Interface BlockingDeque<E>

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

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

All Known Implementing Classes:: LinkedBlockingDeque

public interface BlockingDeque<E> extends BlockingQueue<E>, Deque<E>

A Deque 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.

BlockingDeque methods come in four forms, with different ways of handling operations that cannot be satisfied immediately, but may be satisfied at some point in the future: one throws an exception, the second returns a special value (either null or false, depending on the operation), the third blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up. These methods are summarized in the following table:

Summary of BlockingDeque methods
First Element (Head)
	Throws exception	Special value	Blocks	Times out
Insert	`addFirst(e)`	`offerFirst(e)`	`putFirst(e)`	`offerFirst(e, time, unit)`
Remove	`removeFirst()`	`pollFirst()`	`takeFirst()`	`pollFirst(time, unit)`
Examine	`getFirst()`	`peekFirst()`	not applicable	not applicable
Last Element (Tail)
	Throws exception	Special value	Blocks	Times out
Insert	`addLast(e)`	`offerLast(e)`	`putLast(e)`	`offerLast(e, time, unit)`
Remove	`removeLast()`	`pollLast()`	`takeLast()`	`pollLast(time, unit)`
Examine	`getLast()`	`peekLast()`	not applicable	not applicable

Like any BlockingQueue, a BlockingDeque is thread safe, does not permit null elements, and may (or may not) be capacity-constrained.

A BlockingDeque implementation may be used directly as a FIFO BlockingQueue. The methods inherited from the BlockingQueue interface are precisely equivalent to BlockingDeque methods as indicated in the following table:

Comparison of BlockingQueue and BlockingDeque methods
	`BlockingQueue` Method	Equivalent `BlockingDeque` Method
Insert	`add(e)`	`addLast(e)`
	`offer(e)`	`offerLast(e)`
	`put(e)`	`putLast(e)`
	`offer(e, time, unit)`	`offerLast(e, time, unit)`
Remove	`remove()`	`removeFirst()`
	`poll()`	`pollFirst()`
	`take()`	`takeFirst()`
	`poll(time, unit)`	`pollFirst(time, unit)`
Examine	`element()`	`getFirst()`
Examine	`peek()`	`peekFirst()`

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a BlockingDeque happen-before actions subsequent to the access or removal of that element from the BlockingDeque in another thread.

This interface is a member of the Java Collections Framework.

Since:: 1.6

Method Summary

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.
`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.
`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).
`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`	`offer(E e, long timeout, TimeUnit unit)`	Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque), waiting up to the specified wait time if necessary for space to become available.
`boolean`	`offerFirst(E e)`	Inserts the specified element at the front 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, long timeout, TimeUnit unit)`	Inserts the specified element at the front of this deque, waiting up to the specified wait time if necessary for space to become available.
`boolean`	`offerLast(E e)`	Inserts the specified element at the end 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`	`offerLast(E e, long timeout, TimeUnit unit)`	Inserts the specified element at the end of this deque, waiting up to the specified wait time if necessary for space to become available.
`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`	`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`	`poll(long timeout, TimeUnit unit)`	Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), waiting up to the specified wait time if necessary for an element to become available.
`E`	`pollFirst(long timeout, TimeUnit unit)`	Retrieves and removes the first element of this deque, waiting up to the specified wait time if necessary for an element to become available.
`E`	`pollLast(long timeout, TimeUnit unit)`	Retrieves and removes the last element of this deque, waiting up to the specified wait time if necessary for an element to become available.
`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.
`void`	`put(E e)`	Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque), waiting if necessary for space to become available.
`void`	`putFirst(E e)`	Inserts the specified element at the front of this deque, waiting if necessary for space to become available.
`void`	`putLast(E e)`	Inserts the specified element at the end of this deque, waiting if necessary for space to become 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.
`boolean`	`removeFirstOccurrence(Object o)`	Removes the first occurrence of the specified element from 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.
`E`	`take()`	Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), waiting if necessary until an element becomes available.
`E`	`takeFirst()`	Retrieves and removes the first element of this deque, waiting if necessary until an element becomes available.
`E`	`takeLast()`	Retrieves and removes the last element of this deque, waiting if necessary until an element becomes available.

