ArrayDeque

public class ArrayDeque 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 and Iterator interfaces.

Public Constructor Summary

	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.

Public Method Summary

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

Inherited Method Summary

From class java.util.AbstractCollection

boolean	add(E e) Ensures that this collection contains the specified element (optional operation). This implementation always throws an UnsupportedOperationException.
boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
void	clear() Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the Iterator.remove operation.
boolean	contains(Object o) Returns true if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
boolean	containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
boolean	isEmpty() Returns true if this collection contains no elements. This implementation returns size() == 0.
abstract Iterator<E>	iterator() Returns an iterator over the elements contained in this collection.
boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
abstract int	size() Returns the number of elements in this collection.
<T> T[]	toArray(T[] contents) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index 0.
Object[]	toArray() Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index 0.
String	toString() Returns a string representation of this collection.

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.util.Collection

abstract boolean	add(E e) Ensures that this collection contains the specified element (optional operation).
abstract boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).
abstract void	clear() Removes all of the elements from this collection (optional operation).
abstract boolean	contains(Object o) Returns true if this collection contains the specified element.
abstract boolean	containsAll(Collection<?> c) Returns true if this collection contains all of the elements in the specified collection.
abstract boolean	equals(Object o) Compares the specified object with this collection for equality.
abstract int	hashCode() Returns the hash code value for this collection.
abstract boolean	isEmpty() Returns true if this collection contains no elements.
abstract Iterator<E>	iterator() Returns an iterator over the elements in this collection.
Stream<E>	parallelStream() Returns a possibly parallel Stream with this collection as its source.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
abstract boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).
boolean	removeIf(Predicate<? super E> filter) Removes all of the elements of this collection that satisfy the given predicate.
abstract boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
Spliterator<E>	spliterator() Creates a Spliterator over the elements in this collection.
Stream<E>	stream() Returns a sequential Stream with this collection as its source.
abstract <T> T[]	toArray(T[] a) Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
abstract Object[]	toArray() Returns an array containing all of the elements in this collection.

From interface java.util.Deque

abstract boolean	add(E e) Inserts the specified element into the queue represented by this deque (in other words, at the tail 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.
abstract void	addFirst(E e) Inserts the specified element at the front of this deque if it is possible to do so immediately without violating capacity restrictions, throwing an IllegalStateException if no space is currently available.
abstract void	addLast(E e) Inserts the specified element at the end of this deque if it is possible to do so immediately without violating capacity restrictions, throwing an IllegalStateException if no space is currently available.
abstract boolean	contains(Object o) Returns true if this deque contains the specified element.
abstract Iterator<E>	descendingIterator() Returns an iterator over the elements in this deque in reverse sequential order.
abstract 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).
abstract E	getFirst() Retrieves, but does not remove, the first element of this deque.
abstract E	getLast() Retrieves, but does not remove, the last element of this deque.
abstract Iterator<E>	iterator() Returns an iterator over the elements in this deque in proper sequence.
abstract boolean	offer(E e) Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque) if it is possible to do so immediately without violating capacity restrictions, returning true upon success and false if no space is currently available.
abstract boolean	offerFirst(E e) Inserts the specified element at the front of this deque unless it would violate capacity restrictions.
abstract boolean	offerLast(E e) Inserts the specified element at the end of this deque unless it would violate capacity restrictions.
abstract 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.
abstract E	peekFirst() Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.
abstract E	peekLast() Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.
abstract 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.
abstract E	pollFirst() Retrieves and removes the first element of this deque, or returns null if this deque is empty.
abstract E	pollLast() Retrieves and removes the last element of this deque, or returns null if this deque is empty.
abstract E	pop() Pops an element from the stack represented by this deque.
abstract 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.
abstract E	remove() Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).
abstract boolean	remove(Object o) Removes the first occurrence of the specified element from this deque.
abstract E	removeFirst() Retrieves and removes the first element of this deque.
abstract boolean	removeFirstOccurrence(Object o) Removes the first occurrence of the specified element from this deque.
abstract E	removeLast() Retrieves and removes the last element of this deque.
abstract boolean	removeLastOccurrence(Object o) Removes the last occurrence of the specified element from this deque.
abstract int	size() Returns the number of elements in this deque.

From interface java.lang.Iterable

void	forEach(Consumer<? super T> action) Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
abstract Iterator<E>	iterator() Returns an iterator over elements of type T.
Spliterator<E>	spliterator() Creates a Spliterator over the elements described by this Iterable.

