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.
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.
This interface is a member of the
../../../technotes/guides/collections/index.html">
Java Collections Framework.