Methods declared in interface java.util.concurrent.BlockingQueue

drainTo, drainTo, remainingCapacity

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.util.Deque

addAll, descendingIterator, getFirst, getLast, peekFirst, peekLast, pollFirst, pollLast, pop, removeFirst, removeLast

Methods declared in interface java.lang.Iterable

forEach

Method Details

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 an IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use offerFirst.

Specified by:: addFirst in interface Deque<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; 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 an IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use offerLast.

Specified by:: addLast in interface Deque<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; 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 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 addFirst method, which can fail to insert an element only by throwing an exception.

Specified by:: offerFirst in interface Deque<E>
Parameters:: e - the element to add
Returns:: true if the element was added to this deque, else false
Throws:: ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null; 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 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 addLast method, which can fail to insert an element only by throwing an exception.

Specified by:: offerLast in interface Deque<E>
Parameters:: e - the element to add
Returns:: true if the element was added to this deque, else false
Throws:: ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

putFirst

void putFirst(E e) throws InterruptedException

Inserts the specified element at the front of this deque, waiting if necessary for space to become available.

Parameters:: e - the element to add
Throws:: InterruptedException - if interrupted while waiting; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

putLast

void putLast(E e) throws InterruptedException

Inserts the specified element at the end of this deque, waiting if necessary for space to become available.

Parameters:: e - the element to add
Throws:: InterruptedException - if interrupted while waiting; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

offerFirst

boolean offerFirst(E e, long timeout, TimeUnit unit) throws InterruptedException

Inserts the specified element at the front of this deque, waiting up to the specified wait time if necessary for space to become available.

Parameters:: e - the element to add; timeout - how long to wait before giving up, in units of unit; unit - a TimeUnit determining how to interpret the timeout parameter
Returns:: true if successful, or false if the specified waiting time elapses before space is available
Throws:: InterruptedException - if interrupted while waiting; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

offerLast

boolean offerLast(E e, long timeout, TimeUnit unit) throws InterruptedException

Inserts the specified element at the end of this deque, waiting up to the specified wait time if necessary for space to become available.

Parameters:: e - the element to add; timeout - how long to wait before giving up, in units of unit; unit - a TimeUnit determining how to interpret the timeout parameter
Returns:: true if successful, or false if the specified waiting time elapses before space is available
Throws:: InterruptedException - if interrupted while waiting; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

takeFirst

E takeFirst() throws InterruptedException

Retrieves and removes the first element of this deque, waiting if necessary until an element becomes available.

Returns:: the head of this deque
Throws:: InterruptedException - if interrupted while waiting

takeLast

E takeLast() throws InterruptedException

Retrieves and removes the last element of this deque, waiting if necessary until an element becomes available.

Returns:: the tail of this deque
Throws:: InterruptedException - if interrupted while waiting

pollFirst

E pollFirst(long timeout, TimeUnit unit) throws InterruptedException

Retrieves and removes the first element of this deque, waiting up to the specified wait time if necessary for an element to become available.

Parameters:: timeout - how long to wait before giving up, in units of unit; unit - a TimeUnit determining how to interpret the timeout parameter
Returns:: the head of this deque, or null if the specified waiting time elapses before an element is available
Throws:: InterruptedException - if interrupted while waiting

pollLast

E pollLast(long timeout, TimeUnit unit) throws InterruptedException

Retrieves and removes the last element of this deque, waiting up to the specified wait time if necessary for an element to become available.

Parameters:: timeout - how long to wait before giving up, in units of unit; unit - a TimeUnit determining how to interpret the timeout parameter
Returns:: the tail of this deque, or null if the specified waiting time elapses before an element is available
Throws:: InterruptedException - if interrupted while waiting

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 element e such that o.equals(e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).

Specified by:: removeFirstOccurrence in interface Deque<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 (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 element e such that o.equals(e) (if such an element exists). Returns true if this deque contained the specified element (or equivalently, if this deque changed as a result of the call).

Specified by:: removeLastOccurrence in interface Deque<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 (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, returning true upon success and throwing an IllegalStateException if no space is currently available. When using a capacity-restricted deque, it is generally preferable to use offer.

This method is equivalent to addLast.

Specified by:: add in interface BlockingQueue<E>
Specified by:: add in interface Collection<E>
Specified by:: add in interface Deque<E>
Specified by:: add in interface Queue<E>
Parameters:: e - the element to add
Returns:: true (as specified by Collection.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; 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, returning true upon success and false if no space is currently available. When using a capacity-restricted deque, this method is generally preferable to the add(E) method, which can fail to insert an element only by throwing an exception.

