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- 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 basicCollectionoperations, 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 (eithernullorfalse, depending on the operation). The latter form of the insert operation is designed specifically for use with capacity-restrictedQueueimplementations; in most implementations, insert operations cannot fail.Summary of Queue methods 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()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. EveryQueueimplementation must specify its ordering properties.The
offermethod inserts an element if possible, otherwise returningfalse. This differs from theCollection.addmethod, which can fail to add an element only by throwing an unchecked exception. Theoffermethod is designed for use when failure is a normal, rather than exceptional occurrence, for example, in fixed-capacity (or "bounded") queues.The
remove()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.The
element()andpeek()methods return, but do not remove, the head of the queue.The
Queueinterface 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 theBlockingQueueinterface, which extends this interface.Queueimplementations generally do not allow insertion ofnullelements, although some implementations, such asLinkedList, do not prohibit insertion ofnull. Even in the implementations that permit it,nullshould not be inserted into aQueue, asnullis also used as a special return value by thepollmethod to indicate that the queue contains no elements.Queueimplementations generally do not define element-based versions of methodsequalsandhashCodebut 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
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Method Summary
All Methods Instance Methods Abstract Methods Modifier and Type Method Description booleanadd(E e)Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrueupon success and throwing anIllegalStateExceptionif no space is currently available.Eelement()Retrieves, but does not remove, the head of this queue.booleanoffer(E e)Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.Epeek()Retrieves, but does not remove, the head of this queue, or returnsnullif this queue is empty.Epoll()Retrieves and removes the head of this queue, or returnsnullif this queue is empty.Eremove()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
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Method Detail
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add
boolean add(E e)
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returningtrueupon success and throwing anIllegalStateExceptionif no space is currently available.- Specified by:
addin 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 restrictionsClassCastException- if the class of the specified element prevents it from being added to this queueNullPointerException- if the specified element is null and this queue does not permit null elementsIllegalArgumentException- if some property of this element prevents it from being added to this queue
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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:
trueif the element was added to this queue, elsefalse- Throws:
ClassCastException- if the class of the specified element prevents it from being added to this queueNullPointerException- if the specified element is null and this queue does not permit null elementsIllegalArgumentException- if some property of this element prevents it from being added to this queue
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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
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poll
E poll()
Retrieves and removes the head of this queue, or returnsnullif this queue is empty.- Returns:
- the head of this queue, or
nullif this queue is empty
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element
E element()
Retrieves, but does not remove, the head of this queue. This method differs frompeekonly 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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peek
E peek()
Retrieves, but does not remove, the head of this queue, or returnsnullif this queue is empty.- Returns:
- the head of this queue, or
nullif this queue is empty
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