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Interface Deque<E>

Type Parameters:: E - the type of elements held in this deque

All Superinterfaces:: Collection<E>,Iterable<E>,Queue<E>

All Known Subinterfaces:: BlockingDeque<E>

All Known Implementing Classes:: ArrayDeque,ConcurrentLinkedDeque,LinkedBlockingDeque,LinkedList

public interfaceDeque<E>extendsQueue<E>

A linear collection that supports element insertion and removal at both ends. The namedeque is short for "double ended queue" and is usually pronounced "deck". MostDeque 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 (eithernull orfalse, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restrictedDeque implementations; in most implementations, insert operations cannot fail.

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

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

This interface extends theQueue 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 theQueue interface are precisely equivalent toDeque methods as indicated in the following table:

Comparison of Queue and Deque methods
`Queue` Method	Equivalent`Deque` Method
`add(e)`	`addLast(e)`
`offer(e)`	`offerLast(e)`
`remove()`	`removeFirst()`
`poll()`	`pollFirst()`
`element()`	`getFirst()`
`peek()`	`peekFirst()`

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

Comparison of Stack and Deque methods
Stack Method	Equivalent`Deque` Method
`push(e)`	`addFirst(e)`
`pop()`	`removeFirst()`
`peek()`	`getFirst()`

Note that thepeek 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 andremoveLastOccurrence.

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

WhileDeque implementations are not strictly required to prohibit the insertion of null elements, they are strongly encouraged to do so. Users of anyDeque implementations that do allow null elements are strongly encouragednot to take advantage of the ability to insert nulls. This is so becausenull 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 theequals andhashCode methods, but instead inherit the identity-based versions from classObject.

This interface is a member of the Java Collections Framework.

Since:: 1.6

Method Summary

All Methods Instance Methods Abstract 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<? extendsE> 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.
`int`	`size()`	Returns the number of elements in this deque.

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

Methods declared in interface java.lang.Iterable
forEach

- Method Detail
  - 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 anIllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use methodofferFirst(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 anIllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use methodofferLast(E).
    This method is equivalent toadd(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 theaddFirst(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, elsefalse
    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 theaddLast(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, elsefalse
    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 frompollFirst only in that it throws an exception if this deque is empty.
    Returns:
    the head of this deque
    Throws:
    NoSuchElementException - if this deque is empty
  - removeLast
```
E removeLast()
```
    Retrieves and removes the last element of this deque. This method differs frompollLast only in that it throws an exception if this deque is empty.
    Returns:
    the tail of this deque
    Throws:
    NoSuchElementException - if this deque is empty
  - pollFirst
```
E pollFirst()
```
    Retrieves and removes the first element of this deque, or returnsnull if this deque is empty.
    Returns:
    the head of this deque, ornull if this deque is empty
  - pollLast
```
E pollLast()
```
    Retrieves and removes the last element of this deque, or returnsnull if this deque is empty.
    Returns:
    the tail of this deque, ornull if this deque is empty
  - getFirst
```
E getFirst()
```
    Retrieves, but does not remove, the first element of this deque. This method differs frompeekFirst only in that it throws an exception if this deque is empty.
    Returns:
    the head of this deque
    Throws:
    NoSuchElementException - if this deque is empty
  - getLast
```
E getLast()
```
    Retrieves, but does not remove, the last element of this deque. This method differs frompeekLast only in that it throws an exception if this deque is empty.
    Returns:
    the tail of this deque
    Throws:
    NoSuchElementException - if this deque is empty
  - peekFirst
```
E peekFirst()
```
    Retrieves, but does not remove, the first element of this deque, or returnsnull if this deque is empty.
    Returns:
    the head of this deque, ornull if this deque is empty
  - peekLast
```
E peekLast()
```
    Retrieves, but does not remove, the last element of this deque, or returnsnull if this deque is empty.
    Returns:
    the tail of this deque, ornull 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 elemente such thatObjects.equals(o, e) (if such an element exists). Returnstrue 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 elemente such thatObjects.equals(o, e) (if such an element exists). Returnstrue 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, returningtrue upon success and throwing anIllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to useoffer.
    This method is equivalent toaddLast(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 byCollection.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, returningtrue upon success andfalse if no space is currently available. When using a capacity-restricted deque, this method is generally preferable to theadd(E) method, which can fail to insert an element only by throwing an exception.
    This method is equivalent toofferLast(E).
    Specified by:
    offer in interface Queue<E>
    Parameters:
    e - the element to add
    Returns:
    true if the element was added to this deque, elsefalse
    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 frompoll() only in that it throws an exception if this deque is empty.
    This method is equivalent toremoveFirst().
    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 returnsnull if this deque is empty.
    This method is equivalent topollFirst().
    Specified by:
    poll in interface Queue<E>
    Returns:
    the first element of this deque, ornull 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 frompeek only in that it throws an exception if this deque is empty.
    This method is equivalent togetFirst().
    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 returnsnull if this deque is empty.
    This method is equivalent topeekFirst().
    Specified by:
    peek in interface Queue<E>
    Returns:
    the head of the queue represented by this deque, ornull if this deque is empty
  - addAll
```
boolean addAll(Collection<? extendsE> c)
```
    Adds all of the elements in the specified collection at the end of this deque, as if by callingaddLast(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 calloffer 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 anIllegalStateException if no space is currently available.
    This method is equivalent toaddFirst(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 toremoveFirst().
    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 elemente such thatObjects.equals(o, e) (if such an element exists). Returnstrue if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).
    This method is equivalent toremoveFirstOccurrence(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)
```
    Returnstrue if this deque contains the specified element. More formally, returnstrue if and only if this deque contains at least one elemente such thatObjects.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

Movatterモバイル変換

Interface Deque<E>

Method Summary

Methods declared in interface java.util.Collection

Methods declared in interface java.lang.Iterable

Method Detail

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