java Java 中的 ArrayBlockingQueue poll()方法

Java 中的 ArrayBlockingQueue poll()方法

原文:https://www . geeksforgeeks . org/arrayblockingqueue-poll-method-in-Java/

ArrayBlockingQueue是有界的，阻塞队列存储由数组支持的内部元素。

ArrayBlockingQueue类是 Java Collections 框架的成员。
有界意味着它将有一个固定的大小，你不能存储的元素数量超过队列的容量。
队列还遵循先进先出规则来存储和移除队列中的元素。
如果你试图把一个元素放到一个满的队列中，或者从一个空的队列中取出一个元素，那么这个队列就会阻塞你。

根据传递的参数数量，有两种轮询()方法。

The poll() method retrieves and removes element from head of this queue.If queue is empty then method will return null. Syntax:

java public E poll()

返回值:该方法从该队列的头部返回元素，如果该队列为空，则返回 null。

下面的程序说明了 ArrayBlockingQueue 的 poll()方法。

程序 1:

```java / Program Demonstrate poll() method of ArrayBlockingQueue. */

import java.util.concurrent.ArrayBlockingQueue;

public class GFG {

public static void main(String[] args)     {         // define capacity of ArrayBlockingQueue         int capacity = 5;

// create object of ArrayBlockingQueue         ArrayBlockingQueue queue = new                ArrayBlockingQueue(capacity);

// Add elements to ArrayBlockingQueue         queue.offer(423);         queue.offer(233);         queue.offer(356);

// print elements         System.out.println("Queue Contains" + queue);

// try to poll elements         System.out.println("Removing From head: " +                                        queue.poll());         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                                        queue.poll());         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                                        queue.poll());         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                                        queue.poll());         System.out.println("Queue Contains" + queue);     } } ```

Output:

```java Queue Contains[423, 233, 356] Removing From head: 423 Queue Contains[233, 356] Removing From head: 233 Queue Contains[356] Removing From head: 356 Queue Contains[] Removing From head: null Queue Contains[]

```

程序 2:

```java / * Program Demonstrate poll() method of ArrayBlockingQueue. /

import java.util.concurrent.ArrayBlockingQueue;

public class GFG {

// Create a User Object with name and age as an attribute     public class User {

public String name;         public String age;         User(String name, String age)         {             this.name = name;             this.age = age;         }     }

// Main Method     public static void main(String[] args)     {         GFG gfg = new GFG();         gfg.pollMethodExample();     }

// Method to give example of contains function     public void pollMethodExample()     {

// Define the capacity of ArrayBlockingQueue         int capacity = 5;

// Create object of ArrayBlockingQueue         ArrayBlockingQueue queue =                new ArrayBlockingQueue(capacity);

// Create user objects         User user1 = new User("Aman", "24");         User user3 = new User("Sanjeet", "25");

// Add Objects to ArrayBlockingQueue         queue.offer(user1);         queue.offer(user3);

// Poll users from queue         User user = queue.poll();         System.out.println("removing user having name = "                                             + user.name);         user = queue.poll();         System.out.println("removing user having name = "                                             + user.name);

// Now queue is empty         // Try to remove it will return null         user = queue.poll();         System.out.println("removing user having name = "                                                  + user);     } } ```

Output:

```java removing user having name = Aman removing user having name = Sanjeet removing user having name = null

```
The poll(long timeout, TimeUnit unit) method retrieves and removes element from head of this queue. If the queue is empty then it will, wait till a specified time for an element to become available. Syntax:

java public E poll(*long timeout, TimeUnit unit*) throws InterruptedException

参数:该方法取两个参数:
- 超时(长)–放弃前需要等待多长时间，单位为单位。
- 单位(时间单位)-决定如何解释超时参数的时间单位。
返回值:该方法返回该队列的头，如果在元素可用之前经过了指定的等待时间，则返回空值。 异常:该方法会抛出中断异常如果在等待时被中断。

下面的程序说明了 ArrayBlockingQueue 的轮询(长超时，时间单位)方法。

```java / * Program Demonstrate offer(E e, long timeout, TimeUnit unit) * method of ArrayBlockingQueue. /

import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.TimeUnit;

public class GFG {

public static void main(String[] args)                        throws InterruptedException     {         // Define capacity of ArrayBlockingQueue         int capacity = 5;

// Create object of ArrayBlockingQueue         ArrayBlockingQueue queue =             new ArrayBlockingQueue(capacity);

// Add elements to ArrayBlockingQueue         queue.offer(423);         queue.offer(233);         queue.offer(356);

// Print elements         System.out.println("Queue Contains" + queue);

// Try to poll elements         System.out.println("Removing From head: "                   + queue.poll(10, TimeUnit.SECONDS));         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: "                   + queue.poll(10, TimeUnit.SECONDS));         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                     queue.poll(10, TimeUnit.SECONDS));         System.out.println("Queue Contains" + queue);         System.out.println("Removing From head: " +                    queue.poll(10, TimeUnit.SECONDS));     } } ```

Output:

```java Queue Contains[423, 233, 356] Removing From head: 423 Queue Contains[233, 356] Removing From head: 233 Queue Contains[356] Removing From head: 356 Queue Contains[] Removing From head: null

```