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
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 SummaryModifier and TypeMethodDescriptionbooleanInserts 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.element()Retrieves, but does not remove, the head of this queue.booleanInserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.peek()Retrieves, but does not remove, the head of this queue, or returnsnullif this queue is empty.poll()Retrieves and removes the head of this queue, or returnsnullif this queue is empty.remove()Retrieves and removes the head of this queue.Methods declared in interface java.util.CollectionaddAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, remove, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray, toArray
- 
Method Details- 
addInserts 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 interface- Collection<E>
- Parameters:
- e- the element to add
- Returns:
- true(as specified by- Collection.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
 
- 
offerInserts 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, else- false
- 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
 
- 
removeE 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
 
- 
pollE 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
 
- 
elementE 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
 
- 
peekE 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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