Deque

public interface Deque implements Queue<E>

Known Indirect Subclasses ArrayDeque<E>, BlockingDeque<E>, ConcurrentLinkedDeque<E>, LinkedBlockingDeque<E>, LinkedList<E> ArrayDeque<E> Resizable-array implementation of the Deque interface.  BlockingDeque<E> A Deque that additionally supports blocking operations that wait for the deque to become non-empty when retrieving an element, and wait for space to become available in the deque when storing an element.  ConcurrentLinkedDeque<E> An unbounded concurrent {@linkplain Deque deque} based on linked nodes.  LinkedBlockingDeque<E> An optionally-bounded {@linkplain BlockingDeque blocking deque} based on linked nodes.  LinkedList<E> Doubly-linked list implementation of the List and Deque interfaces.

A linear collection that supports element insertion and removal at both ends. The name deque is short for "double ended queue" and is usually pronounced "deck". Most Deque implementations place no fixed limits on the number of elements they may contain, but this interface supports capacity-restricted deques as well as those with no fixed size limit.

This interface defines methods to access the elements at both ends of the deque. Methods are provided to insert, remove, and examine the element. Each of these methods exists in two forms: one throws an exception if the operation fails, the other returns a special value (either null or false, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restricted Deque implementations; in most implementations, insert operations cannot fail.

The twelve methods described above are summarized in the following table:

Summary of Deque methods

	First Element (Head)		Last Element (Tail)
	Throws exception	Special value	Throws exception	Special value
Insert	addFirst(e)	offerFirst(e)	addLast(e)	offerLast(e)
Remove	removeFirst()	pollFirst()	removeLast()	pollLast()
Examine	getFirst()	peekFirst()	getLast()	peekLast()

This interface extends the Queue interface. When a deque is used as a queue, FIFO (First-In-First-Out) behavior results. Elements are added at the end of the deque and removed from the beginning. The methods inherited from the Queue interface are precisely equivalent to Deque methods as indicated in the following table:

Comparison of Queue and Deque methods

Queue Method	Equivalent Deque Method
add(e)	addLast(e)
offer(e)	offerLast(e)
remove()	removeFirst()
poll()	pollFirst()
element()	getFirst()
peek()	peekFirst()

Deques can also be used as LIFO (Last-In-First-Out) stacks. This interface should be used in preference to the legacy Stack class. When a deque is used as a stack, elements are pushed and popped from the beginning of the deque. Stack methods are precisely equivalent to Deque methods as indicated in the table below:

Comparison of Stack and Deque methods

Stack Method	Equivalent Deque Method
push(e)	addFirst(e)
pop()	removeFirst()
peek()	peekFirst()

Note that the peek method works equally well when a deque is used as a queue or a stack; in either case, elements are drawn from the beginning of the deque.

This interface provides two methods to remove interior elements, removeFirstOccurrence and removeLastOccurrence.

Unlike the List interface, this interface does not provide support for indexed access to elements.

While Deque implementations are not strictly required to prohibit the insertion of null elements, they are strongly encouraged to do so. Users of any Deque implementations that do allow null elements are strongly encouraged not to take advantage of the ability to insert nulls. This is so because null is used as a special return value by various methods to indicated that the deque is empty.

Deque implementations generally do not define element-based versions of the equals and hashCode methods, but instead inherit the identity-based versions from class Object.

Public Method Summary

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.

Inherited Method Summary

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.

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.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.

Public Methods

public 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. When using a capacity-restricted deque, it is generally preferable to use offer.

This method is equivalent to addLast(E).

Parameters

e	the element to add

Returns

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

Throws

IllegalStateException	if the element cannot be added at this time due to capacity restrictions
ClassCastException	if the class of the specified element prevents it from being added to this deque
NullPointerException	if the specified element is null and this deque does not permit null elements
IllegalArgumentException	if some property of the specified element prevents it from being added to this deque

public 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. When using a capacity-restricted deque, it is generally preferable to use method offerFirst(E).

Parameters

e	the element to add

Throws

IllegalStateException	if the element cannot be added at this time due to capacity restrictions
ClassCastException	if the class of the specified element prevents it from being added to this deque
NullPointerException	if the specified element is null and this deque does not permit null elements
IllegalArgumentException	if some property of the specified element prevents it from being added to this deque

public 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. When using a capacity-restricted deque, it is generally preferable to use method offerLast(E).

