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Deque (Java SE 26 & JDK 26)

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Hide sidebar Show sidebar Type Parameters: E - the type of elements held in this deque All Superinterfaces: Collection<E>, Iterable<E>, Queue<E>, SequencedCollection<E> All Known Subinterfaces: BlockingDeque<E> All Known Implementing Classes: ArrayDeque, ConcurrentLinkedDeque, LinkedBlockingDeque, LinkedList

public interface Deque<E> extends Queue<E>, SequencedCollection<E>

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 Remove Examine



addFirst(e)	offerFirst(e)	addLast(e)	offerLast(e)
removeFirst()	pollFirst()	removeLast()	pollLast()
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)offer(e)remove()poll()element()peek()

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 equivalent to Deque methods as indicated in the table below:

Comparison of Stack and Deque methods Stack Method Equivalent Deque Method push(e)pop()peek()

addFirst(e)

removeFirst()

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 indicate 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.

This interface is a member of the Java Collections Framework.

Since: 1.6

Method Summary

All MethodsInstance MethodsAbstract MethodsDefault Methods

Modifier and Type

Method

Description

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.

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 if it is possible to do so immediately without violating capacity restrictions, throwing an IllegalStateException if no space is currently available.

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.

boolean

contains(Object o)

Returns true if this deque contains the specified element.

Iterator<E>

descendingIterator()

Returns an iterator over the elements in this deque in reverse sequential order.

E

element()

Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque).

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.

Iterator<E>

iterator()

Returns an iterator over the elements in this deque in proper sequence.

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.

boolean

offerFirst(E e)

Inserts the specified element at the front of this deque unless it would violate capacity restrictions.

boolean

offerLast(E e)

Inserts the specified element at the end of this deque unless it would violate capacity restrictions.

E

peek()

Retrieves, but does not remove, the head of the queue represented by this deque (in other words, the first element of this deque), or returns null if this deque is empty.

E

peekFirst()

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

E

peekLast()

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

E

poll()

Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), or returns null if this deque is empty.

E

pollFirst()

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

E

pollLast()

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

E

pop()

Pops an element from the stack represented by this deque.

void

push(E e)

Pushes an element onto the stack represented by this deque (in other words, at the head of this deque) if it is possible to do so immediately without violating capacity restrictions, throwing an IllegalStateException if no space is currently available.

E

remove()

Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque).

boolean

remove(Object o)

Removes the first occurrence of the specified element from this deque.

E

removeFirst()

Retrieves and removes the first element of this deque.

boolean

removeFirstOccurrence(Object o)

Removes the first occurrence of the specified element from this deque.

E

removeLast()

Retrieves and removes the last element of this deque.

boolean

removeLastOccurrence(Object o)

Removes the last occurrence of the specified element from this deque.

default Deque<E>

reversed()

Returns a reverse-ordered view of this collection.

int

size()

Returns the number of elements in this deque.

Methods declared in interface Collection
clear, containsAll, equals, hashCode, isEmpty, parallelStream, removeAll, removeIf, retainAll, spliterator, stream, toArray, toArray, toArray

Modifier and Type

Method

Description

void

clear()

Removes all of the elements from this collection (optional operation).

boolean

containsAll(Collection<?> c)

Returns true if this collection contains all of the elements in the specified collection.

boolean

equals(Object o)

Compares the specified object with this collection for equality.

int

hashCode()

Returns the hash code value for this collection.

boolean

isEmpty()

Returns true if this collection contains no elements.

default Stream<E>

parallelStream()

Returns a possibly parallel Stream with this collection as its source.

boolean

removeAll(Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection (optional operation).

default boolean

removeIf(Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate (optional operation).

boolean

retainAll(Collection<?> c)

Retains only the elements in this collection that are contained in the specified collection (optional operation).

default Spliterator<E>

spliterator()

Creates a Spliterator over the elements in this collection.

default Stream<E>

stream()

Returns a sequential Stream with this collection as its source.

Object[]

toArray()

Returns an array containing all of the elements in this collection.

default <T> T[]

toArray(IntFunction<T[]> generator)

Returns an array containing all of the elements in this collection, using the provided generator function to allocate the returned array.

<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.

Methods declared in interface Iterable
forEach

Modifier and Type

Method

Description

default 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.

