Queue interface in java
Interface Queue
- Type Parameters: E — the type of elements held in this queue All Superinterfaces: Collection, IterableAll Known Subinterfaces: BlockingDeque, BlockingQueue, Deque, TransferQueueAll Known Implementing Classes: AbstractQueue , ArrayBlockingQueue , ArrayDeque , ConcurrentLinkedDeque , ConcurrentLinkedQueue , DelayQueue , LinkedBlockingDeque , LinkedBlockingQueue , LinkedList , LinkedTransferQueue , PriorityBlockingQueue , PriorityQueue , SynchronousQueue
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 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.
Method Summary
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.
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
The Queue Interface
A Queue is a collection for holding elements prior to processing. Besides basic Collection operations, queues provide additional insertion, removal, and inspection operations. The Queue interface follows.
public interface Queue extends Collection
Each Queue method exists in two forms: (1) one throws an exception if the operation fails, and (2) the other returns a special value if the operation fails (either null or false , depending on the operation). The regular structure of the interface is illustrated in the following table .
| Type of Operation | Throws exception | Returns special value |
|---|---|---|
| Insert | add(e) | offer(e) |
| Remove | remove() | poll() |
| Examine | element() | peek() |
Queues typically, but not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to their values see the Object Ordering section for details). Whatever ordering is used, the head of the queue is the element that 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.
It is possible for a Queue implementation to restrict the number of elements that it holds; such queues are known as bounded. Some Queue implementations in java.util.concurrent are bounded, but the implementations in java.util are not.
The add method, which Queue inherits from Collection , inserts an element unless it would violate the queue’s capacity restrictions, in which case it throws IllegalStateException . The offer method, which is intended solely for use on bounded queues, differs from add only in that it indicates failure to insert an element by returning false .
The remove and poll methods both remove and return the head of the queue. Exactly which element gets removed is a function of the queue’s ordering policy. The remove and poll methods differ in their behavior only when the queue is empty. Under these circumstances, remove throws NoSuchElementException , while poll returns null .
The element and peek methods return, but do not remove, the head of the queue. They differ from one another in precisely the same fashion as remove and poll : If the queue is empty, element throws NoSuchElementException , while peek returns null .
Queue implementations generally do not allow insertion of null elements. The LinkedList implementation, which was retrofitted to implement Queue , is an exception. For historical reasons, it permits null elements, but you should refrain from taking advantage of this, because null is used as a special return value by the poll and peek methods.
Queue implementations generally do not define element-based versions of the equals and hashCode methods but instead inherit the identity-based versions from Object .
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 interface java.util.concurrent.BlockingQueue , which extends Queue .
In the following example program, a queue is used to implement a countdown timer. The queue is preloaded with all the integer values from a number specified on the command line to zero, in descending order. Then, the values are removed from the queue and printed at one-second intervals. The program is artificial in that it would be more natural to do the same thing without using a queue, but it illustrates the use of a queue to store elements prior to subsequent processing.
import java.util.*; public class Countdown < public static void main(String[] args) throws InterruptedException < int time = Integer.parseInt(args[0]); Queuequeue = new LinkedList(); for (int i = time; i >= 0; i--) queue.add(i); while (!queue.isEmpty()) < System.out.println(queue.remove()); Thread.sleep(1000); >> >
In the following example, a priority queue is used to sort a collection of elements. Again this program is artificial in that there is no reason to use it in favor of the sort method provided in Collections , but it illustrates the behavior of priority queues.
static List heapSort(Collection c) < Queuequeue = new PriorityQueue(c); List result = new ArrayList(); while (!queue.isEmpty()) result.add(queue.remove()); return result; >
Queue interface in java
Interface Queue
- Type Parameters: E — the type of elements held in this collection All Superinterfaces: Collection, IterableAll Known Subinterfaces: BlockingDeque, BlockingQueue, Deque, TransferQueueAll Known Implementing Classes: AbstractQueue, ArrayBlockingQueue, ArrayDeque, ConcurrentLinkedDeque, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQueue, LinkedList, LinkedTransferQueue, PriorityBlockingQueue, PriorityQueue, SynchronousQueue
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 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.
Method Summary
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.
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.