# Deque

public interface **Deque** implements [Queue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html)\<E\>

|---|---|---|
| Known Indirect Subclasses [ArrayDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/ArrayDeque.html)\<E\>, [BlockingDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/BlockingDeque.html)\<E\>, [ConcurrentLinkedDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedDeque.html)\<E\>, [LinkedBlockingDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/LinkedBlockingDeque.html)\<E\>, [LinkedList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/LinkedList.html)\<E\> |---|---| | [ArrayDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/ArrayDeque.html)\<E\> | Resizable-array implementation of the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html` interface. | | [BlockingDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/BlockingDeque.html)\<E\> | A `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html` 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/ConcurrentLinkedDeque.html)\<E\> | An unbounded concurrent {@linkplain Deque deque} based on linked nodes. | | [LinkedBlockingDeque](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/LinkedBlockingDeque.html)\<E\> | An optionally-bounded {@linkplain BlockingDeque blocking deque} based on linked nodes. | | [LinkedList](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/LinkedList.html)\<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:

|---|---|---|---|---|
|   | **First Element (Head)** || **Last Element (Tail)** ||
|   | *Throws exception* | *Special value* | *Throws exception* | *Special value* |
| **Insert** | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#addFirst(E)` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offerFirst(E)` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#addLast(E)` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offerLast(E)` |
| **Remove** | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeFirst()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#pollFirst()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeLast()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#pollLast()` |
| **Examine** | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#getFirst()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peekFirst()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#getLast()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peekLast()` |
[Summary of Deque methods]

This interface extends the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html` 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** |
| `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#add(E)` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#addLast(E)` |
| `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#offer(E)` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offerLast(E)` |
| `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#remove()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeFirst()` |
| `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#poll()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#pollFirst()` |
| `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#element()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#getFirst()` |
| `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#peek()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peek()` |
[Comparison of Queue and Deque methods]

Deques can also be used as LIFO (Last-In-First-Out) stacks. This
interface should be used in preference to the legacy `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Stack.html` 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** |
| `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#push(E)` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#addFirst(E)` |
| `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#pop()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeFirst()` |
| `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peek()` | `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peekFirst()` |
[Comparison of Stack and Deque methods]

Note that the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#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, `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeFirstOccurrence(java.lang.Object)` and
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeLastOccurrence(java.lang.Object)`.

Unlike the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/List.html` 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#add(E))(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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#addFirst(E))(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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#addLast(E))(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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#contains(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Returns `true` if this deque contains the specified element. |
| abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [descendingIterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#descendingIterator())() Returns an iterator over the elements in this deque in reverse sequential order. |
| abstract E | [element](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#getFirst())() Retrieves, but does not remove, the first element of this deque. |
| abstract E | [getLast](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#getLast())() Retrieves, but does not remove, the last element of this deque. |
| abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#iterator())() Returns an iterator over the elements in this deque in proper sequence. |
| abstract boolean | [offer](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offer(E))(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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offerFirst(E))(E e) Inserts the specified element at the front of this deque unless it would violate capacity restrictions. |
| abstract boolean | [offerLast](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offerLast(E))(E e) Inserts the specified element at the end of this deque unless it would violate capacity restrictions. |
| abstract E | [peek](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peekFirst())() Retrieves, but does not remove, the first element of this deque, or returns `null` if this deque is empty. |
| abstract E | [peekLast](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peekLast())() Retrieves, but does not remove, the last element of this deque, or returns `null` if this deque is empty. |
| abstract E | [poll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#pollFirst())() Retrieves and removes the first element of this deque, or returns `null` if this deque is empty. |
| abstract E | [pollLast](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#pollLast())() Retrieves and removes the last element of this deque, or returns `null` if this deque is empty. |
| abstract E | [pop](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#pop())() Pops an element from the stack represented by this deque. |
| abstract void | [push](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#push(E))(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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#remove(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Removes the first occurrence of the specified element from this deque. |
| abstract E | [removeFirst](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeFirst())() Retrieves and removes the first element of this deque. |
| abstract boolean | [removeFirstOccurrence](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeFirstOccurrence(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Removes the first occurrence of the specified element from this deque. |
| abstract E | [removeLast](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeLast())() Retrieves and removes the last element of this deque. |
| abstract boolean | [removeLastOccurrence](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeLastOccurrence(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Removes the last occurrence of the specified element from this deque. |
| abstract int | [size](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#size())() Returns the number of elements in this deque. |

