com.mongodb

Interface Deque<E>

Type Parameters:

E - the type of elements held in this collection

All Superinterfaces:

java.util.Collection<E>, java.lang.Iterable<E>, java.util.Queue<E>

All Known Implementing Classes:

ConcurrentLinkedDeque

interface Deque<E>
extends java.util.Queue<E>

A linear collection that supports element insertion and removal at both ends. The name deque is short for "double ended queue" and is usually pronounced "deck". Most Deque implementations place no fixed limits on the number of elements they may contain, but this interface supports capacity-restricted deques as well as those with no fixed size limit.

This interface defines methods to access the elements at both ends of the deque. Methods are provided to insert, remove, and examine the element. Each of these methods exists in two forms: one throws an exception if the operation fails, the other returns a special value (either null or false, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restricted Deque implementations; in most implementations, insert operations cannot fail.

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

	First Element (Head)		Last Element (Tail)
	Throws exception	Returns special value	Throws exception	Returns 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 to 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
offer(e)	offerLast(e)
add(e)	addLast(e)
poll()	pollFirst()
remove()	removeFirst()
peek()	peekFirst()
element()	getFirst()

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 dequeue 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.

This interface is a member of the Java Collections Framework.

Since:

1.6

Method Summary

Modifier and Type	Method and Description
boolean	add(E e) Inserts the specified element into the queue represented by this deque unless it would violate capacity restrictions.
void	addFirst(E e) Inserts the specified element to the front of this deque unless it would violate capacity restrictions.
void	addLast(E e) Inserts the specified element to the end of this deque unless it would violate capacity restrictions.
E	element() Retrieves, but does not remove, the head of the queue represented by 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.
java.util.Iterator<E>	iterator() Returns an iterator over the elements in this deque.
boolean	offer(E e) Inserts the specified element into the queue represented by this deque unless it would violate capacity restrictions.
boolean	offerFirst(E e) Inserts the specified element to the front this deque unless it would violate capacity restrictions.
boolean	offerLast(E e) Inserts the specified element to 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, returning null if this deque is empty.
E	peekFirst() Retrieves, but does not remove, the first element of this deque, returning null if this deque is empty.
E	peekLast() Retrieves, but does not remove, the last element of this deque, returning null if this deque is empty.
E	poll() Retrieves and removes the head of the queue represented by this deque, or null if this deque is empty.
E	pollFirst() Retrieves and removes the first element of this deque, or null if this deque is empty.
E	pollLast() Retrieves and removes the last element of this deque, or 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.
E	remove() Retrieves and removes the head of the queue represented by this deque.
E	removeFirst() Removes and returns the first element of this deque.
boolean	removeFirstOccurrence(java.lang.Object e) Removes the first occurrence of the specified element in this deque.
E	removeLast() Retrieves and removes the last element of this deque.
boolean	removeLastOccurrence(java.lang.Object e) Removes the last occurrence of the specified element in this deque.

Methods inherited from interface java.util.Collection

addAll, clear, contains, containsAll, equals, hashCode, isEmpty, parallelStream, remove, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray

Methods inherited from interface java.lang.Iterable

forEach

Method Detail

offerFirst

boolean offerFirst(E e)

Inserts the specified element to the front this deque unless it would violate capacity restrictions. When using a capacity-restricted deque, this method is generally preferable to method addFirst, which can fail to insert an element only by throwing an exception.

Parameters:

e - the element to insert

Returns:

true if it was possible to insert the element, else false

Throws:

java.lang.NullPointerException - if e is null and this deque does not permit null elements

offerLast

boolean offerLast(E e)

Inserts the specified element to the end of this deque unless it would violate capacity restrictions. When using a capacity-restricted deque, this method is generally preferable to method addLast which can fail to insert an element only by throwing an exception.

Parameters:

e - the element to insert

Returns:

true if it was possible to insert the element, else false

Throws:

java.lang.NullPointerException - if e is null and this deque does not permit null elements

addFirst

void addFirst(E e)

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

Parameters:

e - the element to insert

Throws:

java.lang.IllegalStateException - if it was not possible to insert the element due to capacity restrictions

java.lang.NullPointerException - if e is null and this deque does not permit null elements

addLast

void addLast(E e)

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

Parameters:

e - the element to insert

Throws:

java.lang.IllegalStateException - if it was not possible to insert the element due to capacity restrictions

java.lang.NullPointerException - if e is null and this deque does not permit null elements

pollFirst

E pollFirst()

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

Returns:

the first element of this deque, or null if this deque is empty

pollLast

E pollLast()

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

Returns:

the last element of this deque, or null if this deque is empty

removeFirst

E removeFirst()

Removes and returns the first element of this deque. This method differs from the pollFirst method only in that it throws an exception if this deque is empty.

