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Type Parameters:
E - the type of elements held in this collection
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 interfaceQueue<E>extendsCollection<E>
```
A collection designed for holding elements prior to processing. Besides basicCollection 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 (eithernull orfalse, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restrictedQueue 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, thehead of the queue is that element which would be removed by a call toremove() orpoll(). In a FIFO queue, all new elements are inserted at the tail of the queue. Other kinds of queues may use different placement rules. EveryQueue implementation must specify its ordering properties.
Theoffer method inserts an element if possible, otherwise returningfalse. This differs from theCollection.add method, which can fail to add an element only by throwing an unchecked exception. Theoffer method is designed for use when failure is a normal, rather than exceptional occurrence, for example, in fixed-capacity (or "bounded") queues.
Theremove() andpoll() 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. Theremove() andpoll() methods differ only in their behavior when the queue is empty: theremove() method throws an exception, while thepoll() method returnsnull.
Theelement() andpeek() methods return, but do not remove, the head of the queue.
TheQueue interface does not define theblocking 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 theBlockingQueue interface, which extends this interface.
Queue implementations generally do not allow insertion ofnull elements, although some implementations, such asLinkedList, do not prohibit insertion ofnull. Even in the implementations that permit it,null should not be inserted into aQueue, asnull is also used as a special return value by thepoll method to indicate that the queue contains no elements.
Queue implementations generally do not define element-based versions of methodsequals andhashCode but instead inherit the identity based versions from classObject, 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

Methods
Modifier and Type	Method and 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`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 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`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
addAll,clear,contains,containsAll,equals,hashCode,isEmpty,iterator,remove,removeAll,retainAll,size,toArray,toArray

- 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, returningtrue upon success and throwing anIllegalStateException if no space is currently available.
    Specified by:
    add in interface Collection<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 toadd(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 frompoll 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 returnsnull if this queue is empty.
    Returns:
    the head of this queue, ornull if this queue is empty
  - element
```
E element()
```
    Retrieves, but does not remove, the head of this queue. This method differs frompeek 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 returnsnull if this queue is empty.
    Returns:
    the head of this queue, ornull if this queue is empty

	Throws exception	Returns special value
Insert	`add(e)`	`offer(e)`
Remove	`remove()`	`poll()`
Examine	`element()`	`peek()`

Movatterモバイル変換

Interface Queue<E>

Method Summary

Methods inherited from interface java.util.Collection

Method Detail

add

offer

remove

poll

element

peek