### Inherited Method Summary

From interface [java.util.Queue](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html)

|---|---|
| abstract boolean | [add](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#add(E))(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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#element())() Retrieves, but does not remove, the head of this queue. |
| abstract boolean | [offer](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#offer(E))(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. |
| abstract E | [peek](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#peek())() Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty. |
| abstract E | [poll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#poll())() Retrieves and removes the head of this queue, or returns `null` if this queue is empty. |
| abstract E | [remove](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Queue.html#remove())() Retrieves and removes the head of this queue. |

From interface [java.util.Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)

|---|---|
| abstract boolean | [add](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#add(E))(E e) Ensures that this collection contains the specified element (optional operation). |
| abstract boolean | [addAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#addAll(java.util.Collection<?%20extends%20E>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<? extends E\> c) Adds all of the elements in the specified collection to this collection (optional operation). |
| abstract void | [clear](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#clear())() Removes all of the elements from this collection (optional operation). |
| abstract boolean | [contains](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#contains(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Returns true if this collection contains the specified element. |
| abstract boolean | [containsAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#containsAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> c) Returns true if this collection contains all of the elements in the specified collection. |
| abstract boolean | [equals](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#equals(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Compares the specified object with this collection for equality. |
| abstract int | [hashCode](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#hashCode())() Returns the hash code value for this collection. |
| abstract boolean | [isEmpty](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#isEmpty())() Returns true if this collection contains no elements. |
| abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#iterator())() Returns an iterator over the elements in this collection. |
| [Stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/stream/Stream.html)\<E\> | [parallelStream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#parallelStream())() Returns a possibly parallel `Stream` with this collection as its source. |
| abstract boolean | [remove](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#remove(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) o) Removes a single instance of the specified element from this collection, if it is present (optional operation). |
| abstract boolean | [removeAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#removeAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation). |
| boolean | [removeIf](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#removeIf(java.util.function.Predicate<?%20super%20E>))([Predicate](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/function/Predicate.html)\<? super E\> filter) Removes all of the elements of this collection that satisfy the given predicate. |
| abstract boolean | [retainAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#retainAll(java.util.Collection<?>))([Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<?\> c) Retains only the elements in this collection that are contained in the specified collection (optional operation). |
| abstract int | [size](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#size())() Returns the number of elements in this collection. |
| [Spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html)\<E\> | [spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#spliterator())() Creates a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html` over the elements in this collection. |
| [Stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/stream/Stream.html)\<E\> | [stream](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#stream())() Returns a sequential `Stream` with this collection as its source. |
| abstract \<T\> T\[\] | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#toArray(T[]))(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\[\]](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) | [toArray](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#toArray())() Returns an array containing all of the elements in this collection. |

From interface [java.lang.Iterable](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html)

|---|---|
| void | [forEach](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html#forEach(java.util.function.Consumer<?%20super%20T>))([Consumer](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/function/Consumer.html)\<? 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<E\> | [iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html#iterator())() Returns an iterator over elements of type `T`. |
| [Spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html)\<E\> | [spliterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Iterable.html#spliterator())() Creates a `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Spliterator.html` 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offer(E)`.

This method is equivalent to `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#addLast(E)`.

##### Parameters

| e | the element to add |
|---|---|

##### Returns

- `true` (as specified by `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html#add(E)`)

##### Throws

| [IllegalStateException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalStateException.html) | if the element cannot be added at this time due to capacity restrictions |
| [ClassCastException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element prevents it from being added to this deque |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offerFirst(E)`.

