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public interface Queue
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() |
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 edu.emory.mathcs.backport.java.util.concurrent.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.
Collection
,
LinkedList
,
PriorityQueue
,
edu.emory.mathcs.backport.java.util.concurrent.LinkedBlockingQueue
,
edu.emory.mathcs.backport.java.util.concurrent.BlockingQueue
,
edu.emory.mathcs.backport.java.util.concurrent.ArrayBlockingQueue
,
edu.emory.mathcs.backport.java.util.concurrent.LinkedBlockingQueue
,
edu.emory.mathcs.backport.java.util.concurrent.PriorityBlockingQueue
Method Summary | |
---|---|
boolean |
add(Object 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. |
Object |
element()
Retrieves, but does not remove, the head of this queue. |
boolean |
offer(Object e)
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. |
Object |
peek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty. |
Object |
poll()
Retrieves and removes the head of this queue, or returns null if this queue is empty. |
Object |
remove()
Retrieves and removes the head of this queue. |
Methods inherited from interface java.util.Collection |
---|
addAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toArray |
Method Detail |
---|
boolean add(Object e)
add
in interface Collection
e
- the element to add
Collection.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 not permit null elements
IllegalArgumentException
- if some property of this element
prevents it from being added to this queueboolean offer(Object e)
add(java.lang.Object)
, 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 queueObject remove()
poll
only in that it throws an exception if this
queue is empty. is empty.
NoSuchElementException
- if this queue is emptyObject poll()
Object element()
peek
only in that it throws an exception
if this queue is empty.
NoSuchElementException
- if this queue is emptyObject peek()
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