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 (optional operation).
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 (optional operation). 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