From interface java.util.Queue

abstract boolean	add(E e) Inserts the specified element into this queue 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.
abstract E	element() Retrieves, but does not remove, the head of this queue.
abstract boolean	offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
abstract E	peek() Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
abstract E	poll() Retrieves and removes the head of this queue, or returns null if this queue is empty.
abstract E	remove() Retrieves and removes the head of this queue.

Public Constructors

public ArrayDeque ()

Constructs an empty array deque with an initial capacity sufficient to hold 16 elements.

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

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

Public Methods

public boolean add (E e)

Inserts the specified element at the end of this deque.

This method is equivalent to addLast(E).

Parameters

e	the element to add

Returns

• true (as specified by Collection.add(E))

Throws

NullPointerException	if the specified element is null

public void addFirst (E e)

Inserts the specified element at the front of this deque.

Parameters

e	the element to add

Throws

NullPointerException	if the specified element is null

public void addLast (E e)

Inserts the specified element at the end of this deque.

This method is equivalent to add(E).

Parameters

e	the element to add

Throws

NullPointerException	if the specified element is null

public void clear ()

Removes all of the elements from this deque. The deque will be empty after this call returns.

public ArrayDeque<E> clone ()

Returns a copy of this deque.

Returns

• a copy of this deque

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).

Parameters

o	object to be checked for containment in this deque

Returns

• true if this deque contains the specified element

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).

Returns

• an iterator over the elements in this deque in reverse sequence

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().

Returns

• the head of the queue represented by this deque

Throws

NoSuchElementException

public E getFirst ()

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.

Returns

• the head of this deque

Throws

NoSuchElementException

public E getLast ()

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.

Returns

• the tail of this deque

Throws

NoSuchElementException

public boolean isEmpty ()

Returns true if this deque contains no elements.

Returns

• true if this deque contains no elements

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()).

Returns

• an iterator over the elements in this deque

public boolean offer (E e)

Inserts the specified element at the end of this deque.

This method is equivalent to offerLast(E).

Parameters

e	the element to add

Returns

• true (as specified by Queue.offer(E))

Throws

NullPointerException	if the specified element is null

public boolean offerFirst (E e)

Inserts the specified element at the front of this deque.

Parameters

e	the element to add

Returns

• true (as specified by Deque.offerFirst(E))

Throws

NullPointerException	if the specified element is null

public boolean offerLast (E e)

Inserts the specified element at the end of this deque.

Parameters

e	the element to add

Returns

• true (as specified by Deque.offerLast(E))

Throws

NullPointerException	if the specified element is null

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().

Returns

• the head of the queue represented by this deque, or null if this deque is empty

public E peekFirst ()

Retrieves, but does not remove, the first element of this deque, or returns null if this deque is empty.

Returns

• the head of this deque, or null if this deque is empty

public E peekLast ()

Retrieves, but does not remove, the last element of this deque, or returns null if this deque is empty.

Returns

• the tail of this deque, or null if this deque is empty

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().

Returns

• the head of the queue represented by this deque, or null if this deque is empty

public E pollFirst ()

Retrieves and removes the first element of this deque, or returns null if this deque is empty.

Returns

• the head of this deque, or null if this deque is empty

public E pollLast ()

Retrieves and removes the last element of this deque, or returns null if this deque is empty.

Returns

• the tail of this deque, or null if this deque is empty

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().

Returns

• the element at the front of this deque (which is the top of the stack represented by this deque)

Throws

NoSuchElementException

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).

Parameters

e	the element to push

Throws

NullPointerException	if the specified element is null

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().

Returns

• the head of the queue represented by this deque

Throws

NoSuchElementException

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).

Parameters

o	element to be removed from this deque, if present

Returns

• true if this deque contained the specified element

public E removeFirst ()

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.

Returns

• the head of this deque

Throws

NoSuchElementException

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).

Parameters

o	element to be removed from this deque, if present

Returns

• true if the deque contained the specified element

public E removeLast ()

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.

Returns

• the tail of this deque

Throws

NoSuchElementException

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).

Parameters

o	element to be removed from this deque, if present

Returns

• true if the deque contained the specified element

public int size ()

Returns the number of elements in this deque.

Returns

• the number of elements in this deque

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.

Returns

• a Spliterator over the elements in this deque

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.

Returns

• an array containing all of the elements in this deque

public 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().

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