `true` (as specified by `Collection.add(E)`)
    Throws:
      `NullPointerException` - if the specified element is null

  - ### offer

    public boolean offer(E e)
    Inserts the specified element at the end of this deque.

    This method is equivalent to `offerLast(E)`.

    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

  - ### remove

    public E remove()
    Retrieves and removes the head of the queue represented by 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()`.

    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

  - ### poll

    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()`.

    Specified by:
      `poll` in interface `Deque<E>`
    Specified by:
      `poll` 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()
    Retrieves, but does not remove, the head of the queue represented by 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()`.

    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

  - ### peek

    public E peek()
    Retrieves, but does not remove, the head of the queue represented by this deque, or returns `null` if this deque is empty.

    This method is equivalent to `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

  - ### push

    public void push(E e)
    Pushes an element onto the stack represented by this deque. In other words, inserts the element at the front of this deque.

    This method is equivalent to `addFirst(E)`.

    Specified by:
      `push` in interface `Deque<E>`
    Parameters:
      `e` - the element to push
    Throws:
      `NullPointerException` - if the specified element is null

  - ### pop

    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()`.

    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

  - ### size

    public int size()
    Returns the number of elements in this deque.
    Specified by:
      `size` in interface `Collection<E>`
    Specified by:
      `size` in interface `Deque<E>`
    Returns:
      the number of elements in this deque

  - ### isEmpty

    public boolean isEmpty()
    Returns `true` if this deque contains no elements.
    Specified by:
      `isEmpty` in interface `Collection<E>`
    Overrides:
      `isEmpty` in class `AbstractCollection<E>`
    Returns:
      `true` if this deque contains no elements

  - ### iterator

    public Iterator<E> iterator()
    Returns an iterator over the elements in this deque. The elements will be ordered from first (head) to last (tail). This is the same order that elements would be dequeued (via successive calls to `remove()` or popped (via successive calls to `pop()`).
    Specified by:
      `iterator` in interface `Collection<E>`
    Specified by:
      `iterator` in interface `Deque<E>`
    Specified by:
      `iterator` in interface `Iterable<E>`
    Specified by:
      `iterator` in class `AbstractCollection<E>`
    Returns:
      an iterator over the elements in this deque

  - ### descendingIterator

    public Iterator<E> descendingIterator()
    Description copied from interface: `Deque`
    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).
    Specified by:
      `descendingIterator` in interface `Deque<E>`
    Returns:
      an iterator over the elements in this deque in reverse sequence

  - ### spliterator

    public Spliterator<E> spliterator()
    Creates a late-binding and fail-fast `Spliterator` over the elements in this deque.

    The `Spliterator` reports `Spliterator.SIZED`, `Spliterator.SUBSIZED`, `Spliterator.ORDERED`, and `Spliterator.NONNULL`. Overriding implementations should document the reporting of additional characteristic values.

    Specified by:
      `spliterator` in interface `Collection<E>`
    Specified by:
      `spliterator` in interface `Iterable<E>`
    Returns:
      a `Spliterator` over the elements in this deque
    Since:
      1.8

  - ### 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

  - ### 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)`

  - ### 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` - object to be checked for containment in this deque
    Returns:
      `true` if this deque contains the specified element

  - ### remove

    public boolean remove(Object o)
    Removes a single instance 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 this deque contained the specified element

  - ### clear

    public void clear()
    Removes all of the elements from this deque. The deque will be empty after this call returns.
    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 component 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

  - ### clone

    public ArrayDeque<E> clone()
    Returns a copy of this deque.
    Overrides:
      `clone` in class `Object`
    Returns:
      a copy of this deque
    See Also:
      - `Cloneable`