Queue
interface Queue<E : Any!> : MutableCollection<E>
java.util.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() |
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 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.
Summary
Public methods | |
---|---|
abstract Boolean |
add(element: E) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning |
abstract E |
element() Retrieves, but does not remove, the head of this queue. |
abstract Boolean |
offer(e: E) Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions. |
abstract E? |
peek() Retrieves, but does not remove, the head of this queue, or returns |
abstract E? |
poll() Retrieves and removes the head of this queue, or returns |
abstract E |
remove() Retrieves and removes the head of this queue. |
Public methods
add
abstract fun add(element: E): Boolean
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.
Parameters | |
---|---|
e |
the element to add |
Return | |
---|---|
Boolean |
true (as specified by Collection#add ) |
Exceptions | |
---|---|
java.lang.UnsupportedOperationException |
if the add operation is not supported by this collection |
java.lang.ClassCastException |
if the class of the specified element prevents it from being added to this queue |
java.lang.NullPointerException |
if the specified element is null and this queue does not permit null elements |
java.lang.IllegalArgumentException |
if some property of this element prevents it from being added to this queue |
java.lang.IllegalStateException |
if the element cannot be added at this time due to capacity restrictions |
element
abstract fun element(): E
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.
Return | |
---|---|
E |
the head of this queue |
Exceptions | |
---|---|
java.util.NoSuchElementException |
if this queue is empty |
offer
abstract fun offer(e: E): Boolean
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
, which can fail to insert an element only by throwing an exception.
Parameters | |
---|---|
e |
E: the element to add |
Return | |
---|---|
Boolean |
true if the element was added to this queue, else false |
Exceptions | |
---|---|
java.lang.ClassCastException |
if the class of the specified element prevents it from being added to this queue |
java.lang.NullPointerException |
if the specified element is null and this queue does not permit null elements |
java.lang.IllegalArgumentException |
if some property of this element prevents it from being added to this queue |
peek
abstract fun peek(): E?
Retrieves, but does not remove, the head of this queue, or returns null
if this queue is empty.
Return | |
---|---|
E? |
the head of this queue, or null if this queue is empty |
poll
abstract fun poll(): E?
Retrieves and removes the head of this queue, or returns null
if this queue is empty.
Return | |
---|---|
E? |
the head of this queue, or null if this queue is empty |
remove
abstract fun remove(): E
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.
Return | |
---|---|
E |
the head of this queue |
Exceptions | |
---|---|
java.util.NoSuchElementException |
if this queue is empty |