java.util.Queue<E> |
Known Indirect Subclasses
AbstractQueue
<E>,
ArrayBlockingQueue
<E>,
ArrayDeque
<E>,
BlockingDeque
<E>,
BlockingQueue
<E>,
ConcurrentLinkedQueue
<E>,
DelayQueue
<E extends
Delayed
>,
Deque
<E>,
LinkedBlockingDeque
<E>,
LinkedBlockingQueue
<E>,
LinkedList
<E>,
PriorityBlockingQueue
<E>,
PriorityQueue
<E>,
SynchronousQueue
<E>
|
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.
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 Methods | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
|
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.
|
||||||||||
|
Retrieves, but does not remove, the head of this queue.
|
||||||||||
|
Inserts the specified element into this queue if it is possible to do
so immediately without violating capacity restrictions.
|
||||||||||
|
Retrieves, but does not remove, the head of this queue,
or returns
null
if this queue is empty.
|
||||||||||
|
Retrieves and removes the head of this queue,
or returns
null
if this queue is empty.
|
||||||||||
|
Retrieves and removes the head of this queue.
|
[Expand]
Inherited Methods
|
|||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
From interface
java.lang.Iterable
|
|||||||||||
From interface
java.util.Collection
|
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.
e | the element to add |
---|
add(E)
)
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 |
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.
NoSuchElementException | if this queue is empty |
---|
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.
e | the element to add |
---|
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 |
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
Retrieves and removes the head of this queue, or returns null if this queue is empty.
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.
NoSuchElementException | if this queue is empty |
---|