This method is equivalent to add(E).

Parameters

e	the element to add

Throws

IllegalStateException	if the element cannot be added at this time due to capacity restrictions
ClassCastException	if the class of the specified element prevents it from being added to this deque
NullPointerException	if the specified element is null and this deque does not permit null elements
IllegalArgumentException	if some property of the specified element prevents it from being added to this deque

public abstract 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 Objects.equals(o, e).

Parameters

o	element whose presence in this deque is to be tested

Returns

• true if this deque contains the specified element

Throws

ClassCastException	if the class of the specified element is incompatible with this deque (optional)
NullPointerException	if the specified element is null and this deque does not permit null elements (optional)

public abstract 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 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). 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	if this deque is empty

public abstract 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	if this deque is empty

public abstract 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	if this deque is empty

public abstract Iterator<E> iterator ()

Returns an iterator over the elements in this deque in proper sequence. The elements will be returned in order from first (head) to last (tail).

Returns

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

public 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. When using a capacity-restricted deque, this method is generally preferable to the add(E) method, which can fail to insert an element only by throwing an exception.

This method is equivalent to offerLast(E).

Parameters

e	the element to add

Returns

• true if the element was added to this deque, else false

Throws

ClassCastException	if the class of the specified element prevents it from being added to this deque
NullPointerException	if the specified element is null and this deque does not permit null elements
IllegalArgumentException	if some property of the specified element prevents it from being added to this deque

public abstract boolean offerFirst (E e)

Inserts the specified element at the front of this deque unless it would violate capacity restrictions. When using a capacity-restricted deque, this method is generally preferable to the addFirst(E) method, which can fail to insert an element only by throwing an exception.

Parameters

e	the element to add

Returns

• true if the element was added to this deque, else false

Throws

ClassCastException	if the class of the specified element prevents it from being added to this deque
NullPointerException	if the specified element is null and this deque does not permit null elements
IllegalArgumentException	if some property of the specified element prevents it from being added to this deque

public abstract boolean offerLast (E e)

Inserts the specified element at the end of this deque unless it would violate capacity restrictions. When using a capacity-restricted deque, this method is generally preferable to the addLast(E) method, which can fail to insert an element only by throwing an exception.

Parameters

e	the element to add

Returns

• true if the element was added to this deque, else false

Throws

ClassCastException	if the class of the specified element prevents it from being added to this deque
NullPointerException	if the specified element is null and this deque does not permit null elements
IllegalArgumentException	if some property of the specified element prevents it from being added to this deque

public 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.

This method is equivalent to peekFirst().

Returns

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

public abstract 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 abstract 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 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.

This method is equivalent to pollFirst().

Returns

• the first element of this deque, or null if this deque is empty

public abstract 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 abstract 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 abstract 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	if this deque is empty

public 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.

This method is equivalent to addFirst(E).

Parameters

e	the element to push

Throws

IllegalStateException	if the element cannot be added at this time due to capacity restrictions
ClassCastException	if the class of the specified element prevents it from being added to this deque
NullPointerException	if the specified element is null and this deque does not permit null elements
IllegalArgumentException	if some property of the specified element prevents it from being added to this deque

public abstract E remove ()

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 removeFirst().

Returns

• the head of the queue represented by this deque

Throws

NoSuchElementException	if this deque is empty

public abstract 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 Objects.equals(o, 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 an element was removed as a result of this call

Throws

ClassCastException	if the class of the specified element is incompatible with this deque (optional)
NullPointerException	if the specified element is null and this deque does not permit null elements (optional)

public abstract 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	if this deque is empty

public abstract 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 Objects.equals(o, 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 an element was removed as a result of this call

Throws

ClassCastException	if the class of the specified element is incompatible with this deque (optional)
NullPointerException	if the specified element is null and this deque does not permit null elements (optional)

public abstract 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	if this deque is empty

public abstract 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 Objects.equals(o, 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 an element was removed as a result of this call

Throws

ClassCastException	if the class of the specified element is incompatible with this deque (optional)
NullPointerException	if the specified element is null and this deque does not permit null elements (optional)

public abstract int size ()

Returns the number of elements in this deque.

Returns

• the number of elements in this deque