A collection designed for holding elements prior to processing.
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
false, depending on the
operation). The latter form of the insert operation is designed
specifically for use with capacity-restricted
implementations; in most implementations, insert operations cannot
|Throws exception||Returns special value|
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
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
must specify its ordering properties.
offer method inserts an element if possible,
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.
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
poll() methods differ only in their behavior when the
queue is empty: the
remove() method throws an exception,
poll() method returns
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
java.util.concurrent.BlockingQueue interface, which
extends this interface.
Queue implementations generally do not allow insertion
null elements, although some implementations, such as
LinkedList, do not prohibit insertion of
Even in the implementations that permit it,
not be inserted into a
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
hashCode but instead inherit the identity based versions
Object, because element-based equality is not
always well-defined for queues with the same elements but different
This interface is a member of the Java Collections Framework.
|the type of elements held in this collection|
java.util.Collection, LinkedList, PriorityQueue, java.util.concurrent.LinkedBlockingQueue, java.util.concurrent.BlockingQueue, java.util.concurrent.ArrayBlockingQueue, java.util.concurrent.LinkedBlockingQueue, java.util.concurrent.PriorityBlockingQueue