Queue poll() method in Java

The poll() method of Queue Interface returns and removes the element at the front end of the container. It deletes the element in the container. The method does not throws an exception when the Queue is empty, it returns null instead.

Syntax:

E poll()

Returns: This method returns the element at the front of the container or the head of the Queue. It returns null when the Queue is empty.

Below programs illustrate poll() method of Queue:

Program 1: With the help of LinkedList.

// 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());
    }
}

Output:

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

Program 2: To Demonstrate poll() method of Queue when the Queue becomes empty

// 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());
    }
}

Output:

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

Program 3: With the help of ArrayDeque.

// 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());
    }
}

Output:

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

Program 4: With the help of ConcurrentLinkedDeque.

// 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());
    }
}

Output:

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

Program 5: With the help of LinkedBlockingDeque.

// 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());
    }
}

Output:

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

Reference: https://docs.oracle.com/javase/8/docs/api/java/util/Queue.html#poll–