Deque  |  API reference  |  Android Developers (original) (raw)

interface Deque<E : Any!> : Queue, SequencedCollection

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

This interface extends the [Queue](/reference/kotlin/java/util/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

Deques can also be used as LIFO (Last-In-First-Out) stacks. This interface should be used in preference to the legacy [Stack](/reference/kotlin/java/util/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

Note that the [peek](#peek%28%29) 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](#removeFirstOccurrence%28kotlin.Any%29) and [removeLastOccurrence](#removeLastOccurrence%28kotlin.Any%29).

Unlike the [List](/reference/kotlin/java/util/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.

Summary

Public methods

add

abstract fun add(element: E): Boolean

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](#offer%28java.util.Deque.E%29).

This method is equivalent to [addLast](#addLast%28java.util.Deque.E%29).

addAll

abstract fun addAll(elements: Collection): Boolean

Adds all of the elements in the specified collection at the end of this deque, as if by calling [addLast](#addLast%28java.util.Deque.E%29) 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](#offer%28java.util.Deque.E%29) 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.

addFirst

abstract fun addFirst(e: E): Unit

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](#offerFirst%28java.util.Deque.E%29).

addLast

abstract fun addLast(e: E): Unit

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](#offerLast%28java.util.Deque.E%29).

This method is equivalent to [add](#add%28java.util.Deque.E%29).

contains

abstract fun contains(element: E?): Boolean

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).

descendingIterator

abstract fun descendingIterator(): MutableIterator

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).

element

abstract fun element(): E

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](#peek%28%29) only in that it throws an exception if this deque is empty.

This method is equivalent to [getFirst()](#getFirst%28%29).

getFirst

abstract fun getFirst(): E

Retrieves, but does not remove, the first element of this deque. This method differs from [peekFirst](#peekFirst%28%29) only in that it throws an exception if this deque is empty.

getLast

abstract fun getLast(): E

Retrieves, but does not remove, the last element of this deque. This method differs from [peekLast](#peekLast%28%29) only in that it throws an exception if this deque is empty.

iterator

abstract fun iterator(): MutableIterator

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).

offer

abstract fun offer(e: E): Boolean

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](#add%28java.util.Deque.E%29) method, which can fail to insert an element only by throwing an exception.

This method is equivalent to [offerLast](#offerLast%28java.util.Deque.E%29).

offerFirst

abstract fun offerFirst(e: E): Boolean

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](#addFirst%28java.util.Deque.E%29) method, which can fail to insert an element only by throwing an exception.

offerLast

abstract fun offerLast(e: E): Boolean

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](#addLast%28java.util.Deque.E%29) method, which can fail to insert an element only by throwing an exception.

peek

abstract fun peek(): E?

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()](#peekFirst%28%29).

peekFirst

abstract fun peekFirst(): E?

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

peekLast

abstract fun peekLast(): E?

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

poll

abstract fun poll(): E?

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()](#pollFirst%28%29).

pollFirst

abstract fun pollFirst(): E?

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

pollLast

abstract fun pollLast(): E?

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

pop

abstract fun pop(): E

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()](#removeFirst%28%29).

push

abstract fun push(e: E): Unit

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](#addFirst%28java.util.Deque.E%29).

remove

abstract fun remove(): E

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()](#poll%28%29) only in that it throws an exception if this deque is empty.

This method is equivalent to [removeFirst()](#removeFirst%28%29).

remove

abstract fun remove(element: E?): Boolean

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(java.lang.Object)](#removeFirstOccurrence%28kotlin.Any%29).

removeFirst

abstract fun removeFirst(): E

Retrieves and removes the first element of this deque. This method differs from [pollFirst](#pollFirst%28%29) only in that it throws an exception if this deque is empty.

removeFirstOccurrence

abstract fun removeFirstOccurrence(o: Any?): Boolean

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).

removeLast

abstract fun removeLast(): E

Retrieves and removes the last element of this deque. This method differs from [pollLast](#pollLast%28%29) only in that it throws an exception if this deque is empty.

removeLastOccurrence

abstract fun removeLastOccurrence(o: Any?): Boolean

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).

reversed

open fun reversed(): Deque

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.

Properties

size

abstract val size: Int

Returns the number of elements in this deque.