##### Parameters

| e | the element to add |
|---|---|

##### Throws

| [IllegalStateException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalStateException.html) | if the element cannot be added at this time due to capacity restrictions |
| [ClassCastException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element prevents it from being added to this deque |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offerLast(E)`.

This method is equivalent to `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#add(E)`.

##### Parameters

| e | the element to add |
|---|---|

##### Throws

| [IllegalStateException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalStateException.html) | if the element cannot be added at this time due to capacity restrictions |
| [ClassCastException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element prevents it from being added to this deque |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if some property of the specified element prevents it from being added to this deque |
|---|---|

#### public abstract boolean
**contains**
([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element is incompatible with this deque ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
|---|---|

#### public abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peek()` only in that it throws an
exception if this deque is empty.

This method is equivalent to `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#getFirst()`.

##### Returns

- the head of the queue represented by this deque

##### Throws

| [NoSuchElementException](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NoSuchElementException.html) | if this deque is empty |
|---|---|

#### public abstract E
**getFirst**
()

Retrieves, but does not remove, the first element of this deque.

This method differs from `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peekFirst()` only in that it
throws an exception if this deque is empty.

##### Returns

- the head of this deque

##### Throws

| [NoSuchElementException](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NoSuchElementException.html) | if this deque is empty |
|---|---|

#### public abstract E
**getLast**
()

Retrieves, but does not remove, the last element of this deque.
This method differs from `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#peekLast()` only in that it
throws an exception if this deque is empty.

##### Returns

- the tail of this deque

##### Throws

| [NoSuchElementException](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NoSuchElementException.html) | if this deque is empty |
|---|---|

#### public abstract [Iterator](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Iterator.html)\<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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#add(E)` method, which can fail to
insert an element only by throwing an exception.

This method is equivalent to `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#offerLast(E)`.

##### Parameters

| e | the element to add |
|---|---|

##### Returns

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

##### Throws

| [ClassCastException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element prevents it from being added to this deque |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element prevents it from being added to this deque |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element prevents it from being added to this deque |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeFirst()`.

##### Returns

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

##### Throws

| [NoSuchElementException](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NoSuchElementException.html) | 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#addFirst(E)`.

##### Parameters

| e | the element to push |
|---|---|

##### Throws

| [IllegalStateException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalStateException.html) | if the element cannot be added at this time due to capacity restrictions |
| [ClassCastException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element prevents it from being added to this deque |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#poll()` only in that it throws an
exception if this deque is empty.

This method is equivalent to `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeFirst()`.

##### Returns

- the head of the queue represented by this deque

##### Throws

| [NoSuchElementException](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NoSuchElementException.html) | if this deque is empty |
|---|---|

#### public abstract boolean
**remove**
([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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 `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#removeFirstOccurrence(java.lang.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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element is incompatible with this deque ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
|---|---|

#### public abstract E
**removeFirst**
()

Retrieves and removes the first element of this deque. This method
differs from `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#pollFirst()` only in that it throws an
exception if this deque is empty.

##### Returns

- the head of this deque

##### Throws

| [NoSuchElementException](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NoSuchElementException.html) | if this deque is empty |
|---|---|

#### public abstract boolean
**removeFirstOccurrence**
([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element is incompatible with this deque ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
|---|---|

#### public abstract E
**removeLast**
()

Retrieves and removes the last element of this deque. This method
differs from `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Deque.html#pollLast()` only in that it throws an
exception if this deque is empty.

##### Returns

- the tail of this deque

##### Throws

| [NoSuchElementException](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/NoSuchElementException.html) | if this deque is empty |
|---|---|

#### public abstract boolean
**removeLastOccurrence**
([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) 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](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/ClassCastException.html) | if the class of the specified element is incompatible with this deque ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
| [NullPointerException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/NullPointerException.html) | if the specified element is null and this deque does not permit null elements ([optional](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection#optional-restrictions)) |
|---|---|

#### public abstract int
**size**
()

Returns the number of elements in this deque.

##### Returns

- the number of elements in this deque