This method is equivalent to offerLast.

Specified by:: offer in interface BlockingQueue<E>
Specified by:: offer in interface Deque<E>
Specified by:: offer in interface Queue<E>
Parameters:: e - the element to add
Returns:: true if the element was added to this queue, else false
Throws:: ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

put

void put(E e) throws InterruptedException

Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque), waiting if necessary for space to become available.

This method is equivalent to putLast.

Specified by:: put in interface BlockingQueue<E>
Parameters:: e - the element to add
Throws:: InterruptedException - if interrupted while waiting; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

offer

boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException

Inserts the specified element into the queue represented by this deque (in other words, at the tail of this deque), waiting up to the specified wait time if necessary for space to become available.

This method is equivalent to offerLast.

Specified by:: offer in interface BlockingQueue<E>
Parameters:: e - the element to add; timeout - how long to wait before giving up, in units of unit; unit - a TimeUnit determining how to interpret the timeout parameter
Returns:: true if the element was added to this deque, else false
Throws:: InterruptedException - if interrupted while waiting; ClassCastException - if the class of the specified element prevents it from being added to this deque; NullPointerException - if the specified element is null; 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 from poll() only in that it throws an exception if this deque is empty.

This method is equivalent to removeFirst.

Specified by:: remove in interface Deque<E>
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 returns null if this deque is empty.

This method is equivalent to Deque.pollFirst().

Specified by:: poll in interface Deque<E>
Specified by:: poll in interface Queue<E>
Returns:: the head of this deque, or null if this deque is empty

take

E take() throws InterruptedException

Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), waiting if necessary until an element becomes available.

This method is equivalent to takeFirst.

Specified by:: take in interface BlockingQueue<E>
Returns:: the head of this deque
Throws:: InterruptedException - if interrupted while waiting

poll

E poll(long timeout, TimeUnit unit) throws InterruptedException

Retrieves and removes the head of the queue represented by this deque (in other words, the first element of this deque), waiting up to the specified wait time if necessary for an element to become available.

This method is equivalent to pollFirst.

Specified by:: poll in interface BlockingQueue<E>
Parameters:: timeout - how long to wait before giving up, in units of unit; unit - a TimeUnit determining how to interpret the timeout parameter
Returns:: the head of this deque, or null if the specified waiting time elapses before an element is available
Throws:: InterruptedException - if interrupted while waiting

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 from peek only in that it throws an exception if this deque is empty.

This method is equivalent to getFirst.

Specified by:: element in interface Deque<E>
Specified by:: element in interface Queue<E>
Returns:: the head of 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 returns null if this deque is empty.

This method is equivalent to peekFirst.

Specified by:: peek in interface Deque<E>
Specified by:: peek in interface Queue<E>
Returns:: the head of this deque, or null if this deque is empty

remove

boolean remove(Object o)

This method is equivalent to removeFirstOccurrence.

Specified by:: remove in interface BlockingQueue<E>
Specified by:: remove in interface Collection<E>
Specified by:: remove in interface Deque<E>
Parameters:: o - element to be removed from this deque, if present
Returns:: true if this deque changed as a result of the call
Throws:: ClassCastException - if the class of the specified element is incompatible with this deque (optional); NullPointerException - if the specified element is null (optional)

contains

boolean contains(Object o)

Returns true if this deque contains the specified element. More formally, returns true if and only if this deque contains at least one element e such that o.equals(e).

Specified by:: contains in interface BlockingQueue<E>
Specified by:: contains in interface Collection<E>
Specified by:: contains in interface Deque<E>
Parameters:: o - object to be checked for containment in this deque
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 (optional)

size

int size()

Returns the number of elements in this deque.

Specified by:: size in interface Collection<E>
Specified by:: size in interface Deque<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 Deque<E>
Specified by:: iterator in interface Iterable<E>
Returns:: an iterator over the elements in this deque in proper sequence

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 an IllegalStateException if no space is currently available.

This method is equivalent to addFirst.

Specified by:: push in interface Deque<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; IllegalArgumentException - if some property of the specified element prevents it from being added to this deque

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https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/concurrent/BlockingDeque.html