Java ArrayBlockingQueue poll()方法

ArrayBlockingQueue 是有边界的、阻塞的队列，它在内部存储着由数组支持的元素。

ArrayBlockingQueue 类是Java集合框架的一个成员。
有界意味着它将有一个固定的大小，你不能存储超过队列容量的元素数量。
该队列也遵循FIFO（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入一个满的队列或从一个空的队列中取出一个元素，那么队列将阻止你。

有两种类型的poll()方法，取决于传递的参数数量。

poll()方法检索并删除该队列头部的元素**.如果队列是空的，那么该方法将返回空。

语法:

public E poll()

返回值：该方法返回该队列头部的元素，如果该队列是空的，则返回空。

下面的程序说明了ArrayBlockingQueue的poll()方法。

示例 1:

/*
*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<Integer> queue = new 
               ArrayBlockingQueue<Integer>(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);
    }
}

输出:

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[]

示例 2:

/*
* 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<User> queue = 
               new ArrayBlockingQueue<User>(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);
    }
}

输出:

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

poll(long timeout, TimeUnit unit)方法检索并删除该队列头部的元素。如果队列是空的，那么它将等待到指定的时间，以获得一个元素。

语法:

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

参数：该方法需要两个参数。

timeout (long) – 在放弃之前要等待多长时间，单位为unit。
unit (TimeUnit)- 一个决定如何解释超时参数的TimeUnit。

返回值：该方法返回这个队列的头，如果在指定的等待时间过后才有一个元素，则返回空。

异常：如果在等待时被打断，该方法会抛出InterruptedException。

下面的程序说明了ArrayBlockingQueue的poll(long timeout, TimeUnit unit)方法。

/*
* 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<Integer> queue = 
            new ArrayBlockingQueue<Integer>(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));
    }
}

输出:

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

参考资料:
https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ArrayBlockingQueue.html#poll()