Queue

AI-generated Key Takeaways

• The Queue interface in Java represents a collection designed for holding elements prior to processing, typically following a FIFO (first-in-first-out) structure but allowing for variations like priority or LIFO queues.

• It extends the Collection interface and provides methods for insertion (add, offer), extraction (remove, poll), and inspection (element, peek) of elements.

• Two forms of each method are available: one throws an exception upon failure (e.g., removing from an empty queue), while the other returns a special value (like null or false).

• Queue doesn't define blocking queue methods; those are found in the BlockingQueue interface which extends this one, offering features for concurrent programming.

• Implementations generally disallow null element insertion as it's used as a special return value by poll indicating an empty queue.

public interface Queue implements Collection<E>

Known Indirect Subclasses AbstractQueue<E>, ArrayBlockingQueue<E>, ArrayDeque<E>, BlockingDeque<E>, BlockingQueue<E>, ConcurrentLinkedDeque<E>, ConcurrentLinkedQueue<E>, DelayQueue<E extends Delayed>, Deque<E>, LinkedBlockingDeque<E>, LinkedBlockingQueue<E>, LinkedList<E>, LinkedTransferQueue<E>, PriorityBlockingQueue<E>, PriorityQueue<E>, SynchronousQueue<E>, TransferQueue<E> AbstractQueue<E> This class provides skeletal implementations of some Queue operations.  ArrayBlockingQueue<E> A bounded {@linkplain BlockingQueue blocking queue} backed by an array.  ArrayDeque<E> Resizable-array implementation of the Deque interface.  BlockingDeque<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.  BlockingQueue<E> A Queue that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.  ConcurrentLinkedDeque<E> An unbounded concurrent {@linkplain Deque deque} based on linked nodes.  ConcurrentLinkedQueue<E> An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.  DelayQueue<E extends Delayed> An unbounded {@linkplain BlockingQueue blocking queue} of Delayed elements, in which an element can only be taken when its delay has expired.  Deque<E> A linear collection that supports element insertion and removal at both ends.  LinkedBlockingDeque<E> An optionally-bounded {@linkplain BlockingDeque blocking deque} based on linked nodes.  LinkedBlockingQueue<E> An optionally-bounded {@linkplain BlockingQueue blocking queue} based on linked nodes.  LinkedList<E> Doubly-linked list implementation of the List and Deque interfaces.  LinkedTransferQueue<E> An unbounded TransferQueue based on linked nodes.  PriorityBlockingQueue<E> An unbounded {@linkplain BlockingQueue blocking queue} that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations.  PriorityQueue<E> An unbounded priority {@linkplain Queue queue} based on a priority heap.  SynchronousQueue<E> A {@linkplain BlockingQueue blocking queue} in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa.  TransferQueue<E> A BlockingQueue in which producers may wait for consumers to receive elements.

A collection designed for holding elements prior to processing. Besides basic Collection operations, queues provide additional insertion, extraction, and inspection operations. 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 Queue implementations; in most implementations, insert operations cannot fail.

Summary of Queue methods

	Throws exception	Returns special value
Insert	add(e)	offer(e)
Remove	remove()	poll()
Examine	element()	peek()

Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering, and LIFO queues (or stacks) which order the elements LIFO (last-in-first-out). Whatever the ordering used, the head of the queue is that element which would be removed by a call to remove() or poll(). In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. Every Queue implementation must specify its ordering properties.

The offer method inserts an element if possible, otherwise returning false. This differs from the Collection.add method, which can fail to add an element only by throwing an unchecked exception. The offer method is designed for use when failure is a normal, rather than exceptional occurrence, for example, in fixed-capacity (or "bounded") queues.

The remove() and poll() methods remove and return the head of the queue. Exactly which element is removed from the queue is a function of the queue's ordering policy, which differs from implementation to implementation. The remove() and poll() methods differ only in their behavior when the queue is empty: the remove() method throws an exception, while the poll() method returns null.

The element() and peek() methods return, but do not remove, the head of the queue.

The Queue interface does not define the blocking queue methods, which are common in concurrent programming. These methods, which wait for elements to appear or for space to become available, are defined in the BlockingQueue interface, which extends this interface.

Queue implementations generally do not allow insertion of null elements, although some implementations, such as LinkedList, do not prohibit insertion of null. Even in the implementations that permit it, null should not be inserted into a Queue, as null is also used as a special return value by the poll method to indicate that the queue contains no elements.

Queue implementations generally do not define element-based versions of methods equals and hashCode but instead inherit the identity based versions from class Object, because element-based equality is not always well-defined for queues with the same elements but different ordering properties.

Public Method Summary

abstract boolean	add(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() Retrieves, but does not remove, the head of this queue.
abstract boolean	offer(E e) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
abstract E	peek() Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
abstract E	poll() Retrieves and removes the head of this queue, or returns null if this queue is empty.
abstract E	remove() Retrieves and removes the head of this queue.

Inherited Method Summary

From interface java.util.Collection

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

From interface java.lang.Iterable

void	forEach(Consumer<? 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<E>	iterator() Returns an iterator over elements of type T.
Spliterator<E>	spliterator() Creates a Spliterator over the elements described by this Iterable.

Public Methods

public abstract boolean add (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.

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 queue
NullPointerException	if the specified element is null and this queue does not permit null elements
IllegalArgumentException	if some property of this element prevents it from being added to this queue

public abstract E element ()

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

Returns

• the head of this queue

Throws

NoSuchElementException	if this queue is empty

public abstract boolean offer (E e)

Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. When using a capacity-restricted queue, this method is generally preferable to add(E), 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 queue, else false

Throws

ClassCastException	if the class of the specified element prevents it from being added to this queue
NullPointerException	if the specified element is null and this queue does not permit null elements
IllegalArgumentException	if some property of this element prevents it from being added to this queue

public abstract E peek ()

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

Returns

• the head of this queue, or null if this queue is empty

public abstract E poll ()

Retrieves and removes the head of this queue, or returns null if this queue is empty.

Returns

• the head of this queue, or null if this queue is empty

public abstract E remove ()

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

Returns

• the head of this queue

Throws

NoSuchElementException	if this queue is empty