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Added in API level 9

Deque

public interface Deque
implements Queue<E>

java.util.Deque<E>
Known Indirect Subclasses


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:

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:

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:

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.

Summary

Public methods

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.

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.

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, returning true upon success and 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 methods

From interface java.util.Queue
From interface java.util.Collection
From interface java.lang.Iterable

Public methods

add

Added in API level 11
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 E: the element to add
Returns
boolean true (as specified by 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

addFirst

Added in API level 9
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. When using a capacity-restricted deque, it is generally preferable to use method offerFirst(E).

Parameters
e 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

Added in API level 9
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. When using a capacity-restricted deque, it is generally preferable to use method offerLast(E).

This method is equivalent to add(E).

Parameters
e 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

contains

Added in API level 11
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==null ? e==null : o.equals(e)).

Parameters
o Object: element whose presence in this deque is to be tested
Returns
boolean true if this deque contains the specified element
Throws
ClassCastException if the type 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)

descendingIterator

Added in API level 9
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
Iterator<E> an iterator over the elements in this deque in reverse sequence

element

Added in API level 11
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
E the head of the queue represented by this deque
Throws
NoSuchElementException if this deque is empty

getFirst

Added in API level 9
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
E the head of this deque
Throws
NoSuchElementException if this deque is empty

getLast

Added in API level 9
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
E the tail of this deque
Throws
NoSuchElementException if this deque is empty

iterator

Added in API level 11
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
Iterator<E> an iterator over the elements in this deque in proper sequence

offer

Added in API level 11
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 E: the element to add
Returns
boolean 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

offerFirst

Added in API level 9
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 E: the element to add
Returns
boolean 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

Added in API level 9
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 E: the element to add
Returns
boolean 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

peek

Added in API level 11
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
E the head of the queue represented by this deque, or null if this deque is empty

peekFirst

Added in API level 9
E peekFirst ()

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

Returns
E the head of this deque, or null if this deque is empty

peekLast

Added in API level 9
E peekLast ()

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

Returns
E the tail of this deque, or null if this deque is empty

poll

Added in API level 11
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
E the first element of this deque, or null if this deque is empty

pollFirst

Added in API level 9
E pollFirst ()

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

Returns
E the head of this deque, or null if this deque is empty

pollLast

Added in API level 9
E pollLast ()

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

Returns
E the tail of this deque, or null if this deque is empty

pop

Added in API level 9
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
E 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

push

Added in API level 9
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, returning true upon success and throwing an IllegalStateException if no space is currently available.

This method is equivalent to addFirst(E).

Parameters
e 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

remove

Added in API level 11
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
E the head of the queue represented by this deque
Throws
NoSuchElementException if this deque is empty

remove

Added in API level 11
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 (o==null ? e==null : 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 Object: element to be removed from this deque, if present
Returns
boolean 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)

removeFirst

Added in API level 9
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
E the head of this deque
Throws
NoSuchElementException if this deque is empty

removeFirstOccurrence

Added in API level 9
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 (o==null ? e==null : 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 Object: element to be removed from this deque, if present
Returns
boolean 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)

removeLast

Added in API level 9
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
E the tail of this deque
Throws
NoSuchElementException if this deque is empty

removeLastOccurrence

Added in API level 9
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 (o==null ? e==null : 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 Object: element to be removed from this deque, if present
Returns
boolean 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)

size

Added in API level 11
int size ()

Returns the number of elements in this deque.

Returns
int the number of elements in this deque
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