Interface Queue<E>
- 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
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.
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 Type | Method | Description |
---|---|---|
boolean | add(E e) | Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning |
E | element() | Retrieves, but does not remove, the head of this queue. |
boolean | offer(E e) | Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. |
E | peek() | Retrieves, but does not remove, the head of this queue, or returns |
E | poll() | Retrieves and removes the head of this queue, or returns |
E | remove() | Retrieves and removes the head of this queue. |
Methods declared in interface java.util.Collection
addAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, remove, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray, toArray
Methods declared in interface java.lang.Iterable
forEach
Method Detail
add
boolean add(E 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.
- 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 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
offer
boolean offer(E e)
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.
- 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 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
remove
E remove()
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.
- 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 returns null
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 from peek
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 returns null
if this queue is empty.
- Returns:
- the head of this queue, or
null
if this queue is empty