Method Details
- addFirst
  
  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).
  Specified by: addFirst in interface SequencedCollection<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
- addLast
  
  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).
  Specified by: addLast in interface SequencedCollection<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
- offerFirst
  
  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
- offerLast
  
  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
- removeFirst
  
  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.
  Specified by: removeFirst in interface SequencedCollection<E> Returns: the head of this deque Throws: NoSuchElementException - if this deque is empty
- removeLast
  
  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.
  Specified by: removeLast in interface SequencedCollection<E> Returns: the tail of this deque Throws: NoSuchElementException - if this deque is empty
- pollFirst
  
  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
- pollLast
  
  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
- getFirst
  
  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.
  Specified by: getFirst in interface SequencedCollection<E> Returns: the head of this deque Throws: NoSuchElementException - if this deque is empty
- getLast
  
  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.
  Specified by: getLast in interface SequencedCollection<E> Returns: the tail of this deque Throws: NoSuchElementException - if this deque is empty
- peekFirst
  
  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
- peekLast
  
  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
- removeFirstOccurrence
  
  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)
- removeLastOccurrence
  
  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)
- add
  
  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).
  Specified by: add in interface Collection<E> Specified by: add in interface Queue<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
- offer
  
  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).
  Specified by: offer in interface Queue<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
- remove
  
  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().
  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
  
  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 Queue<E> Returns: the first element of this deque, or null if this deque is empty
- element
  
  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().
  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
  
  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().
  Specified by: peek in interface Queue<E> Returns: the head of the queue represented by this deque, or null if this deque is empty
- addAll
  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.
  When using a capacity-restricted deque, it is generally preferable to call offer separately on each element.
  An exception encountered while trying to add an element may result in only some of the elements having been successfully added when the associated exception is thrown.
  Specified by: addAll in interface Collection<E> Parameters: c - the elements to be inserted into this deque Returns: true if this deque changed as a result of the call Throws: IllegalStateException - if not all the elements can be added at this time due to insertion restrictions ClassCastException - if the class of an element of the specified collection prevents it from being added to this deque NullPointerException - if the specified collection contains a null element and this deque does not permit null elements, or if the specified collection is null IllegalArgumentException - if some property of an element of the specified collection prevents it from being added to this deque See Also:
  - Collection.add(Object)
- push
  
  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
- pop
  
  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
- remove
  
  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).
  Specified by: remove in interface Collection<E> 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)
- contains
  
  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).
  Specified by: contains in interface Collection<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)
- size
  
  int size()
  
  Returns the number of elements in this deque.
  Specified by: size in interface Collection<E> Returns: the number of elements in this deque
- iterator
  
  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).
  Specified by: iterator in interface Collection<E> Specified by: iterator in interface Iterable<E> Returns: an iterator over the elements in this deque in proper sequence
- descendingIterator
  
  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
- reversed
  
  default Deque<E> reversed()
  
  Returns a reverse-ordered view of this collection. The encounter order of elements in the returned view is the inverse of the encounter order of elements in this collection. The reverse ordering affects all order-sensitive operations, including those on the view collections of the returned view. If the collection implementation permits modifications to this view, the modifications "write through" to the underlying collection. Changes to the underlying collection might or might not be visible in this reversed view, depending upon the implementation.
  Specified by: reversed in interface SequencedCollection<E> Implementation Requirements: The implementation in this interface returns a reverse-ordered Deque view. The reversed() method of the view returns a reference to this Deque. Other operations on the view are implemented via calls to public methods on this Deque. The exact relationship between calls on the view and calls on this Deque is unspecified. However, order-sensitive operations generally behave as if they delegate to the appropriate method with the opposite orientation. For example, calling getFirst on the view might result in a call to getLast on this Deque. Returns: a reverse-ordered view of this collection, as a Deque Since: 21

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Interface Deque<E>

Method Summary

Methods declared in interface Collection

Methods declared in interface Iterable

Method Details

addFirst

addLast

offerFirst

offerLast

removeFirst

removeLast

pollFirst

pollLast

getFirst

getLast

peekFirst

peekLast

removeFirstOccurrence

removeLastOccurrence

add

offer

remove

poll

element

peek

addAll

push

pop

remove

contains

size

iterator

descendingIterator

reversed