Java PriorityBlockingQueue poll()方法

1.poll()方法

PriorityBlockingQueue的 poll() 方法从这个PriorityBlockingQueue的 头部检索和删除元素 。这个方法返回它从PriorityBlockingQueue中移除的元素，但是当队列是空的时候，方法将返回null。

语法

public E poll()

返回： 该方法返回来自该PriorityBlockingQueue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PriorityBlockingQueue的poll()方法。

例1： 演示PriorityBlockingQueue的poll()方法的程序，从一个数字列表中删除一个元素。

// Java Program Demonstrate poll()
// method of PriorityBlockingQueue
  
import java.util.concurrent.PriorityBlockingQueue;
  
public class GFG {
    public static void main(String[] args)
    {
  
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 5;
  
        // create object of PriorityBlockingQueue
        PriorityBlockingQueue<Integer> PrioQueue
            = new PriorityBlockingQueue<Integer>(capacityOfQueue);
  
        // Add numbers to PriorityBlockingQueue
        PrioQueue.offer(35658786);
        PrioQueue.offer(5278367);
        PrioQueue.offer(74381793);
        PrioQueue.offer(87625142);
  
        // remove numbers from head using poll()
        // and print removed number
        int removedItem = PrioQueue.poll();
  
        // print details
        System.out.println("Removed Element: " + removedItem);
        System.out.println("Now Queue Contains:");
        System.out.println(PrioQueue.toString());
    }
}

输出:

Removed Element: 5278367
Now Queue Contains:
[35658786, 87625142, 74381793]

例2： 演示poll()方法的程序，从一个字符串列表中删除字符串，如果列表为空，则返回null。

// Java Program Demonstrate poll()
// method of PriorityBlockingQueue
  
import java.util.concurrent.PriorityBlockingQueue;
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
    {
  
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 5;
  
        // create object of PriorityBlockingQueue which contains
        // name of students
        PriorityBlockingQueue<String> names
            = new PriorityBlockingQueue<String>(capacityOfQueue);
  
        // Add names of students of girls college
        names.offer("Joyita");
        names.offer("Priyanka");
  
        // remove two names from list of names
        // and print removed name
        String removedName1 = names.poll();
        String removedName2 = names.poll();
  
        // print details
        System.out.println("Removed Name 1: " + removedName1);
        System.out.println("Removed Name 2: " + removedName2);
        System.out.println("Now Queue Contains:");
        System.out.println(names.toString());
  
        // try to remove from empty PriorityBlockingQueue
        String removedName3 = names.poll();
        System.out.println("Removed Name 3: " + removedName3);
    }
}

输出:

Removed Name 1: Joyita
Removed Name 2: Priyanka
Now Queue Contains:
[]
Removed Name 3: null

2. poll(long timeout, TimeUnit unit) 方法

PriorityBlockingQueue的 poll(long timeout, TimeUnit unit) 方法从这个PriorityBlockingQueue的 头部检索并移除元素 。如果PriorityBlockingQueue是空的，那么它将等待到指定的时间，让一个元素变得可用。

语法

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

参数：

这个方法接受两个参数。

timeout(long) : 在放弃之前要等待多长时间，单位为unit。
unit(TimeUnit) : 决定如何解释timeout参数的TimeUnit。

返回： 该方法返回该PriorityBlockingQueue头部的元素，如果在指定的等待时间过后才有元素，则返回空。

异常： 该方法只抛出一个异常 InterruptedException --如果在等待时被打断。

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

例1： 演示PriorityBlockingQueue的poll(long timeout, TimeUnit unit)方法的程序，从一个数字列表中删除一个元素。

// Java Program Demonstrate poll(long timeout, TimeUnit unit)
// method of PriorityBlockingQueue
  
import java.util.concurrent.PriorityBlockingQueue;
import java.util.concurrent.TimeUnit;
  
public class GFG {
    public static void main(String[] args)
        throws InterruptedException
    {
  
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 5;
  
        // create object of PriorityBlockingQueue
        PriorityBlockingQueue<Integer> PrioQueue
            = new PriorityBlockingQueue<Integer>(capacityOfQueue);
  
        // Add numbers to PriorityBlockingQueue
        PrioQueue.offer(35658786);
        PrioQueue.offer(5278367);
  
        // Try to poll  elements from PriorityBlockingQueue
        System.out.println("Removed Number: "
                           + PrioQueue.poll(10, TimeUnit.SECONDS));
        System.out.println("List Contains" + PrioQueue);
  
        System.out.println("Removed Number: "
                           + PrioQueue.poll(10, TimeUnit.SECONDS));
        System.out.println("List Contains" + PrioQueue);
  
        System.out.println("Removed Number: "
                           + PrioQueue.poll(10, TimeUnit.SECONDS));
        System.out.println("List Contains" + PrioQueue);
    }
}

输出:

Removed Number: 5278367
List Contains[35658786]
Removed Number: 35658786
List Contains[]
Removed Number: null
List Contains[]

参考资料

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/PriorityBlockingQueue.html#poll-
https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/PriorityBlockingQueue.html#poll-long-java.util.concurrent.TimeUnit-