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Queue

public interface Queue
implements Collection<E>

java.util.Queue<E>
Known Indirect Subclasses


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.

Summary

Public methods

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 methods

From interface java.util.Collection
From interface java.lang.Iterable

Public methods

add

Added in API level 11
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 E: the element to add
Returns
boolean true (as specified by 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

element

Added in API level 1
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
E the head of this queue
Throws
NoSuchElementException if this queue is empty

offer

Added in API level 1
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 E: the element to add
Returns
boolean 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

peek

Added in API level 1
E peek ()

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

Returns
E the head of this queue, or null if this queue is empty

poll

Added in API level 1
E poll ()

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

Returns
E the head of this queue, or null if this queue is empty

remove

Added in API level 1
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
E the head of this queue
Throws
NoSuchElementException if this queue is empty
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