Queue (Java 2 Platform SE 5.0) - JDK 5 Documentation v1.5.0, Java 2 SDK 英文文档
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JavaTM 2 Platform Standard Ed. 5.0 |
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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:
- BlockingQueue<E>
- All Known Implementing Classes:
- AbstractQueue, ArrayBlockingQueue, ConcurrentLinkedQueue, DelayQueue, 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.
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 | |
|---|---|
E |
element()
Retrieves, but does not remove, the head of this queue. |
boolean |
offer(E o)
Inserts the specified element into this queue, if possible. |
E |
peek()
Retrieves, but does not remove, the head of this queue, returning null if this queue is empty. |
E |
poll()
Retrieves and removes the head of this queue, or 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 |
| Method Detail |
|---|
offer
boolean offer(E o)
- Inserts the specified element into this queue, if possible. When
using queues that may impose insertion restrictions (for
example capacity bounds), method offer is generally
preferable to method
Collection.add(E), which can fail to insert an element only by throwing an exception.- Parameters:
o- the element to insert.- Returns:
- true if it was possible to add the element to this queue, else false
poll
E poll()
- Retrieves and removes the head of this queue, or null
if this queue is empty.
- Returns:
- the head of this queue, or null if this queue is empty.
remove
E remove()
- Retrieves and removes the head of this queue. This method
differs from the poll method 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,
returning 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 the peek method 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.
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JavaTM 2 Platform Standard Ed. 5.0 |
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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
Copyright 2004 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.