- Type Parameters:
E
- the type of elements held in this queue
- All Superinterfaces:
Collection<E>
,Iterable<E>
- All Known Subinterfaces:
BlockingDeque<E>
,BlockingQueue<E>
,Deque<E>
,TransferQueue<E>
- All Known Implementing Classes:
AbstractQueue
,ArrayBlockingQueue
,ArrayDeque
,ConcurrentLinkedDeque
,ConcurrentLinkedQueue
,DelayQueue
,LinkedBlockingDeque
,LinkedBlockingQueue
,LinkedList
,LinkedTransferQueue
,PriorityBlockingQueue
,PriorityQueue
,SynchronousQueue
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.
This interface is a member of the Java Collections Framework.
- Since:
- 1.5
-
Method Summary
Modifier and TypeMethodDescriptionboolean
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrue
upon success and throwing anIllegalStateException
if no space is currently available.element()
Retrieves, but does not remove, the head of this queue.boolean
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.peek()
Retrieves, but does not remove, the head of this queue, or returnsnull
if this queue is empty.poll()
Retrieves and removes the head of this queue, or returnsnull
if this queue is empty.remove()
Retrieves and removes the head of this queue.Methods inherited from interface java.util.Collection
addAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, remove, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray, toArray
-
Method Details
-
add
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrue
upon success and throwing anIllegalStateException
if no space is currently available.- Specified by:
add
in interfaceCollection<E>
- Parameters:
e
- the element to add- Returns:
true
(as specified byCollection.add(E)
)- Throws:
IllegalStateException
- if the element cannot be added at this time due to capacity restrictionsClassCastException
- if the class of the specified element prevents it from being added to this queueNullPointerException
- if the specified element is null and this queue does not permit null elementsIllegalArgumentException
- if some property of this element prevents it from being added to this queue
-
offer
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 toadd(E)
, which can fail to insert an element only by throwing an exception.- Parameters:
e
- the element to add- Returns:
true
if the element was added to this queue, elsefalse
- Throws:
ClassCastException
- if the class of the specified element prevents it from being added to this queueNullPointerException
- if the specified element is null and this queue does not permit null elementsIllegalArgumentException
- if some property of this element prevents it from being added to this queue
-
remove
E remove()Retrieves and removes the head of this queue. This method differs frompoll()
only in that it throws an exception if this queue is empty.- Returns:
- the head of this queue
- Throws:
NoSuchElementException
- if this queue is empty
-
poll
E poll()Retrieves and removes the head of this queue, or returnsnull
if this queue is empty.- Returns:
- the head of this queue, or
null
if this queue is empty
-
element
E element()Retrieves, but does not remove, the head of this queue. This method differs frompeek
only in that it throws an exception if this queue is empty.- Returns:
- the head of this queue
- Throws:
NoSuchElementException
- if this queue is empty
-
peek
E peek()Retrieves, but does not remove, the head of this queue, or returnsnull
if this queue is empty.- Returns:
- the head of this queue, or
null
if this queue is empty
-