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());     } } |
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());     } } |
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());     } } |
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());     } } |
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());     } } |
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–