Java Queue poll()方法

Queue接口 的 poll() 方法返回并删除容器前端的元素。它删除了容器中的元素。当队列为空时，该方法不会抛出一个异常，而是返回null。

语法

E poll()

返回： 该方法返回容器前面的元素或队列的头部。当队列为空时，它返回null。

下面的程序说明了Queue的poll()方法。

程序1： 在 LinkedList 的帮助下 。

// Java Program Demonstrate poll()
// method of Queue
  
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Queue
        Queue<Integer> Q
            = new LinkedList<Integer>();
  
        // Add numbers to end of Queue
        Q.add(7855642);
        Q.add(35658786);
        Q.add(5278367);
        Q.add(74381793);
  
        // print queue
        System.out.println("Queue: " + Q);
  
        // print head and deletes the head
        System.out.println("Queue's head: " + Q.poll());
  
        // print head and deleted the head
        System.out.println("Queue's head: " + Q.poll());
    }
}

输出:

Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642
Queue's head: 35658786

程序2： 演示当队列变空时队列的poll()方法。

// Java Program Demonstrate poll()
// method of Queue when the Queue becomes empty
  
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Queue
        Queue<Integer> Q
            = new LinkedList<Integer>();
  
        // Add numbers to end of Queue
        Q.add(423);
        Q.add(3432);
  
        // print queue
        System.out.println("Queue: " + Q);
  
        // print head and deletes the head
        System.out.println("Queue's head: " + Q.poll());
  
        // print head and deleted the head
        System.out.println("Queue's head: " + Q.poll());
  
        // print queue
        System.out.println("Queue: " + Q);
  
        // print null as Queue is empty now
        System.out.println("Queue's head: " + Q.poll());
    }
}

输出:

Queue: [423, 3432]
Queue's head: 423
Queue's head: 3432
Queue: []
Queue's head: null

程序3： 在 ArrayDeque 的帮助下 。

// Java Program Demonstrate poll()
// method of Queue
  
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Queue
        Queue<Integer> Q
            = new ArrayDeque<Integer>();
  
        // Add numbers to end of Queue
        Q.add(7855642);
        Q.add(35658786);
        Q.add(5278367);
        Q.add(74381793);
  
        // print queue
        System.out.println("Queue: " + Q);
  
        // print head and deletes the head
        System.out.println("Queue's head: " + Q.poll());
  
        // print head and deleted the head
        System.out.println("Queue's head: " + Q.poll());
    }
}

输出:

Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642
Queue's head: 35658786

程序4： 在 ConcurrentLinkedDeque 的帮助下 。

// Java Program Demonstrate poll()
// method of Queue
  
import java.util.*;
import java.util.concurrent.ConcurrentLinkedDeque;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Queue
        Queue<Integer> Q
            = new ConcurrentLinkedDeque<Integer>();
  
        // Add numbers to end of Queue
        Q.add(7855642);
        Q.add(35658786);
        Q.add(5278367);
        Q.add(74381793);
  
        // print queue
        System.out.println("Queue: " + Q);
  
        // print head and deletes the head
        System.out.println("Queue's head: " + Q.poll());
  
        // print head and deleted the head
        System.out.println("Queue's head: " + Q.poll());
    }
}

输出:

Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642
Queue's head: 35658786

程序5： 在 LinkedBlockingDeque 的帮助下 。

// Java Program Demonstrate poll()
// method of Queue
  
import java.util.*;
import java.util.concurrent.LinkedBlockingDeque;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Queue
        Queue<Integer> Q
            = new LinkedBlockingDeque<Integer>();
  
        // Add numbers to end of Queue
        Q.add(7855642);
        Q.add(35658786);
        Q.add(5278367);
        Q.add(74381793);
  
        // print queue
        System.out.println("Queue: " + Q);
  
        // print head and deletes the head
        System.out.println("Queue's head: " + Q.poll());
  
        // print head and deleted the head
        System.out.println("Queue's head: " + Q.poll());
    }
}

输出:

Queue: [7855642, 35658786, 5278367, 74381793]
Queue's head: 7855642
Queue's head: 35658786

参考资料: https://docs.oracle.com/javase/8/docs/api/java/util/Queue.html#poll-