Java中的 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。此方法返回它从 PriorityBlockingQueue 中删除的元素，但是当队列为空时，该方法将返回 null。

句法：

public E poll()

返回：此方法返回此 PriorityBlockingQueue 头部的元素，如果此队列为空，则返回 null。

下面的程序说明了 PriorityBlockingQueue 的 poll() 方法：

示例 1：演示 PriorityBlockingQueue 上的 poll() 方法从数字列表中删除元素的程序。

// 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 PrioQueue
            = new PriorityBlockingQueue(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]

示例 2：演示 poll() 方法从 String 值列表中删除 String 并在列表为空时返回 null 的程序。

// 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 names
            = new PriorityBlockingQueue(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 为空则它将等待到指定的时间让元素变为可用。等待时间和时间单位是作为方法的参数给出。

句法：

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

范围：
该方法接受两个参数：

1. timeout(long) : 放弃前等待多长时间，以unit为单位。
2. unit(TimeUnit) : 确定如何解释超时参数的 TimeUnit。

返回：此方法从此 PriorityBlockingQueue 的头部返回元素，如果在元素可用之前经过指定的等待时间，则返回 null。

异常：此方法仅抛出一个异常InterruptedException - 如果在等待时被中断

以下程序说明 PriorityBlockingQueue 的 poll(long timeout, TimeUnit unit) 方法：

示例 1：演示 PriorityBlockingQueue 上的 poll(long timeout, TimeUnit unit) 方法从数字列表中删除元素的程序。

// 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 PrioQueue
            = new PriorityBlockingQueue(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：

• https://docs.oracle.com/javase/8/docs/api/java Java
• https://docs.oracle.com/javase/8/docs/api/java Java