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public interfaceQueue<E>extendsCollection<E>

A collection designed for holding elements prior to processing. Besides basicCollection 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 (eithernull orfalse, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restrictedQueue 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, thehead of the queue is that element which would be removed by a call toremove() orpoll(). In a FIFO queue, all new elements are inserted at thetail of the queue. Other kinds of queues may use different placement rules. EveryQueue implementation must specify its ordering properties.

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

Theremove() andpoll() 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. Theremove() andpoll() methods differ only in their behavior when the queue is empty: theremove() method throws an exception, while thepoll() method returnsnull.

Theelement() andpeek() methods return, but do not remove, the head of the queue.

TheQueue interface does not define theblocking 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 theBlockingQueue interface, which extends this interface.

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

Queue implementations generally do not define element-based versions of methodsequals andhashCode but instead inherit the identity based versions from classObject, because element-based equality is not always well-defined for queues with the same elements but different ordering properties.

This interface is a member of the Java Collections Framework.