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java.util
Interface Queue<E>
-
-
Type Parameters:
-
E - the type of elements held in this collection
-
All Superinterfaces:
-
Collection
<E>,
Iterable
<E>
-
All Known Subinterfaces:
-
BlockingDeque
BlockingQueue
<E>,
BlockingQueue
<E>,
Deque
<E>
-
All Known Implementing Classes:
-
AbstractQueue
,
ArrayBlockingQueue
,
ArrayDeque
,
ConcurrentLinkedQueue
,
DelayQueue
,
LinkedBlockingDeque
,
LinkedBlockingQueue
,
LinkedList
,
PriorityBlockingQueue
,
PriorityQueue
,
SynchronousQueue
-
public interface Queue<E>
- extends Collection<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.
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
-
See Also:
-
Collection
,
LinkedList
,
PriorityQueue
,
LinkedBlockingQueue
,
BlockingQueue
,
ArrayBlockingQueue
,
LinkedBlockingQueue
,
PriorityBlockingQueue
Method Summary
|
boolean
|
add
(
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.
|
E
|
element
()
Retrieves, but does not remove, the head of this queue. |
boolean |
offer
(
E
Inserts the specified element into this
queue
queue,
if
it is possible to do so immediately without violating capacity restrictions.
possible.
|
E
|
peek
()
Retrieves, but does not remove, the head of this queue,
or returns
returning
null
if this queue is empty. |
E
|
poll
()
Retrieves and removes the head of this queue, or
returns
null
if this queue is empty. |
E
|
remove
()
Retrieves and removes the head of this queue. |
Methods inherited from interface java.util.
Collection
|
add
,
addAll
,
clear
,
contains
,
containsAll
,
equals
,
hashCode
,
isEmpty
,
iterator
,
remove
,
removeAll
,
retainAll
,
size
,
toArray
,
toArray
|
add
offer
boolean
add ![](../../diffpics/oold.gif)
offer (E
e) ![](../../diffpics/oold.gif)
o)
-
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning
Inserts the specified element into this queue, if possible. When using queues that may impose insertion restrictions (for example capacity bounds), method
true
offer
upon success and throwing an
IllegalStateException
if no space is currently available.
is generally preferable to method
Collection.add(E)
, which can fail to insert an element only by throwing an exception.
-
-
Specified by:
-
add
in interface
Collection
<
E
>
-
-
Parameters:
-
e
o
- the element to
add
insert.
-
Returns:
-
true
(as specified by
Collection.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 not permit null elements
-
IllegalArgumentException
- if some property of this element prevents it from being added to this queue
if it was possible to add the element to this queue, else
false
offer
poll
boolean
![](../../diffpics/oold.gif)
E
offer ![](../../diffpics/oold.gif)
poll
(
E
e) ![](../../diffpics/oold.gif)
()
-
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.
Retrieves and removes the head of this queue, or
null
if this queue is empty.
-
-
-
Parameters:
-
e - the element to add
-
Returns:
-
the head of this queue, or
true
null
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
if this queue is empty.
remove
E remove()
-
Retrieves and removes the head of this queue. This method differs from
poll
only
the
poll
method
in that it throws an exception if this queue is empty.
-
-
-
Returns:
-
the head of this
queue
queue.
-
Throws:
-
NoSuchElementException
- if this queue is
empty
empty.
poll
peek
E
poll ![](../../diffpics/oold.gif)
peek ()
-
Retrieves and removes the head of this queue, or returns
Retrieves, but does not remove, the head of this queue, returning
null
if this queue is empty.
-
-
-
Returns:
-
the head of this queue, or
null
if this queue is
empty
empty.
element
E element()
-
Retrieves, but does not remove, the head of this queue. This method differs from
peek
the
peek
method
only in that it throws an exception if this queue is empty.
-
-
-
Returns:
-
the head of this
queue
queue.
-
Throws:
-
NoSuchElementException
- if this queue is
empty
empty.
peek
E
peek
()
-
Retrieves, but does not remove, the head of this queue, or returns
null
if this queue is empty.
-
-
-
Returns:
-
the head of this queue, or
null
if this queue is empty