Returns:

the first element of this deque

Throws:

java.util.NoSuchElementException - if this deque is empty

removeLast

E removeLast()

Retrieves and removes the last element of this deque. This method differs from the pollLast method only in that it throws an exception if this deque is empty.

Returns:

the last element of this deque

Throws:

java.util.NoSuchElementException - if this deque is empty

peekFirst

E peekFirst()

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

Returns:

the first element of this deque, or null if this deque is empty

peekLast

E peekLast()

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

Returns:

the last element of this deque, or null if this deque is empty

getFirst

E getFirst()

Retrieves, but does not remove, the first element of this deque. This method differs from the peek method only in that it throws an exception if this deque is empty.

Returns:

the first element of this deque

Throws:

java.util.NoSuchElementException - if this deque is empty

getLast

E getLast()

Retrieves, but does not remove, the last element of this deque. This method differs from the peek method only in that it throws an exception if this deque is empty.

Returns:

the last element of this deque

Throws:

java.util.NoSuchElementException - if this deque is empty

removeFirstOccurrence

boolean removeFirstOccurrence(java.lang.Object e)

Removes the first occurrence of the specified element in 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).

Parameters:

e - element to be removed from this deque, if present

Returns:

true if the deque contained the specified element

Throws:

java.lang.NullPointerException - if the specified element is null

removeLastOccurrence

boolean removeLastOccurrence(java.lang.Object e)

Removes the last occurrence of the specified element in 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).

Parameters:

e - element to be removed from this deque, if present

Returns:

true if the deque contained the specified element

Throws:

java.lang.NullPointerException - if the specified element is null

offer

boolean offer(E e)

Inserts the specified element into the queue represented by this deque unless it would violate capacity restrictions. In other words, inserts the specified element to the end of this deque. When using a capacity-restricted deque, this method is generally preferable to the add(E) method, which can fail to insert an element only by throwing an exception.

This method is equivalent to offerLast(E).

Specified by:

offer in interface java.util.Queue<E>

Parameters:

e - the element to insert

Returns:

true if it was possible to insert the element, else false

Throws:

java.lang.NullPointerException - if e is null and this deque does not permit null elements

add

boolean add(E e)

Inserts the specified element into the queue represented by this deque unless it would violate capacity restrictions. In other words, inserts the specified element as the last element of this deque.

This method is equivalent to addLast(E).

Specified by:

add in interface java.util.Collection<E>

Specified by:

add in interface java.util.Queue<E>

Parameters:

e - the element to insert

Returns:

true (as per the spec for Collection.add(E))

Throws:

java.lang.IllegalStateException - if it was not possible to insert the element due to capacity restrictions

java.lang.NullPointerException - if e is null and this deque does not permit null elements

poll

E poll()

Retrieves and removes the head of the queue represented by this deque, or null if this deque is empty. In other words, retrieves and removes the first element of this deque, or null if this deque is empty.

This method is equivalent to pollFirst().

Specified by:

poll in interface java.util.Queue<E>

Returns:

the first element of this deque, or null if this deque is empty

remove

E remove()

Retrieves and removes the head of the queue represented by this deque. This method differs from the poll method only in that it throws an exception if this deque is empty.

This method is equivalent to removeFirst().

Specified by:

remove in interface java.util.Queue<E>

Returns:

the head of the queue represented by this deque

Throws:

java.util.NoSuchElementException - if this deque is empty

peek

E peek()

Retrieves, but does not remove, the head of the queue represented by this deque, returning null if this deque is empty.

This method is equivalent to peekFirst().

Specified by:

peek in interface java.util.Queue<E>

Returns:

the head of the queue represented by this deque, or null if this deque is empty

element

E element()

Retrieves, but does not remove, the head of the queue represented by this deque. This method differs from the peek method only in that it throws an exception if this deque is empty.

This method is equivalent to getFirst().

Specified by:

element in interface java.util.Queue<E>

Returns:

the head of the queue represented by this deque

Throws:

java.util.NoSuchElementException - if this deque is empty

push

void push(E e)

Pushes an element onto the stack represented by this deque. In other words, inserts the element to the front this deque unless it would violate capacity restrictions.

This method is equivalent to addFirst(E).

Throws:

java.lang.IllegalStateException - if it was not possible to insert the element due to capacity restrictions

java.lang.NullPointerException - if e is null and this deque does not permit null elements

pop

E pop()

Pops an element from the stack represented by this deque. In other words, removes and returns the first element of this deque.

This method is equivalent to removeFirst().

Returns:

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

Throws:

java.util.NoSuchElementException - if this deque is empty

iterator

java.util.Iterator<E> iterator()

Returns an iterator over the elements in this deque. The elements will be ordered from first (head) to last (tail).

Specified by:

iterator in interface java.util.Collection<E>

Specified by:

iterator in interface java.lang.Iterable<E>

Returns:

an Iterator over the elements in this deque